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#41
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: On Jun 24, 3:34*pm, John Byrns wrote: I want to see a schematic with all test results before I make up my mind on Alex's FB "trick." It could be a clever trick, or a swindle. However the network introduces both a zero and pole into the response, with the zero at a higher frequency than the pole. *Remember this network is just another tool in your toolbox; it is not a cure all and requires some sophistication in its application. *Now the one thing I know about stabilizing the low frequency response of a feedback system is that it is all about correctly placing the poles, and zeros if there are any. In any amp where there are say 2 CR coupled stages and a final stage with LR then you have a recipe for LF instability and a poor margin of stability at LF. OK, I have worked through some of the math and understand more fully what is going on with Alex's feedback network. As I said the network introduces both a zero and a pole in the loop gain. Ignoring the added pole for a moment, the zero can be placed so that it exactly cancels the effect of one of the three poles in the amplifier you describe above, with 2 CR coupled stages and a final stage with LR, effectively reducing the number of low frequency poles by 1, making the LF stability problem easier to deal with. The zero effectively cancels both the phase shift and amplitude roll off caused by the pole that is being canceled. Unfortunately it is impossible, at least so far as I know, to build a network with an isolated zero such as I have described, so an actual network, such as Alex's, must include a pole at a lower frequency. Hopefully this new pole won't cause us too much trouble if we place it at a very low frequency where the loop gain has already fallen well below 1.0 as a result of the other two remaining poles that weren't canceled. Now the obvious question is, why bother with this extra complexity when we could simply directly move one of the 3 poles to a very low frequency, as would probably be part of the normal pole staggering process anyway? I will leave that for others to comment on as I have not personally mucked about in my workshop with amplifiers that have 3 LF poles. I suspect that one reason may have to do with LF overload when using a Bean Counter approved OPT. I can see how Alex's network has the potential to resolve a problem I have encountered when mucking about with simpler amplifiers having only 2 poles. When using OPTs designed by Bean Counters, especially SE OPTs, there is a tendency towards LF overload in the OPT and final tube(s). I have attempted to mitigate this problem by choosing a relatively high pole frequency for the interstage coupling network to keep LF signals out of the OPT and final tube(s). This puts the interstage pole too close to the pole caused by the OPT which then causes a bump in the ³CLG² low frequency response, plus of course it isn't really a very good solution to the LF overload problem. It occurs to me that Alex's feedback network might also offer a solution to the OPT saturation problem in Bean Counter designed OPTs, just as it offers a solution to the input stage problem. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#42
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 26, 2:25*am, John Byrns wrote:
In article , *Patrick Turner wrote: On Jun 24, 3:34*pm, John Byrns wrote: I want to see a schematic with all test results before I make up my mind on Alex's FB "trick." It could be a clever trick, or a swindle. However the network introduces both a zero and pole into the response, with the zero at a higher frequency than the pole. *Remember this network is just another tool in your toolbox; it is not a cure all and requires some sophistication in its application. *Now the one thing I know about stabilizing the low frequency response of a feedback system is that it is all about correctly placing the poles, and zeros if there are any. In any amp where there are say 2 CR coupled stages and a final stage with LR then you have a recipe for LF instability and a poor margin of stability at LF. OK, I have worked through some of the math and understand more fully what is going on with Alex's feedback network. *As I said the network introduces both a zero and a pole in the loop gain. Ignoring the added pole for a moment, the zero can be placed so that it exactly cancels the effect of one of the three poles in the amplifier you describe above, with 2 CR coupled stages and a final stage with LR, effectively reducing the number of low frequency poles by 1, making the LF stability problem easier to deal with. I wish I could be so sure. Just because you have in theory "cancelled out" one devil it doesn't mean you've done much, especially considering that the Laex FB path renders the FB amp to be a cathode follower at DC, when you have no ability at all for the V0 to follow and input at DC. The circuit needs to be built and tested. But notice how silent and bone lazy everyone is about the issue. They just don't want to get out of the their arm chairs. How can I have respect for laziness? *The zero effectively cancels both the phase shift and amplitude roll off caused by the pole that is being canceled. *Unfortunately it is impossible, at least so far as I know, to build a network with an isolated zero such as I have described, so an actual network, such as Alex's, must include a pole at a lower frequency. *Hopefully this new pole won't cause us too much trouble if we place it at a very low frequency where the loop gain has already fallen well below 1.0 as a result of the other two remaining poles that weren't canceled. Now the obvious question is, why bother with this extra complexity when we could simply directly move one of the 3 poles to a very low frequency, as would probably be part of the normal pole staggering process anyway? *I will leave that for others to comment on as I have not personally mucked about in my workshop with amplifiers that have 3 LF poles. *I suspect that one reason may have to do with LF overload when using a Bean Counter approved OPT. Its not always convenient to move poles down, and often it merely moves the oscillaton F lower. It can be exasperating to find that even if you increase coupling caps from say 0.22uF to 2.2uF, the trace on the CRO still slowly rises and falls because Fo has just gone lower. But you don't need a bean counter designed OPT to give a pole that is at an F too high. You could have a well designed OPT for an SE amp which will is designed to saturate at 20Hz at full PO and yet it oscillates at LF. This is because the permeability of the core has been much reduced by the air gap from the maximum one might find in a PP OPT with fully interleaved laminations. So I have found my shelving network especially effective in SE amps. The Williamson PP amp required Lp = 100H minimum with triode Ra-a of 3k2 plus RLa-a = 10k in parallel, ie, the ratio of RAA to Lp = 2k4:100H, giving a pole at 3.8Hz. With say 4 x EL34 in parallel SET, Ra = 310 ohms, RL might be 1k0, so RA = 236 ohms. Lp might be 8H to give XLp = 1k0 at 20Hz, so the -3dB response pole is at 236 / ( 6.28 x 8 ) = 4.7Hz. The SE amp is then in theory slightly worse off with regard to phase shift at LF. My shelving network is especially effective where the OPT has far less inductance than it should and usually the Fsat is also way too high, eg, in most old radios the OPTs are allowed to saturate at 70Hz. This is why they are so small in size; Afe is 1/4 of the size needed for hi- fi, for the number of turns used. In the case of a pentode or beam tetrode SE OP tube, the open loop LF pole is determined by the RLa and the Lp, and Ra has little effect because Ra is such a high R. So say you have 4 x EL34 in parallel pentode then RLa = 1k2, and Lp needs to be 9.6H and the response is down 3dB at 20Hz. If the speaker load is high, or there is no speaker connected, then the LF open loop pole is between Ra and Lp, and if Ra for the 4 x EL34 = 4k0, then pole rises to 66Hz. One cannot easily move the pole. So, just how Alexe's brainchild works in all conditions remains to be established. Unconditional stability is mandatory in all my amp designs as it is in all other reputable brand names. I can see how Alex's network has the potential to resolve a problem I have encountered when mucking about with simpler amplifiers having only 2 poles. * When using OPTs designed by Bean Counters, especially SE OPTs, there is a tendency towards LF overload in the OPT and final tube(s). * Nearly all the SE amps from brand names have Fsat too high and Lp too low. And so do many PP amps. amps with only 2 stages like Quad-II have better LF stability with shelving networks. They are inherently stable because of their single CR coupling and OPT LR but the trace wobbles after going from high level to zero level. It remains to be seen if they'd enjoy a cap in the FB network. I have attempted to mitigate this problem by choosing a relatively high pole frequency for the interstage coupling network to keep LF signals out of the OPT and final tube(s). * This puts the interstage pole too close to the pole caused by the OPT which then causes a bump in the ³CLG² low frequency response, plus of course it isn't really a very good solution to the LF overload problem. *It occurs to me that Alex's feedback network might also offer a solution to the OPT saturation problem in Bean Counter designed OPTs, just as it offers a solution to the input stage problem. Mere postulations John. The only real truth is known when theory is applied. Patrick Turner. |
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#43
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
"John Byrns" wrote in message ... In article , Patrick Turner wrote: On Jun 24, 3:34 pm, John Byrns wrote: I want to see a schematic with all test results before I make up my mind on Alex's FB "trick." It could be a clever trick, or a swindle. However the network introduces both a zero and pole into the response, with the zero at a higher frequency than the pole. Remember this network is just another tool in your toolbox; it is not a cure all and requires some sophistication in its application. Now the one thing I know about stabilizing the low frequency response of a feedback system is that it is all about correctly placing the poles, and zeros if there are any. In any amp where there are say 2 CR coupled stages and a final stage with LR then you have a recipe for LF instability and a poor margin of stability at LF. OK, I have worked through some of the math and understand more fully what is going on with Alex's feedback network. As I said the network introduces both a zero and a pole in the loop gain. Ignoring the added pole for a moment, the zero can be placed so that it exactly cancels the effect of one of the three poles in the amplifier you describe above, with 2 CR coupled stages and a final stage with LR, effectively reducing the number of low frequency poles by 1, making the LF stability problem easier to deal with. The zero effectively cancels both the phase shift and amplitude roll off caused by the pole that is being canceled. Unfortunately it is impossible, at least so far as I know, to build a network with an isolated zero such as I have described, so an actual network, such as Alex's, must include a pole at a lower frequency. Hopefully this new pole won't cause us too much trouble if we place it at a very low frequency where the loop gain has already fallen well below 1.0 as a result of the other two remaining poles that weren't canceled. Now the obvious question is, why bother with this extra complexity when we could simply directly move one of the 3 poles to a very low frequency, as would probably be part of the normal pole staggering process anyway? I will leave that for others to comment on as I have not personally mucked about in my workshop with amplifiers that have 3 LF poles. I suspect that one reason may have to do with LF overload when using a Bean Counter approved OPT. I can see how Alex's network has the potential to resolve a problem I have encountered when mucking about with simpler amplifiers having only 2 poles. When using OPTs designed by Bean Counters, especially SE OPTs, there is a tendency towards LF overload in the OPT and final tube(s). I have attempted to mitigate this problem by choosing a relatively high pole frequency for the interstage coupling network to keep LF signals out of the OPT and final tube(s). This puts the interstage pole too close to the pole caused by the OPT which then causes a bump in the ³CLG² low frequency response, plus of course it isn't really a very good solution to the LF overload problem. It occurs to me that Alex's feedback network might also offer a solution to the OPT saturation problem in Bean Counter designed OPTs, just as it offers a solution to the input stage problem. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ You grasped the idea perfectly well John. This frequency compensation is useful to prevent a lousy OPT and 6AQ5 overloading in a lousy boring audio amp found in those boring AA5 style radios. Below is the sequence of mods you do to improve the radio. 1. In an AM detector you remove a cap coupling a detector load (470K||100pF) to the volume control (1M pot). In fact you are making the volume control the detector load. This is for the detector to be able to handle nearly 100% modulation. But now you det DC on the input of the audio amp. 2. You decouple this DC from the pot wiper by a 0.02...0.05uF, but you need 10M input impedance (grid leak) so that the AM detector is not loaded and 100% modulation is still handled. 3. On most stations your AM detector delivers 1...4V of audio, while sensitivity of a typical boring feedbackless two-stage audio amp is 100...200mV. The radio always works with volume control close to minimum. The speaker is boomy (not damped), distortion is high. You want to trade the sensitivity excess for distortion and apply NFB, reducing sensitivity to 0.5...1V. You are enjoying tight crispy sound, but THAT IS WHERE PROBLEMS BEGIN. 4. Now the bandwidth of your amp is in theory goes to 5...10Hz, but because of the lousy OPT, these low frequencies do not reach your ears, but only overload the 6AQ5, since the error signal becomes too large at low frequencies. 5. Note that neither the american aggressive LF cutoff by reducing interstage cap to 2000pF nor the Patrick's shelving does not work, since still yjr NFB is pushing to maintaim unmaintainable LF output, the error signal is large, the first stage is overloading, and (IMPORTANT!) since the shelving is virtually a differentiator, it accentuates all the harmonics generated in the overloaded 1-st stage and feeds them to the 6AQ5. Instead of overloading the 6AQ5 we have emphasized distortion from the first stage. 6. That is where you need this RRC compensation in the feedback to roll the low-frequency response of the amp in line with the capability of the lousy OPT. Typically from 80....100Hz down. Applying the RRC divider provides undistorted output down to 10...25Hz, because the error signal remains under control. 7. However at the frequencies lower than 10...25Hz, where the NFB RRC network levels out at 100% beta, the error signal continues to rise. This VLF content caused by fadings, AGC knocked by atmospheric interference, etc. is smaller, and some might stop here, but a purist might like to prevent overloading even at VLF. 8. To do the above one needs to degenerate the first stage gain, rather than to use a Partick's attenuator after it. To degenerate the 1-st stage gain you need to place say 22K resistor in series with the 1-st stage cathode, and shunt this resistor by a 0.22...0.47uF capacitor. Thus for medium frequencies the 1-st stage will be working as usual, but ay low frequencies the transconductance will be degenerating with the perfect linearity of the first stage maintained. It is the same shelving, but implemented in a wise linear mode. (Of course the grid leak can not be taken to GND any more, it should be connected to the cathode or a tap in this 22K resistor.) At infinitely low frequencies the gain of the first stage is to be degenerated to 10...20 so that the 1-st stage output is just below the negative bias of 6AQ5 and the later is never overloaded. Usually I apply mods up to #7 and sometimes #8 too if I can find enough free solder lugs around the 1-st audio stage tube. Of course, all of the above implies the cathode of the 1-st audio tube has to be free of duo-diode functionality. That forces to use separate diodes for AGC and the AM detector. In some cases instead of 6Q7, 6B8, 6AV6, etc. I might use a 6SL7 tube with one triode as the 1-st audio and the other triode as a diode for the AM detector, and a silicon diode as an AGC detector. Regards, Alex |
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#44
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Quote:
These were not always perfect so that rumble was a concern. Another concern had to do with the possibility of driving the loudspeaker at frequencies below it’s normal operating range, perhaps by the above mentioned turntable rumble. These two problems either separately or together are a good recipe for Intermodulation Distortion both in the speaker & the amp. I was also troubled by possible damage to the output tubes or the speakers caused by switching transients. As a result, I devised a passive HP filter for the amps front end which is two stage rather than one. To do this you need to arrange the impedance of the second section to be in the range five to ten times that of the first. That’s how I arranged the gain control in the first section & something of higher impedance in the second section. I did not believe a low pass filter made sense at this point. Here is the response of the high pass network High Pass Section- 3 db down @ 20 hz 44 db down @ One hz 170 degree phase shift 12 db / octave |
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#45
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Quote:
Cheers, John |
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#46
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: On Jun 26, 2:25*am, John Byrns wrote: Now the obvious question is, why bother with this extra complexity when we could simply directly move one of the 3 poles to a very low frequency, as would probably be part of the normal pole staggering process anyway? *I will leave that for others to comment on as I have not personally mucked about in my workshop with amplifiers that have 3 LF poles. *I suspect that one reason may have to do with LF overload when using a Bean Counter approved OPT. Its not always convenient to move poles down, and often it merely moves the oscillaton F lower. It can be exasperating to find that even if you increase coupling caps from say 0.22uF to 2.2uF, the trace on the CRO still slowly rises and falls because Fo has just gone lower. Hi Patrick, The phrasing of your comment suggests that you are speaking of moving all the poles down, in which case as you say "Fo has just gone lower". What I was considering was moving just one pole down in frequency. This leads directly to the question of how you choose your pole frequencies? You have explained how your LF shelving network improves LF stability, although with the risk of pushing the input stage closer to overload, what you haven't explained, at least that I remember, is how you choose your pole frequencies, especially in an amplifier with 3 poles like we are discussing? -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#47
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[8. To do the above one needs to degenerate the first stage gain, rather than
to use a Partick's attenuator after it. To degenerate the 1-st stage gain you need to place say 22K resistor in series with the 1-st stage cathode, and shunt this resistor by a 0.22...0.47uF capacitor. Thus for medium frequencies the 1-st stage will be working as usual, but ay low frequencies the transconductance will be degenerating with the perfect linearity of the first stage maintained. It is the same shelving, but implemented in a wise linear mode. (Of course the grid leak can not be taken to GND any more, it should be connected to the cathode or a tap in this 22K resistor.) At infinitely low frequencies the gain of the first stage is to be degenerated to 10...20 so that the 1-st stage output is just below the negative bias of 6AQ5 and the later is never overloaded. Usually I apply mods up to #7 and sometimes #8 too if I can find enough free solder lugs around the 1-st audio stage tube. Of course, all of the above implies the cathode of the 1-st audio tube has to be free of duo-diode functionality. That forces to use separate diodes for AGC and the AM detector. In some cases instead of 6Q7, 6B8, 6AV6, etc. I might use a 6SL7 tube with one triode as the 1-st audio and the other triode as a diode for the AM detector, and a silicon diode as an AGC detector. Regards, Alex[/quote] Here it is again with the 1st AF grid leak corrected. The NFB network should probably be conjugate with the 1st AF cathode network but that gets this one to a 3 microfarad NFB cap. But otherwise would be OK. Cheers, John |
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#48
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Quote:
But otherwise would be OK. Cheers, John[/quote] Hello Again Alex- I checked this cct again & find the cutoff begins at an f1 about 10X too high using the CR network you recommended, 0.22 Ck & 22K Rk. Have a look at Fig. 12.3B, p484 in RDH4 for guidance. The CR time constant appears to predict a point well down on the curve, not the -3db point. Your proposed cct fix still looks quite useful for the application discussed. Cheers, John Cheers, John |
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#49
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
snip for brevity.
John Byrns Surf my web pages at, *http://fmamradios.com/ You grasped the idea perfectly well John. This frequency compensation is useful to prevent a lousy OPT and 6AQ5 overloading in a lousy boring audio amp found in those boring AA5 style radios. I agree with everything Alex says below except for a few things. In most old radios with DC flow across the volume control pot track the adjustment of the volume is often very noisy after the pot has aged a few years. So instead of the conventional arrangements put forward by RDH and most others to avoid parts costs I will have the last IFT coil biased at say +50V at one end, and the live end goes to a triode grid of 1/2 12AU7 CF buffer to remove any loading effect of diode detection on the last IFT LC. Then I use Ge diode feeding RC circuit, and this can directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to any a volume control and while employing time passive poles to give say -3dB at 30Hz before any power amp which has NFB. I often add in another 12AU7 gain stage for tone control to boost/cut treble; bass in AM is usually OK. To slightly widen AF response the Q of all IFTs may be reduced by strapping 100k across each coil. It doesn't work in all sets, but may be tried. There is a simple method explained in RDH4 to add a few turns of fine wire around the primary of IFT1 and switch it to being in series with the sec of IFT1 which will give a doble peak to IFT response which effectively increases IF channel bandwidth from a typical 4kHz to 8 kHz. Quad used it. I've used it - finally there is the the treble which most sets refuse to give me. I like to use paralleled 12AX7 as V1 and EL34 in triode as V2 for the audio amp and with 12dB global NFB. This works far better than anything with 6V6 or EL84 etc. The Ge diode in the detector is biased on with a low current in the R. Its works best with high level signals and up to 100% mod, with 10Vrms available if you want it. But anyone could use the normal arrangement of IFT plus tube rectifiers found in IF amp tubes like 6N8 and thus get say -2Vdc generated by a carrier with 2pk volts of signal. at 100% mod the Vpk- pk of audio = 4V, so max audio = 1.4V, no? The audio signal with negative vdc and some 455kHz ripple could be direct coupled to a CF triode buffer stage but you'd need a -150Vdc rail for the cathode resistor, while the anode would go to a +100V supply. The CF buffer and negative rail was viewed as a waste of money in 1950, so it was never done. But there's no reason why it can't be done NOW. And the screen voltage applied to mixer and IF amp is best regulated at 100Vdc, so that's a good point of supply with CF anodes . I don't much like tubes like the 6AV6 with grounded cathodes and 10M grid leak biasing. The very low F signals generated in AM sets don't seem to cause any problems in anything I have made. Patrick Turner. Below is the sequence of mods you do to improve the radio. 1. In an AM detector you remove a cap coupling a detector load (470K||100pF) to the volume control (1M pot). In fact you are making the volume control the detector load. This is for the detector to be able to handle nearly 100% modulation. But now you det DC on the input of the audio amp. 2. You decouple this DC from the pot wiper by a 0.02...0.05uF, but you need 10M input impedance (grid leak) so that the AM detector is not loaded and 100% modulation is still handled. 3. On most stations your AM detector delivers 1...4V of audio, while sensitivity of a typical boring feedbackless two-stage audio amp is 100...200mV. The radio always works with volume control close to minimum. The speaker is boomy (not damped), distortion is high. You want to trade the sensitivity excess for distortion and apply NFB, reducing sensitivity to 0.5...1V. You are enjoying tight crispy sound, but THAT IS WHERE PROBLEMS BEGIN. 4. Now the bandwidth of your amp is in theory goes to 5...10Hz, but because of the lousy OPT, these low frequencies do not reach your ears, but only overload the 6AQ5, since the error signal becomes too large at low frequencies. 5. Note that neither the american aggressive LF cutoff by reducing interstage cap to 2000pF nor the Patrick's shelving does not work, since still yjr NFB is pushing to maintaim unmaintainable LF output, the error signal is large, the first stage is overloading, and (IMPORTANT!) since the shelving is virtually a differentiator, it accentuates all the harmonics generated in the overloaded 1-st stage and feeds them to the 6AQ5. Instead of overloading the 6AQ5 we have emphasized distortion from the first stage. 6. That is where you need this RRC compensation in the feedback to roll the low-frequency response of the amp in line with the capability of the lousy OPT. Typically from 80....100Hz down. Applying the RRC divider provides undistorted output down to 10...25Hz, because the error signal remains under control. 7. However at the frequencies lower than 10...25Hz, where the NFB RRC network levels out at 100% beta, the error signal continues to rise. This VLF content caused by fadings, AGC knocked by atmospheric interference, etc. is smaller, and some might stop here, but a purist might like to prevent overloading even at VLF. 8. To do the above one needs to degenerate the first stage gain, rather than to use a Partick's attenuator after it. To degenerate the 1-st stage gain you need to place say 22K resistor in series with the 1-st stage cathode, and shunt this resistor by a 0.22...0.47uF capacitor. Thus for medium frequencies the 1-st stage will be working as usual, but ay low frequencies the transconductance will be degenerating with the perfect linearity of the first stage maintained. It is the same shelving, but implemented in a wise linear mode. (Of course the grid leak can not be taken to GND any more, it should be connected to the cathode or a tap in this 22K resistor.) At infinitely low frequencies the gain of the first stage is to be degenerated to 10...20 so that the 1-st stage output is just below the negative bias of 6AQ5 and the later is never overloaded. Usually I apply mods up to #7 and sometimes #8 too if I can find enough free solder lugs around the 1-st audio stage tube. Of course, all of the above implies the cathode of the 1-st audio tube has to be free of duo-diode functionality. That forces to use separate diodes for AGC and the AM detector. In some cases instead of 6Q7, 6B8, 6AV6, etc. I might use a 6SL7 tube with one triode as the 1-st audio and the other triode as a diode for the AM detector, and a silicon diode as an AGC detector. Regards, Alex- Hide quoted text - - Show quoted text - |
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#50
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
"Patrick Turner" wrote in message
... ......................... In most old radios with DC flow across the volume control pot track the adjustment of the volume is often very noisy after the pot has aged a few years. So instead of the conventional arrangements put forward by RDH and most others to avoid parts costs I will have the last IFT coil biased at say +50V at one end, and the live end goes to a triode grid of 1/2 12AU7 CF buffer to remove any loading effect of diode detection on the last IFT LC. Then I use Ge diode feeding RC circuit, and this can directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to any a volume control and while employing time passive poles to give say -3dB at 30Hz before any power amp which has NFB. I often add in another 12AU7 gain stage for tone control to boost/cut treble; bass in AM is usually OK. Alex: Is adding two extra tubes a reasonable "mod"? It is the radio redesign. If one wants to go that far, op-amps would be far better. Take TLE2074. With 10MHz bandwidth and 45V/us slew rate it can work as IF amplifier to increase sensitivity, tone control, buffer, AGC integrator, etc. And after that why do you need to retain a lousy tube amp? Why not to use an IC? All the radio can be powered from a 6.3V winding with a bipolar voltage doubler (four caps, four 1N4004 diodes) .................... I like to use paralleled 12AX7 as V1 and EL34 in triode as V2 for the audio amp and with 12dB global NFB. This works far better than anything with 6V6 or EL84 etc. Alex: Works better in which respect? EL34 takes as many amps for its heater as the whole original radio circuit. What about the poor lousy power transformer? |
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#51
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: snip for brevity. John Byrns Surf my web pages at, *http://fmamradios.com/ You grasped the idea perfectly well John. This frequency compensation is useful to prevent a lousy OPT and 6AQ5 overloading in a lousy boring audio amp found in those boring AA5 style radios. I agree with everything Alex says below except for a few things. In most old radios with DC flow across the volume control pot track the adjustment of the volume is often very noisy after the pot has aged a few years. So instead of the conventional arrangements put forward by RDH and most others to avoid parts costs I will have the last IFT coil biased at say +50V at one end, and the live end goes to a triode grid of 1/2 12AU7 CF buffer to remove any loading effect of diode detection on the last IFT LC. What is the purpose and or advantage of using a cathode follower between the IFT and detector diode, especially if you are going to add 100k resistors to widen the IF response as you discuss below? Then I use Ge diode feeding RC circuit, and this can directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to any a volume control and while employing time passive poles to give say -3dB at 30Hz before any power amp which has NFB. I often add in another 12AU7 gain stage for tone control to boost/cut treble; bass in AM is usually OK. To slightly widen AF response the Q of all IFTs may be reduced by strapping 100k across each coil. It doesn't work in all sets, but may be tried. The coupling between the two coils in an IFT often needs to be increased somewhat when adding resistors, that may explain why it doesn't work in all sets, if the IFTs are over coupled adding resistors may ne counter productive. Related to this is the fact that the instruction manual for Heathkit's first AM tuner kit recommended placing a resistor(s) across the first IFT if a narrower bandwidth was needed, so things don't always work as expected. The reason the resistor(s) narrowed the bandwidth in the Heathkit is because it was a wideband tuner with an over coupled 1st IFT, so adding the resistor(s) takes the transformers close to critical coupling, giving a narrower response. I don't remember if the Heathkit mod for reducing the bandwidth involved adding resistors on both the primary and secondary, or on only one winding, I will have to see if I can find the manual, IIRC they also used a resistor or resistors during alignment to eliminate the over coupling, allowing the tuner to be aligned by peaking the IFTs. There is a simple method explained in RDH4 to add a few turns of fine wire around the primary of IFT1 and switch it to being in series with the sec of IFT1 which will give a doble peak to IFT response which effectively increases IF channel bandwidth from a typical 4kHz to 8 kHz. Quad used it. QUAD's first superheterodyne AM tuner, the Acoustical AM tuner, used a different scheme to vary the bandwidth. Rather than using an IFT they used two separate IF coils with low side capacitive coupling, the low side capacitive coupling could be switched to vary the bandwidth. I've used it - finally there is the the treble which most sets refuse to give me. I like to use paralleled 12AX7 as V1 and EL34 in triode as V2 for the audio amp and with 12dB global NFB. This works far better than anything with 6V6 or EL84 etc. The Ge diode in the detector is biased on with a low current in the R. Its works best with high level signals and up to 100% mod, with 10Vrms available if you want it. But anyone could use the normal arrangement of IFT plus tube rectifiers found in IF amp tubes like 6N8 and thus get say -2Vdc generated by a carrier with 2pk volts of signal. at 100% mod the Vpk- pk of audio = 4V, so max audio = 1.4V, no? The audio signal with negative vdc and some 455kHz ripple could be direct coupled to a CF triode buffer stage but you'd need a -150Vdc rail for the cathode resistor, while the anode would go to a +100V supply. The CF buffer and negative rail was viewed as a waste of money in 1950, so it was never done. But there's no reason why it can't be done NOW. And the screen voltage applied to mixer and IF amp is best regulated at 100Vdc, so that's a good point of supply with CF anodes . I don't much like tubes like the 6AV6 with grounded cathodes and 10M grid leak biasing. The very low F signals generated in AM sets don't seem to cause any problems in anything I have made. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#52
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 30, 8:03*pm, "Alex Pogossov" wrote:
"Patrick Turner" wrote in message ... ........................ In most old radios with DC flow across the volume control pot track the adjustment of the volume is often very noisy after the pot has aged a few years. So instead of the conventional arrangements put forward by RDH and most others to avoid parts costs I will have the last IFT coil biased at say +50V at one end, and the live end goes to a triode grid of 1/2 12AU7 CF buffer to remove any loading effect of diode detection on the last IFT LC. Then I use Ge diode feeding RC circuit, and this can directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to any a volume control and while employing time passive poles to give say -3dB at 30Hz before any power amp which has NFB. I often add in another 12AU7 gain stage for tone control to boost/cut treble; bass in AM is usually OK. Alex: Is adding two extra tubes a reasonable "mod"? It is the radio redesign. I raise my hat to all the designers of tube AM radios. They worked under duress and had no liberty to produce the best radio which could be built because the companies all had to compete with each other for sales so they all agreed to make the lowest common denominator radios. But DIYers were always free to do whatever they damned well liked, and some did, but there's very little record of the 0.000000000000000000001% of radios made by ppl MORE intelligent that 99.99% of staff employed by companies. If you consider the bulk and inefficiency of using pre WW2 octals and most tubes, then why would making a radio with a couple of extra tubes be a hanging offense? 1950's 7pin and 9pin tubes are far smaller and compact than th octals they replaced, so you could use more tubes. When FM radio and TV came into being the number of tubes became a minor concern - you needed plenty of tubes, sure, and a shirt&trouser load of other "stuff". Prices for such goods wasn't based on the cost of production + a margin for profit because the industry had learnt far higher margins over cost were needed to fund the phasing in of Solid State and a huge increase om manufacturing infrastructure. one wants to go that far, op-amps would be far better. Take TLE2074. With 10MHz bandwidth and 45V/us slew rate it can work as IF amplifier to increase sensitivity, tone control, buffer, AGC integrator, etc. Sure. Then you have a solid state radio. **** that. Gimme a coupla triodes. And after that why do you need to retain a lousy tube amp? Why not to use an IC? All the radio can be powered from a 6.3V winding with a bipolar voltage doubler (four caps, four 1N4004 diodes) I build tube radios, OK. ................... I like to use paralleled 12AX7 as V1 and EL34 in triode as V2 for the audio amp and with 12dB global NFB. This works far better than anything with 6V6 or EL84 etc. Alex: Works better in which respect? EL34 takes as many amps for its heater as the whole original radio circuit. What about the poor lousy power transformer? I replace PT, maybe add another if I have to. Anything goes, but what goes out my door gives excellent AM and SW if there are coils, plus I put in a couple of RCA sockets to take CD player L&R or an FM tuner L&R so you get mono sound and real Hi-Fi. The trioded EL34 makes better sound than anu single EL84/6V6. 807 and 6L6 in triode are also excellent, and need less heater power. I have a pile of PTs from which I can use for old radios where the PT has become very fragile and poor insulation after running hot for 70 years. So when someone getds me to fix a big beautiful floor standing radio, they get the glory of the nice woodwork and they are not compelled to listen all day to maybe 1 or 2 stations which sometimes broadcast something worth listening to. They need to be able to play CDs and and listen to AM and then they discover the glories of SET amps and wide AF bandwidth. Most people don't need the sound to be loud, they don't need stereo, and what they want is GOOD sound, and that's what I give them. And in 3 years time they don't have to find a tube type originating from pre-WW2 Europe, they will find spares quite easily for the next 20 years at least. Some radio ideas never became popular. The 1947 Tucker Synchrodyne was such a thing. Synchronous detection really had to wait until the solid state era where you could become more precise with circuits which all too often were a nightmare for anyone to get running easily with tubes. Patrick Turner. |
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#53
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jul 1, 1:58*am, John Byrns wrote:
In article , *Patrick Turner wrote: snip for brevity. John Byrns Surf my web pages at, *http://fmamradios.com/ You grasped the idea perfectly well John. This frequency compensation is useful to prevent a lousy OPT and 6AQ5 overloading in a lousy boring audio amp found in those boring AA5 style radios. I agree with everything Alex says below except for a few things. In most old radios with DC flow across the volume control pot track the adjustment of the volume is often very noisy after the pot has aged a few years. So instead of the conventional arrangements put forward by RDH and most others to avoid parts costs I will have the last IFT coil biased at say +50V at one end, and the live end goes to a triode grid of 1/2 12AU7 CF buffer to remove any loading effect of diode detection on the last IFT LC. What is the purpose and or advantage of using a cathode follower between the IFT and detector diode, especially if you are going to add 100k resistors to widen the IF response as you discuss below? The last IFT coil is a high impedance tuned circuit signal source. If one is going to load it slightly to slightly reduce the Q thus widening the pass band and AF response then using a pure resistance is benign. The CF converts the high Z source to low Z source and all the crapological behaviour of the Ge diode dissappears. One could still use a 6AL5 if one wanted to. Then I use Ge diode feeding RC circuit, and this can directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to any a volume control and while employing time passive poles to give say -3dB at 30Hz before any power amp which has NFB. I often add in another 12AU7 gain stage for tone control to boost/cut treble; bass in AM is usually OK. To slightly widen AF response the Q of all IFTs may be reduced by strapping 100k across each coil. It doesn't work in all sets, but may be tried. The coupling between the two coils in an IFT often needs to be increased somewhat when adding resistors, that may explain why it doesn't work in all sets, if the IFTs are over coupled adding resistors may ne counter productive. Try things. Predjudice don't belong anywhere when you want to make old junk meet modern expectations. In my kichen set I have a knob on front which changes IF gain. This is done by varying the distance between the two coils on IFT1 so they can be slightly over coupled after the set is tuned with coils apart. Magically, the AF bandwidth nearly doubles. Its because the slight over coupling creates a double IF peak in response which complememts the single peak of the IFT2, and you get a wider pass band but skirt selectivity is still very good. I recall Halicrafters had a communications set with all 3 IFTs having mechanically variable selectivity. This was a boon to radio hams. Related to this is the fact that the instruction manual for Heathkit's first AM tuner kit recommended placing a resistor(s) across the first IFT if a narrower bandwidth was needed, so things don't always work as expected. * I've always found R across IFT tanks reduced Q and increased BW, and tended to make circuits less likely to oscillate. The reason the resistor(s) narrowed the bandwidth in the Heathkit is because it was a wideband tuner with an over coupled 1st IFT, so adding the resistor(s) takes the transformers close to critical coupling, giving a narrower response. * Most ordinary radios have lots of selectivity but 2kHz of AF BW - if you are lucky. I don't remember if the Heathkit mod for reducing the bandwidth involved adding resistors on both the primary and secondary, or on only one winding, I will have to see if I can find the manual, IIRC they also used a resistor or resistors during alignment to eliminate the over coupling, allowing the tuner to be aligned by peaking the IFTs. There is a simple method explained in RDH4 to add a few turns of fine wire around the primary of IFT1 and switch it to being in series with the sec of IFT1 which will give a doble peak to IFT response which effectively increases IF channel bandwidth from a typical 4kHz to 8 kHz. Quad used it. QUAD's first superheterodyne AM tuner, the Acoustical AM tuner, used a different scheme to vary the bandwidth. *Rather than using an IFT they used two separate IF coils with low side capacitive coupling, the low side capacitive coupling could be switched to vary the bandwidth. There are a number of things doable to widen IF BW, but doing them is always easier said than done. Patrick Turner. |
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#54
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
"Patrick Turner" wrote in message news:02ac8343-a10e-48e9- I raise my hat to all the designers of tube AM radios. They worked under duress and had no liberty to produce the best radio which could be built because the companies all had to compete with each other for sales so they all agreed to make the lowest common denominator radios. But DIYers were always free to do whatever they damned well liked, and some did, but there's very little record of the 0.000000000000000000001% of radios made by ppl MORE intelligent that 99.99% of staff employed by companies. If you consider the bulk and inefficiency of using pre WW2 octals and most tubes, then why would making a radio with a couple of extra tubes be a hanging offense? 1950's 7pin and 9pin tubes are far smaller and compact than th octals they replaced, so you could use more tubes. When FM radio and TV came into being the number of tubes became a minor concern - you needed plenty of tubes, sure, and a shirt&trouser load of other "stuff". Prices for such goods wasn't based on the cost of production + a margin for profit because the industry had learnt far higher margins over cost were needed to fund the phasing in of Solid State and a huge increase om manufacturing infrastructure. one wants to go that far, op-amps would be far better. Take TLE2074. With 10MHz bandwidth and 45V/us slew rate it can work as IF amplifier to increase sensitivity, tone control, buffer, AGC integrator, etc. Sure. Then you have a solid state radio. **** that. Gimme a coupla triodes. And after that why do you need to retain a lousy tube amp? Why not to use an IC? All the radio can be powered from a 6.3V winding with a bipolar voltage doubler (four caps, four 1N4004 diodes) I build tube radios, OK. ................... I like to use paralleled 12AX7 as V1 and EL34 in triode as V2 for the audio amp and with 12dB global NFB. This works far better than anything with 6V6 or EL84 etc. Alex: Works better in which respect? EL34 takes as many amps for its heater as the whole original radio circuit. What about the poor lousy power transformer? I replace PT, maybe add another if I have to. Anything goes, but what goes out my door gives excellent AM and SW if there are coils, plus I put in a couple of RCA sockets to take CD player L&R or an FM tuner L&R so you get mono sound and real Hi-Fi. The trioded EL34 makes better sound than anu single EL84/6V6. 807 and 6L6 in triode are also excellent, and need less heater power. I have a pile of PTs from which I can use for old radios where the PT has become very fragile and poor insulation after running hot for 70 years. So when someone getds me to fix a big beautiful floor standing radio, they get the glory of the nice woodwork and they are not compelled to listen all day to maybe 1 or 2 stations which sometimes broadcast something worth listening to. They need to be able to play CDs and and listen to AM and then they discover the glories of SET amps and wide AF bandwidth. Most people don't need the sound to be loud, they don't need stereo, and what they want is GOOD sound, and that's what I give them. And in 3 years time they don't have to find a tube type originating from pre-WW2 Europe, they will find spares quite easily for the next 20 years at least. Some radio ideas never became popular. The 1947 Tucker Synchrodyne was such a thing. Synchronous detection really had to wait until the solid state era where you could become more precise with circuits which all too often were a nightmare for anyone to get running easily with tubes. Patrick Turner. ==================== Alex replies: I see your approach to radio design. You are prepared to improve a boring typical aussie radio by adding a cathode follower, another audio stage for tone control, replace PT, drill/punch a couple of holes for the extra 2 tubes, etc. What I can not understand is the craziness. Why go through all this bother if a better improvement can be done with transistors and op-amps, and the stuff can be hidden on a small board under the chassis. Unfortunately there is no area where tubes outperform solid state, except for (literaly) warm feeling. Sometimes I do redesign as well. For example, the best AM detector is the emitter detector. It virtually combines your cathode follower (emitter follower in this case) with a diode detector (emitter junction). It has rather high input impedance: beta times higher than of a bare diode detector. And beta is quite high for the modern transistors. One problem though. Reverse breakdown voltage of an emitter junction is about 8V, so the carrier shall not exceed 1Vm (with a margin). Thus the radio needs to have a perfect AGC. An AGC in a boring radio is bad. On a week station AM detector gives 0.5V, while on a strong local station can reach 5...10V. What I do, I put an integrator in the AGC circuit so that no matter how strong is a station, the carrier level is about 1V, while the AGC can be anywhere from 0V to --30V. The radio becomes very convenient. Set volume and cruse the dial. All the stations will have the same volume. The radio does not blast when running on a strong local station. Now I ask you: how would you make an integrator on a tube? Possible, but cumbersome. Why lika communist, create difficulties and then overcome them with a loud chest-pounding and shouting slogans? Well, normally I do not do deep redesign, but just improve whatever possible keeping the same topology. That is where the idea of low cut off in the feedback came from. Once you begin to apply NFB to a lousy boring amp (to reduce the speaker boom and make crispier sond), you want not to extend the low frequencies below what the lousy OPT can handle naturally. And you can not use RC high-pass filters on the input because the input impedance must be 10M or more (not to load the detector). And you can not use a Hi-Z RC filter of say 470pF and 10M, as hum and noise caused by flickering grid current would be enormous. |
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#55
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
====================
Alex replies: I see your approach to radio design. You are prepared to improve a boring typical aussie radio by adding a cathode follower, another audio stage for tone control, replace PT, drill/punch a couple of holes for the extra 2 tubes, etc. What I can not understand is the craziness. Why go through all this bother if a better improvement can be done with transistors and op-amps, and the stuff can be hidden on a small board under the chassis. I've got a lotta tubes and bits and peices. Most SS AM radios are quite poor performers. I enjoy working with tubes. it seems fraudelent to get someone to pay to have their radio restored by fitting a little board full of chippy crappy thinges. One has to draw a line some place. Unfortunately there is no area where tubes outperform solid state, except for (literaly) warm feeling. Try telling that to ppl who prefer tube gear. It just is not true for them. I have a Marantz AM-FM chip based tuner sitting on top of my AM radio. I can switch to FM if I want to, and the FM is not better sounding than my tube AM set. When I switch the tuner to its AM section its much inferior to the tubes, so I use the tubes for AM and audio, and the chip tuner only for the FM signal. Something like a Leak Troughline FM tuner is excellent, also Quad FM tuners. But the tubed stereo decoders often increased noise and distortion so they were not always good. The add-on MPX made for Leak and Quad with Ge transistors were awful. OK, so a chip based MPX Does Work OK. I once instaled a kit made in the UK into an old Kenwood AM- FM receiver with mono FM - worked very well, and there wasn't a tubed MPX in there already. Sometimes I do redesign as well. For example, the best AM detector is the emitter detector. It virtually combines your cathode follower (emitter follower in this case) with a diode detector (emitter junction). It has rather high input impedance: beta times higher than of a bare diode detector. And beta is quite high for the modern transistors. One problem though. Reverse breakdown voltage of an emitter junction is about 8V, so the carrier shall not exceed 1Vm (with a margin). Thus the radio needs to have a perfect AGC. Recently I used a couple of small bjts in darlington pair EF mode to buffer the signal after Ge diode and RC - worked very well to avoid cut off distortion because of AC loading. But I prefer 1/2 a 12AU7. An AGC in a boring radio is bad. On a week station AM detector gives 0.5V, while on a strong local station can reach 5...10V. What I do, I put an integrator in the AGC circuit so that no matter how strong is a station, the carrier level is about 1V, while the AGC can be anywhere from 0V to --30V. Yes, but today I finally got a re-built 1935 radio working fairly well with 6AN7 triode-hexode mixer and 6N8 IF remote cut off IF amp. To avoid the terrible hum when tuned to AM stations ( due to compact fluorescent lamps) I replace the existing RF input coil designed for antenna = 3 metres of wire. The lamps tend to modulate the electrostatic content of the electromagnetivc wave at the rectifier F, 100Hz plus horrid harmonics. The replacement is a ferrite rod antenna which reacts to the magnetic portion of the wave which seems to have far less hum. But the voltage level is smaller at the input. But with only 2 tubes for the AM tuner, 4 out of 7 local AM stations produce between -5 and -8 Vdc of AGC voltage, and audio level does not change much for the stations even though I have no DC amp to assist the AGC function. On my own AM radio I have IF amp = 6BX6 which is sharp cut off and I have local current FB to linearise the IF amp. AGC is applied to an RF input stage and mixer only, sure the AF levels vary more but it sounds better. I've done a few sets which have had all octal tubes, 6U7 RF, 6A8 mixer, 6U7 IF amp, and because you have THREE stages and Gm is only moderate, they work very well indeed, and give better SW than the two stages with just mixer and IF amp. I've never felt a need for amplified AVC voltage. The radio becomes very convenient. Set volume and cruse the dial. All the stations will have the same volume. The radio does not blast when running on a strong local station. Nor does mine, and I am not alergic to changing volume for the mood, to turn up music and intelligent discussions, and to trun down the crap, adds and lying politicians. Moderate AGC is fine for me. Now I ask you: how would you make an integrator on a tube? Possible, but cumbersome. Why lika communist, create difficulties and then overcome them with a loud chest-pounding and shouting slogans? What's an integrator? Last time I looked, it was something than changed a square wave into a saw tooth wave, ie, RC circuit. Unlike the Chest Pounding Communists, I am no slave to any unruly mob called 'Socialists For Electronic Correctness'. I'll do things my way, or a better way if I am convinced its yet another intelligent use of a tube or two, or three. Well, normally I do not do deep redesign, but just improve whatever possible keeping the same topology. I find it impossible to retain existing circuits when I know that I know better. It is a private thing with me. There are gory details. But usually the music survives better than otherwise. That is where the idea of low cut off in the feedback came from. Once you begin to apply NFB to a lousy boring amp (to reduce the speaker boom and make crispier sond), you want not to extend the low frequencies below what the lousy OPT can handle naturally. But what if the amp is NOT "lousy boring amp" and what if there is NO speaker boom, and no need of the extra NFB at VLF? Feel free to add the cap if you want but I have yet to explore the idea and make careful measurements to verify to myself the idea is worth persuit. So far nobody seems to have devoted a website page with all the details of performance which is too difficult to explain here in words only. And you can not use RC high-pass filters on the input because the input impedance must be 10M or more (not to load the detector). And you can not use a Hi-Z RC filter of say 470pF and 10M, as hum and noise caused by flickering grid current would be enormous. Sure. I get around all the problems you tell me about by abolishing the use of grid leak biasing, and using something different. Today I spent yet another 8 hours on a 1935 radio in which all old tubes were removed and replaced with 1950s tubes. There had been a mix of 3 oddball European tubes wth Queer Sockets plus a couple of octals made in Oz; someone had already modified the set, and done it quite badly, and what was there in 1935 isn't clear, but basically just mixer, IF amp with diodes, AF input, AF output and type 80 rectifier, and a Magic Eye. 6 tubes. Now it has 7, with Si diodes to replace 80. I have yet to re-wire the magic eye. The power lost in the 80 is now used in the EL34. I found that instead of triode on this set I could have a CT on the OPT primary for 50% SE UL mode and 12dB GNFB works fine. B+ is +375V at the same Ia as used for a 6F6 which had been used. The EL34 in SE UL or SE triode gives far nicer sound than 6F6, 42, 6V6, EL84, and other crap in pentode/tetrode mode with very little GNFB. The only down side is that when Rout is high with pentode mode the small amount of treble produced in old radios is "naturally boosted" by the rising signal at the speaker at higher F because the speaker impedance rises with F. Some makers were clever to marry the output tubes to just the right speaker to give a boosted treble. If GNFB is used the treble dissappears. So then one must use a tweeter. But most old sets without GNFB become tiring to me and where there should be clear HF there are competing high levels of "artificial HF", ie, the high levels of THD and IMD - Its OK if all you need to know is the latest news bulletin, but not so hot for music. The set I'm working on will have 2 RCA sockets to combine L&R inputs from an FM tuner or CD player, ipod, or other i-crap. Patrick Turner. |
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#56
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: On Jul 1, 1:58*am, John Byrns wrote: In article , *Patrick Turner wrote: I agree with everything Alex says below except for a few things. In most old radios with DC flow across the volume control pot track the adjustment of the volume is often very noisy after the pot has aged a few years. So instead of the conventional arrangements put forward by RDH and most others to avoid parts costs I will have the last IFT coil biased at say +50V at one end, and the live end goes to a triode grid of 1/2 12AU7 CF buffer to remove any loading effect of diode detection on the last IFT LC. What is the purpose and or advantage of using a cathode follower between the IFT and detector diode, especially if you are going to add 100k resistors to widen the IF response as you discuss below? The last IFT coil is a high impedance tuned circuit signal source. If one is going to load it slightly to slightly reduce the Q thus widening the pass band and AF response then using a pure resistance is benign. The CF converts the high Z source to low Z source and all the crapological behaviour of the Ge diode dissappears. One could still use a 6AL5 if one wanted to. You haven't really answered the question, just spouted a bunch of meaningless gibberish. My question was what is the advantage of using a cathode follower between the secondary of the IFT and the detector diode? The cathode follower is going to reduce the loading on the IFT, which is going in exactly the opposite direction you seem to want to go when you recommend loading the IFTs with 100k resistors! What do you gain by adding the cathode follower to the circuit? What is wrong with simply choosing the diode load to reflect the desired load to the secondary of the IFT, 100k or whatever? Also the coupling coefficient between the primary and secondary of each IFT should be properly coordinated with the selected loading on the IFTs to achieve the desired bandwidth with maximum response flatness. A cathode follower after the diode can be useful as one way to eliminate negative peak clipping due to a poor AC/DC load ratio. What is this "carpological" behavior of the Ge diode that you are talking about? Can you define the nature of this behavior? Then I use Ge diode feeding RC circuit, and this can directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to any a volume control and while employing time passive poles to give say -3dB at 30Hz before any power amp which has NFB. I often add in another 12AU7 gain stage for tone control to boost/cut treble; bass in AM is usually OK. To slightly widen AF response the Q of all IFTs may be reduced by strapping 100k across each coil. It doesn't work in all sets, but may be tried. The coupling between the two coils in an IFT often needs to be increased somewhat when adding resistors, that may explain why it doesn't work in all sets, if the IFTs are over coupled adding resistors may ne counter productive. Try things. Predjudice don't belong anywhere when you want to make old junk meet modern expectations. What and whose prejudice are you talking about here? Are you talking about people who have a "prejudice" against "improving" old radios? I have no such prejudice. I do wonder about the design basis and effectiveness of modifications, whether the modifications are being done in the best and most effective manner. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#57
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jul 6, 10:07*am, John Byrns wrote:
In article , *Patrick Turner wrote: On Jul 1, 1:58*am, John Byrns wrote: In article , *Patrick Turner wrote: I agree with everything Alex says below except for a few things. In most old radios with DC flow across the volume control pot track the adjustment of the volume is often very noisy after the pot has aged a few years. So instead of the conventional arrangements put forward by RDH and most others to avoid parts costs I will have the last IFT coil biased at say +50V at one end, and the live end goes to a triode grid of 1/2 12AU7 CF buffer to remove any loading effect of diode detection on the last IFT LC. What is the purpose and or advantage of using a cathode follower between the IFT and detector diode, especially if you are going to add 100k resistors to widen the IF response as you discuss below? The last IFT coil is a high impedance tuned circuit signal source. If one is going to load it slightly to slightly reduce the Q thus widening the pass band and AF response then using a pure resistance is benign. The CF converts the high Z source to low Z source and all the crapological behaviour of the Ge diode dissappears. One could still use a 6AL5 if one wanted to. You haven't really answered the question, just spouted a bunch of meaningless gibberish. *My question was what is the advantage of using a cathode follower between the secondary of the IFT and the detector diode? * I didn't speak meaningless giberish. But, I do understand that whatever I do say, and whatever I have posted at my website as examples of my craftmanship will always be misunderstood, belitlled, rubbished, dismissed, and ****ed upon shat upon from a great height. I don't care because whoever is downloading the regular 100MB a day from my site might not believe in you gibberish theory, and when they try some of my circuits they find the sound is dramatically better than has been served up by an industry mainly concerned with the cost-of- production rather than audio fidelity. As many of the readers here are approaching nearer to their 90th birthday closer than I am, of course its likely they might find my answers to questions and discussions of 101 things related to audio or the world to be gibberish well before I conceed to myself that I don't understand my position any better, because of increasing possible senility. When blokes here say they were apprentices in some electronics company in 1952, but of course try to hide their age, I have to allow for the inevitable fact that a percentage of them have gone soft up top, and I'd be utterly bamboozling, no matter how simply I explained anything. I suggest you read RDH4 and visit my website more. I just re-wired a horrible Howard radio set made in about 1935. The last IFT secondary is biased at +50V, and it feeds a 1/2 12AU7 CF, with Ge diode, with 680k + 250pF cap to 0V, then 200k plus 100pF LPF then direct coupled to another 1/2 12AU7 CF which poweres the 500k volume control. The sound is HEAD and SHOULDERS above any other crappy radio. I suggest you try my methods to learn why I use the methods, for therin lies the answers to all you testy questions. The cathode follower is going to reduce the loading on the IFT, which is going in exactly the opposite direction you seem to want to go when you recommend loading the IFTs with 100k resistors! Resistance loads the IFTs with resistance, and the effect retains pentode linearity. I don't always use R damping to reduce Q, but its handy when the set has been changed to work with all mains operated 7 pin tubes in lieu of the 1.4V filament types which are fragile, have low gm, and gain, and are becoming hard to obtain, and which ppl don't want to use any more with batteries. The IFTs in such sets usually have very high Q, and very low and thus poor sounding AF BW so R damping is fine. With sets that have an RF stage ahead of the mixer so hence have a total of 6 tuned circuits, side band cutting sometimes limits AF to 2kHz, ie, bloody awful, OK if you are listening to long distance talk back, but useless for music. So with so many stages the IF Q may be reduced quite a bit and there is still plenty of gain for the set overall for what is most listened to, ie, strong local AM stations. Some reduction of Q is best if spread over all 4 coils in 2 IFTs. One must remember that the each LC circuit has only 6dB/octave attenuation away from the Fo. Let's say each IFT LC has Q = 45. The pass band = +/-5kHz, or 10kHz. Maybe you get -15dB at +/- 20kHz so that with 4 such tuned circuits the pass band is 5kHz or less, and at +/- 20kHz the attenuation = -60dB, considered OK, because strong locals are usually separated by 45kHz at least. The nose shape of well done IFTs won't be as sharp as just one one coil because the mutual coupling is supposed to be arranged to flatten the top of the pass band somewhat to give wide BW. Anyway, the nose shape won't much affect the skirt attenuation at more than +/- 20kHz away from Fo. One has to take all these considerations into account before adding R dampers. *What do you gain by adding the cathode follower to the circuit? *What is wrong with simply choosing the diode load to reflect the desired load to the secondary of the IFT, 100k or whatever? * Better sound. Try it sometime. Then you'll know. Also the coupling coefficient between the primary and secondary of each IFT should be properly coordinated with the selected loading on the IFTs to achieve the desired bandwidth with maximum response flatness. *A cathode follower after the diode can be useful as one way to eliminate negative peak clipping due to a poor AC/DC load ratio. *What is this "carpological" behavior of the Ge diode that you are talking about? *Can you define the nature of this behavior? Ge diodes have reverse lekage currents. Try using one in a typical radio circuit and get back to me. Be very careful when observing the whole depressing performance using a high impedance non loading type of probe to your CRO. Then I use Ge diode feeding RC circuit, and this can directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to any a volume control and while employing time passive poles to give say -3dB at 30Hz before any power amp which has NFB. I often add in another 12AU7 gain stage for tone control to boost/cut treble; bass in AM is usually OK. To slightly widen AF response the Q of all IFTs may be reduced by strapping 100k across each coil. It doesn't work in all sets, but may be tried. The coupling between the two coils in an IFT often needs to be increased somewhat when adding resistors, that may explain why it doesn't work in all sets, if the IFTs are over coupled adding resistors may ne counter productive. Try things. Predjudice don't belong anywhere when you want to make old junk meet modern expectations. What and whose prejudice are you talking about here? *Are you talking about people who have a "prejudice" against "improving" old radios? *I have no such prejudice. *I do wonder about the design basis and effectiveness of modifications, whether the modifications are being done in the best and most effective manner. Its far simpler for me to to humbly suggest you try my methods rather than expect a full explanation. If I give an explanation its read, perhaps called gibberish, and forgotten. The reader does not solder the idea into reality. Nothing is learnt. I have wasted my precious time. I've put all my answers at my website in the form of schematics. I've explained all this at rec.audio.tubes several times in the last 11 years and I'm getting tired of repeating myself. There are far better ways to build an AM radio than by slavishly, blindly and unquestioningly following RDH4. Its a great book, but many of the best practices with tubes are omitted because manufacturers could not afford to implement them. Patrick Turner. |
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#58
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: On Jul 6, 10:07*am, John Byrns wrote: *What do you gain by adding the cathode follower to the circuit? *What is wrong with simply choosing the diode load to reflect the desired load to the secondary of the IFT, 100k or whatever? * Better sound. Try it sometime. Then you'll know. OK, I'm sensing a challenge here, similar to Danger Dave's assertions that resulted in the ³Power Amplifier Without Power Transformer². The problem is that with respect to Danger Dave's assertions, SNR was an easy to measure parameter. Unfortunately your claims for the benefits of using a cathode follower to buffer the IFT from the detector diode, is ³Better sound², a sort of vague standard. Before I take up my soldering iron to demonstrate that an AM detector the equal of yours can be built without the cathode follower between the IFT and detector diode, I will need to know how to measure ³Better sound². Can you suggest some valid objective measurements I can use to measure ³Better sound²? -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#59
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Quote:
Pls pick up that soldering iron & show us what you can do. Cheers to all, John L Stewart who still remembers which end is the handle! |
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#60
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VLF stability in Williamson-type amplifiers
On Jul 8, 8:51*am, John Byrns wrote:
In article , *Patrick Turner wrote: On Jul 6, 10:07*am, John Byrns wrote: *What do you gain by adding the cathode follower to the circuit? *What is wrong with simply choosing the diode load to reflect the desired load to the secondary of the IFT, 100k or whatever? * Better sound. Try it sometime. Then you'll know. OK, I'm sensing a challenge here, similar to Danger Dave's assertions that resulted in the ³Power Amplifier Without Power Transformer². *The problem is that with respect to Danger Dave's assertions, SNR was an easy to measure parameter. *Unfortunately your claims for the benefits of using a cathode follower to buffer the IFT from the detector diode, is ³Better sound², a sort of vague standard. Before I take up my soldering iron to demonstrate that an AM detector the equal of yours can be built without the cathode follower between the IFT and detector diode, I will need to know how to measure ³Better sound². *Can you suggest some valid objective measurements I can use to measure ³Better sound²? I suggest you renovate your distortion measuring capabilities, and then compare an average AM adio performance to something designed on the principles I recommend which bean counters in 1950 were likely to say were "uanaffordable". No excuses for your allergy to warm your soldering iron could ever be respected. You resemble the lad at school who turns up with no completed homework, and pleads to teacher, "Please sir, the dog ate my exercise book" or other ficticious sorts of excuses. I recall having no excuse for incomplete homework. I wore teacher's wrath. I was deemed lazy. Pity, they said, while saying I was a walking waste of intelligence. But ya shoulda seen the Meccano models and boat models I made and train sets and the radio sets and transmitters. I was busy, and i'd only learn something if I needed too, then I learnt real good, and real quick. So I wasa **** of a kid, ****in independant. But very busy; Bored ****less with stuff the teachers and parents wanted me to learn, like Latin, French, Geology, English, Economics, and I deliberately sang out of tune to avoid being in the Skool Choir. So when others wasted time singing hyms and crap I got a precious hour to catch up on math or physics. It is upon math and physics that the World turns round! I hated swimming, and on the main comp day I just didn't turn up. I was a traitor. I was a hopeless football player and had no athletic ability. I'm mighty proud I didn't catow to all these folks wanting me to do stuff I just had **** all interest in. I didn't bother to be keen at the one and only social dance held by our school where they bussed in girls from a local non-catholic grammer school. All those ****s didn't ****, wouldn't ****, and would charge such enormous prices for a **** if you married one that I could see they were not worth dancing with. It was useless trying to explain to Christian Brothers. They'd never had a ****. I hadn't either, but I knew what it'd be like, and what the costs would be, and so why try to relate to posh un****able girls prematurely? Those same girls have become old chooks I'd not wanna know. Later I prooved I was right about everything and that education had limited benefits. I got a job as a carpenter's apprentice and spent many years Building A Better Australia With My Own Bare Hands, and I went to night college for 6 years to get a Building Certificate. I found a use for education - to facilitate a noble cause far more worthy than the existance teachers had in mind for me. But being a bloke who worked outside and got mud on his boots was a mighty horrid thing in the eyes of grammar school girls or any girls, even catholic ones. I moved outa home and gave up all the pretense of upper middle class status my parents thought they had, and then I met the Working Class who I have loved ever since, despite all their many little failings. In summer I swim about 5km a week and I ride a bike 200km a week, and I am officially very fit for my age. Most blokes of my age, 64, are 3/4 ****ed in their minds and bodies. That's my explanation or excuse for not knowing everything, what's yours? Patrick Turner. Regards, John Byrns Surf my web pages at, *http://fmamradios.com/ |
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#61
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VLF stability in Williamson-type amplifiers
RAT's confused leftwing ****tard muttered:
I got a precious hour to catch up on math or physics. It is upon math and physics that the World turns round! I'm sure you didn't let little things like mistakes of four orders of magnitude slow your headlong progress in the field. Lord Valve American - and **** you if you don't like it. |
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#62
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: On Jul 8, 8:51*am, John Byrns wrote: In article , *Patrick Turner wrote: On Jul 6, 10:07*am, John Byrns wrote: *What do you gain by adding the cathode follower to the circuit? *What is wrong with simply choosing the diode load to reflect the desired load to the secondary of the IFT, 100k or whatever? * Better sound. Try it sometime. Then you'll know. OK, I'm sensing a challenge here, similar to Danger Dave's assertions that resulted in the ³Power Amplifier Without Power Transformer². *The problem is that with respect to Danger Dave's assertions, SNR was an easy to measure parameter. *Unfortunately your claims for the benefits of using a cathode follower to buffer the IFT from the detector diode, is ³Better sound², a sort of vague standard. Before I take up my soldering iron to demonstrate that an AM detector the equal of yours can be built without the cathode follower between the IFT and detector diode, I will need to know how to measure ³Better sound². *Can you suggest some valid objective measurements I can use to measure ³Better sound²? I suggest you renovate your distortion measuring capabilities, OK, you seem to be saying that it isn't "Better sound" that counts, that it is measured "distortion" that counts? I have next to zero interest in "renovating" my distortion measuring capabilities, how good do my distortion measuring capabilities have to be to adequately measure the performance of your AM detector? What is the measured distortion performance of your AM detector circuit as shown at the following URL? http://www.turneraudio.com.au/am-fm-...iles/schem-tun er-am-tuner-dec-05.gif At what IF signal level, into the detector, did you measure its performance? This is important since I do not intend to try and duplicate your entire tuner, just the detector. and then compare an average AM adio performance Why do I care about the performance of "an average AM adio", why do you even mention it? to something designed on the principles I recommend which bean counters in 1950 were likely to say were "uanaffordable". All I am interested in comparing is "something designed on the principles you recommend" vs. something designed on the principals I have recommended. http://www.turneraudio.com.au/am-fm-...ex-decoder.htm On your web page above, that describes your AM tuner design, you say about the detector, "The grid of the IFT2 sec is biased at +31V to give some idle current in the cathode R9, and to trickle a small current in the 1N914 diode via R16&R17, so that the non-linear turn on character of the diode is avoided, since it is always slightly turned on by the idle dc flow." Can you explain how this bias avoids the "turn on character" of the diode, given that to function as a detector the diode must turn off?! Bias, if correctly applied, will help reduce the slew rate limiting at high modulating frequencies. An interesting web page describing a number of AM detectors used in broadcast modulation monitors and ham radio receivers can be found here. http://www.tonnesoftware.com/appnote...iodedemod.html [Much off topic drivel snipped] -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#63
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
"John Byrns" wrote in message ... An interesting web page describing a number of AM detectors used in broadcast modulation monitors and ham radio receivers can be found here. http://www.tonnesoftware.com/appnote...iodedemod.html Thanks John, This is a very interesting article indeed. |
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#64
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jul 9, 9:19*am, John Byrns wrote:
In article , *Patrick Turner wrote: On Jul 8, 8:51*am, John Byrns wrote: In article , *Patrick Turner wrote: On Jul 6, 10:07*am, John Byrns wrote: *What do you gain by adding the cathode follower to the circuit? *What is wrong with simply choosing the diode load to reflect the desired load to the secondary of the IFT, 100k or whatever? * Better sound. Try it sometime. Then you'll know. OK, I'm sensing a challenge here, similar to Danger Dave's assertions that resulted in the ³Power Amplifier Without Power Transformer². *The problem is that with respect to Danger Dave's assertions, SNR was an easy to measure parameter. *Unfortunately your claims for the benefits of using a cathode follower to buffer the IFT from the detector diode, is ³Better sound², a sort of vague standard. Before I take up my soldering iron to demonstrate that an AM detector the equal of yours can be built without the cathode follower between the IFT and detector diode, I will need to know how to measure ³Better sound². *Can you suggest some valid objective measurements I can use to measure ³Better sound²? I suggest you renovate your distortion measuring capabilities, OK, you seem to be saying that it isn't "Better sound" that counts, that it is measured "distortion" that counts? I have next to zero interest in "renovating" my distortion measuring capabilities, how good do my distortion measuring capabilities have to be to adequately measure the performance of your AM detector? What is the measured distortion performance of your AM detector circuit as shown at the following URL? http://www.turneraudio.com.au/am-fm-...ex-decoder_fil... er-am-tuner-dec-05.gif At what IF signal level, into the detector, did you measure its performance? * This is important since I do not intend to try and duplicate your entire tuner, just the detector. and then compare an average AM adio performance Why do I care about the performance of "an average AM adio", why do you even mention it? to something designed on the principles I recommend which bean counters in 1950 were likely to say were "uanaffordable". All I am interested in comparing is "something designed on the principles you recommend" vs. something designed on the principals I have recommended. http://www.turneraudio.com.au/am-fm-...ex-decoder.htm On your web page above, that describes your AM tuner design, you say about the detector, "The grid of the IFT2 sec is biased at +31V to give some idle current in the cathode R9, and to trickle a small current in the 1N914 diode via R16&R17, so that the non-linear turn on character of the diode is avoided, since it is always slightly turned on by the idle dc flow." *Can you explain how this bias avoids the "turn on character" of the diode, given that to function as a detector the diode must turn off?! *Bias, if correctly applied, will help reduce the slew rate limiting at high modulating frequencies. An interesting web page describing a number of AM detectors used in broadcast modulation monitors and ham radio receivers can be found here. http://www.tonnesoftware.com/appnote...iodedemod.html [Much off topic drivel snipped] You may well delete "drivel" like so many others here who have awful personalities who cannot cope with being human or nice in any way. Ah you going senile? Just can't cope with well rounded discussions? Its OK, you don't have to hide such characteristics. I've read the tonnesoftware.com site before and there's nothing there that works better than my circuits using tubes, and I got less wave form distortion that they show on their oscillograms. I suggest you try my circuit rather than waste time trying to find excuses not to use your soldering iron. I don't really mind if you are rivetted to your lounge room chair in front of your PC, but when I ever about an idea which might be good I rush out and try it out to see if I can replicate claims by the inventor. If you can't manage to build my detector circuit which uses 1 x 12AU7 tube with a few R&C then maybe you've lost your touch. And I have extensively discussed all this stuff before with you here at r.a.t and I refuse to do it all over again. Patrick Turner. |
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#65
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: On Jul 9, 9:19*am, John Byrns wrote: [Much off topic drivel snipped] You may well delete "drivel" like so many others here who have awful personalities who cannot cope with being human or nice in any way. Ah you going senile? Just can't cope with well rounded discussions? Its OK, you don't have to hide such characteristics. Your drivel is a fine topic for discussion, the problem is that it is off topic in this group and you should take it elsewhere. I've read the tonnesoftware.com site before and there's nothing there that works better than my circuits using tubes, and I got less wave form distortion that they show on their oscillograms. The detectors used in the modulation monitors, the General Radio 1931, Gates M5693, and Belar AMM-2/3 don't appear to have any visible waveform distortion, although your eye is probably more sensitive to this type of aberration than mine. In my defense, back in the days before I became senile, when I worked in the radio design factory, where we were designing "white goods" radios and measuring power output at the 5% distortion level, I was consistently able to set the signal level where 5% distortion occurred without reading the meter on the distortion analyzer, looking only at the waveform displayed on the CRO. I have two main problems with the tonnesoft web page. The first is that he uses an ideal diode model for some of his simulations, and doesn't say what model he used in others, although I haven't looked at the web page in two years, and if I read it again might find that he does specify the diode models he used. The second problem, which is very relevant to our disagreement over the use of a cathode follower to buffer the diode from the IFT, is that he assumes that the demodulators are driven by a zero impedance, effectively all his circuits include your cathode follower, which confuses the issue. I suggest you try my circuit rather than waste time trying to find excuses not to use your soldering iron. I probably shouldn't even be thinking about this cathode follower issue, let alone thinking about soldering together your detector circuit. I should be saving my solder for building an improved 25L6 amplifier, however I am curious about the effect of your apparently pointless cathode follower, and it is possible that I will be overcome by a desire to show it to be worthless. You seem to possess a reverse "bean counter" element to your personality which drives you to include circuit elements that serve no real function, thereby needlessly driving costs up and reliability down. I don't really mind if you are rivetted to your lounge room chair in front of your PC, but when I ever about an idea which might be good I rush out and try it out to see if I can replicate claims by the inventor. OK, what exactly is your claim for inserting a cathode follower between the secondary of the IFT and the detector diode? On your web page you make two claims. Claim #1 is that "this CF isolate the loading effects of the diode detector from the secondary of the 6BA6", however you don't say why this isolation might be desirable? The only reason I can see is to increase the Q of the secondary of the 6BA6, however increasing the Q doesn't seem to be your goal as you also talk about adding 100k damping resistors to reduce the Q. You could save the cost of the cathode follower and the loading resistor by simply using neither. Claim #2 is that the cathode follower "provides a low impedance output to drive the 1N914 silicon diode detector." Again you give no hint why this might be useful, although it does seems to have some potential usefulness, on the other hand a higher impedance output also has some advantages in detector design/operation. If you can't manage to build my detector circuit which uses 1 x 12AU7 tube with a few R&C then maybe you've lost your touch. Looking at the schematic of your detector it appears to use only 1/2 x 12AU7 for the cathode follower between the secondary of the IFT and the diode, your design doesn't really use a cathode follower after the diode, instead depending on a voltage divider, 330k/100k, in conjunction with a relatively high AC coupled load of 890k, to minimize negative peak clipping. You do use cathode followers as the audio stage, but they are not connected in a way that they have any influence on negative peak clipping. Your design has given me one idea that I have missed up until now. My design uses a cathode follower directly coupled to the diode detector to eliminate negative peak clipping. The down side of this approach for those of us who have been influenced by bean counters is that it requires a negative supply for the cathode of the cathode follower, impacting both cost and reliability to some extent. On the plus side one advantage of this approach is that the AGC voltage can be easily obtained from a tap a little way down the cathode resistor of the cathode follower. The idea you have given me is to float the secondary of the IFT and the diode at a positive potential allowing the direct coupled cathode follower to operate without a negative supply. The complication is that a separate AGC rectifier is required, which has the potential to negatively affect the sound, especially when a delay circuit is used. And I have extensively discussed all this stuff before with you here at r.a.t and I refuse to do it all over again. Discussed it yes, unfortunately you have never offered an explanation of the usefulness of the cathode follower between the IFT and the detector diode, beyond the subjective claim that it provides the "best sound". Near as I can tell the only purpose this cathode follower serves is to assuage your prejudices against "bean counters". -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#66
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jul 10, 3:48*am, John Byrns wrote:
In article , *Patrick Turner wrote: On Jul 9, 9:19*am, John Byrns wrote: [Much off topic drivel snipped] You may well delete "drivel" like so many others here who have awful personalities who cannot cope with being human or nice in any way. Ah you going senile? Just can't cope with well rounded discussions? Its OK, you don't have to hide such characteristics. Your drivel is a fine topic for discussion, the problem is that it is off topic in this group and you should take it elsewhere. I see no reason not to include some background information which indirectly relates to the subject. Otherwise the group's discussion can be very boring, especially since all this has probably been discussed before. I've read the tonnesoftware.com site before and there's nothing there that works better than my circuits using tubes, and I got less wave form distortion that they show on their oscillograms. The detectors used in the modulation monitors, the General Radio 1931, Gates M5693, and Belar AMM-2/3 don't appear to have any visible waveform distortion, although your eye is probably more sensitive to this type of aberration than mine. *In my defense, back in the days before I became senile, when I worked in the radio design factory, where we were designing "white goods" radios and measuring power output at the 5% distortion level, I was consistently able to set the signal level where 5% distortion occurred without reading the meter on the distortion analyzer, looking only at the waveform displayed on the CRO. Don't worry, when, or if I become senile, i'll not recognise 5% ona CRO when its there. But most waves at that site showed slew distortions and cut off distortions and so forth, all of which can easily be avoided or minimised to be below 1% with my circuit over a wide AF range and if the output voltage 1 Vrms. But even where one listens to short wave where AF detector output 0.1Vrms, the sound is good, although rather mauled by having travelled so far and being so riddles with noise and fading up and down. I have never ever seen any old radio or any old schematic of what was used for detecting AF for use in re-broadcasting. AFAIK, not one single commercial example exists of a radio with "infinite impedance detector" even though the Selsted & Smith example is given in RDH4, page 1,495, Figs 27.56, 27.57 RDH4 does not have anything that works as well as what I invented for myself 14 years ago. Maybe someone else invented it elsewhere, but almost anything is better than the normal bean counter driven way of AM detection in old radios. I have two main problems with the tonnesoft web page. *The first is that he uses an ideal diode model for some of his simulations, and doesn't say what model he used in others, although I haven't looked at the web page in two years, and if I read it again might find that he does specify the diode models he used. I don't care about his modelling or what diodes he used; I just build my REAL stuff and test it. If it measures well, and better than all the websites I've seen online, they have wasted their time. The second problem, which is very relevant to our disagreement over the use of a cathode follower to buffer the diode from the IFT, is that he assumes that the demodulators are driven by a zero impedance, effectively all his circuits include your cathode follower, which confuses the issue. There is no need to load the secondary of the IFT and one may indeed connect it to an "infinite impedance" and it will still act like a tuned transformer as intended. But one may load the IFT. No laws against that. But loading can't be low, and so I have found its better to convert the IF signal to low impedance with the CF and then the diode is not so critical. Point contact GE types with high reverse voltage ratings are probably best to use, forward voltage drop of only 0.2V. I suggest you try my circuit rather than waste time trying to find excuses not to use your soldering iron. I probably shouldn't even be thinking about this cathode follower issue, let alone thinking about soldering together your detector circuit. *I should be saving my solder for building an improved 25L6 amplifier, however I am curious about the effect of your apparently pointless cathode follower, and it is possible that I will be overcome by a desire to show it to be worthless. *You seem to possess a reverse "bean counter" element to your personality which drives you to include circuit elements that serve no real function, thereby needlessly driving costs up and reliability down. Most bean counters justify their employment by being able to reduce the parts and labour needed to make something, and therefore increasing shareholder profits and most often reducing the sound quality and reliability in electronics produced by the company. I do indeed have an anti bean counter attitude, and most of them should be frog-marched in public through the town square to where they should be given a pick and shovel to dig their own grave. Where is Pol Pot when you need the man? But I am not so extreme in practice, and the radios and amps I build are amoung the best sound and most reliable on the planet. You can set out on a wanton voyage of defiance of good sense if you like, but meanwhile I have clients I like to please and who pay me to put in the stuff that the companies left out all those years ago. I don't really mind if you are rivetted to your lounge room chair in front of your PC, but when I ever about an idea which might be good I rush out and try it out to see if I can replicate claims by the inventor. OK, what exactly is your claim for inserting a cathode follower between the secondary of the IFT and the detector diode? *On your web page you make two claims. Claim #1 is that "this CF isolate the loading effects of the diode detector from the secondary of the 6BA6", however you don't say why this isolation might be desirable? *The only reason I can see is to increase the Q of the secondary of the 6BA6, however increasing the Q doesn't seem to be your goal as you also talk about adding 100k damping resistors to reduce the Q. *You could save the cost of the cathode follower and the loading resistor by simply using neither. OK, RDH4 talks about the ill-effects of diode + R&C detection loading and although the book was written to promote tube use it didn't go far enough as I have in this area. Loading should be resistive only, and then you buffer to the following processes. Claim #2 is that the cathode follower "provides a low impedance output to drive the 1N914 silicon diode detector." *Again you give no hint why this might be useful, although it does seems to have some potential usefulness, on the other hand a higher impedance output also has some advantages in detector design/operation. Si diodes for signal detection at highish F should be driven by low Z sources lest you get all manner of distortions you don't want. Just build and measure, and you'll be pleased. If you just won't try such a tiny little idea with just a tiny handful of parts to do something that is so simple because its veracity cannot be proved in advance in words here at this group, then you do indeed possess a moribund mind and one possessing a high amount of sloth. If you can't manage to build my detector circuit which uses 1 x 12AU7 tube with a few R&C then maybe you've lost your touch. Looking at the schematic of your detector it appears to use only 1/2 x 12AU7 for the cathode follower between the secondary of the IFT and the diode, your design doesn't really use a cathode follower after the diode, instead depending on a voltage divider, 330k/100k, in conjunction with a relatively high AC coupled load of 890k, to minimize negative peak clipping. *You do use cathode followers as the audio stage, but they are not connected in a way that they have any influence on negative peak clipping. Your design has given me one idea that I have missed up until now. *My design uses a cathode follower directly coupled to the diode detector to eliminate negative peak clipping. *The down side of this approach for those of us who have been influenced by bean counters is that it requires a negative supply for the cathode of the cathode follower, impacting both cost and reliability to some extent. *On the plus side one advantage of this approach is that the AGC voltage can be easily obtained from a tap a little way down the cathode resistor of the cathode follower. The idea you have given me is to float the secondary of the IFT and the diode at a positive potential allowing the direct coupled cathode follower to operate without a negative supply. *The complication is that a separate AGC rectifier is required, which has the potential to negatively affect the sound, especially when a delay circuit is used. The generation of AGC voltage may be done easily and off the CF output with a cap and diode with its cathode grounded, with the generated - Vdc drained off with 1.5M to bias points. It can also be done from the anode of the IF amp and thus from the primary of IFT. C used is usually 33pF, and loading effect is negligible. The anode IF signal is higher than secondary do more AGC is available and usually there is less sibilance during tuning when the anode is used for AGC dervivation. The rules about invention are anything but strict; just do what works better than you read about in text books or that you see being used in commercial junk designed bt accountants. And I have extensively discussed all this stuff before with you here at r.a.t and I refuse to do it all over again. Discussed it yes, unfortunately you have never offered an explanation of the usefulness of the cathode follower between the IFT and the detector diode, beyond the subjective claim that it provides the "best sound". *Near as I can tell the only purpose this cathode follower serves is to assuage your prejudices against "bean counters". I give a damn if all I've said misses the point asfayac. Those capable of invention and who insist that AM radios and audio amps should sound better and measure better than much of the dumbed down junk that's sold in shops will understand my website and the attitudes I have. Patrick Turner. -- Regards, John Byrns Surf my web pages at, *http://fmamradios.com/ |
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#67
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: On Jul 10, 3:48*am, John Byrns wrote: In article , *Patrick Turner wrote: On Jul 9, 9:19*am, John Byrns wrote: [Much off topic drivel snipped] You may well delete "drivel" like so many others here who have awful personalities who cannot cope with being human or nice in any way. Ah you going senile? Just can't cope with well rounded discussions? Its OK, you don't have to hide such characteristics. Your drivel is a fine topic for discussion, the problem is that it is off topic in this group and you should take it elsewhere. I see no reason not to include some background information which indirectly relates to the subject. I don't see how your so called "background" information relates even "indirectly" to the subject, it just plain doesn't belong in this group. I've read the tonnesoftware.com site before and there's nothing there that works better than my circuits using tubes, and I got less wave form distortion that they show on their oscillograms. The detectors used in the modulation monitors, the General Radio 1931, Gates M5693, and Belar AMM-2/3 don't appear to have any visible waveform distortion, although your eye is probably more sensitive to this type of aberration than mine. *In my defense, back in the days before I became senile, when I worked in the radio design factory, where we were designing "white goods" radios and measuring power output at the 5% distortion level, I was consistently able to set the signal level where 5% distortion occurred without reading the meter on the distortion analyzer, looking only at the waveform displayed on the CRO. Don't worry, when, or if I become senile, i'll not recognise 5% ona CRO when its there. But most waves at that site showed slew distortions and cut off distortions and so forth, all of which can easily be avoided or minimised to be below 1% with my circuit over a wide AF range and if the output voltage 1 Vrms. Yes, there are plenty of distortions shown to illustrate the various problems that can occur in a poorly designed detector, and what causes the distortions. There are also plenty of undistorted waves shown at that site, with distortions of less than 1%, to illustrate the good results that can be achieved with careful design. But even where one listens to short wave where AF detector output 0.1Vrms, the sound is good, although rather mauled by having travelled so far and being so riddles with noise and fading up and down. I have never ever seen any old radio or any old schematic of what was used for detecting AF for use in re-broadcasting. I found a schematic for an old Telefunken AM rebroadcast receiver, a.k.a. "Ballempfänger", on the web. It was a rather complex radio although IIRC it used a straightforward vacuum diode detector. AFAIK, not one single commercial example exists of a radio with "infinite impedance detector" even though the Selsted & Smith example is given in RDH4, page 1,495, Figs 27.56, 27.57 RDH4 does not have anything that works as well as what I invented for myself 14 years ago. In the US J.W. Miller offered both commercial AM tuners and Radios using the infinite impedance detector, I have one of their AM Tuners. Altec Lansing also offered an AM-FM tuner that used the infinite impedance detector in the AM section. Sargent Rayment also offered a line of tuners and receivers that used the Selsted & Smith detector in the AM sections. Ampex also built an AM-FM tuner that used a perverted variant of the Selsted & Smith detector, I also have one of these. It's hard to beat a carefully designed diode detector though. Most bean counters justify their employment by being able to reduce the parts and labour needed to make something, and therefore increasing shareholder profits and most often reducing the sound quality and reliability in electronics produced by the company. The opposite approach, of adding components that serve no useful function is just as bad, actually worse. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#68
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jul 10, 1:09*pm, John Byrns wrote:
In article , *Patrick Turner wrote: On Jul 10, 3:48*am, John Byrns wrote: In article , *Patrick Turner wrote: On Jul 9, 9:19*am, John Byrns wrote: [Much off topic drivel snipped] You may well delete "drivel" like so many others here who have awful personalities who cannot cope with being human or nice in any way. Ah you going senile? Just can't cope with well rounded discussions? Its OK, you don't have to hide such characteristics. Your drivel is a fine topic for discussion, the problem is that it is off topic in this group and you should take it elsewhere. I see no reason not to include some background information which indirectly relates to the subject. I don't see how your so called "background" information relates even "indirectly" to the subject, it just plain doesn't belong in this group. Well then we'll just have to agee to disagree. But I like to talk about Sex, Politics, Religion, and Economics at dinner parties. I find small talk tedious and boring if not spiced up with some off topics. I thus get called a "raving left wing Grade A arsole" and so forth on regular basis for daring to challenge the validity of the limited perceptions of others which seem to be set in concrete. To un-set the concrete, and allow flows of ideas I can come across as smooth as grade 6 sand paper and I don't give a ****. For flows of ideas, readers of this rec.audio.tubes site should be active with a soldering iron and Get Off Fat Arses And Do A Lot Of Something, ie, GOFAADALOS !!!! :-) :-) And it'd help if they were FREE in how they come to the site to talk about anything at all. Someone said to me yesterday, "I'm black or white. I only ever see black or white" She's the wife of a tech I employ, and can be rather a stern old lady. "Ah", I said, "ever thought that there might be other colours, like red, or green? Maybe blue, yellow?" She goes, "Nah, black or white." Maybe that's why her husband don't talk to her much - she's intimidating. But I mentioned, "Surely there'd be some green in how ya see the world, no, because surely there'd be a bitta the Irish in ya". She still wouldn't smile. Like a lemon. No sense of humour. What ****s me are old people who don't ask any sensible questions and seek only to disprove others while NEVER constructing something new and better within the field of their interests, and definately never trying out anything different to see if it works. If you now think I have been off topic, no apologies, because its not wrong to be opinionated. Don't worry, when, or if I become senile, i'll not recognise 5% ona CRO when its there. But most waves at that site showed slew distortions and cut off distortions and so forth, all of which can easily be avoided or minimised to be below 1% with my circuit over a wide AF range and if the output voltage 1 Vrms. Yes, there are plenty of distortions shown to illustrate the various problems that can occur in a poorly designed detector, and what causes the distortions. * There are also plenty of undistorted waves shown at that site, with distortions of less than 1%, to illustrate the good results that can be achieved with careful design. Because my home brew test signal of modulated RF waves isn't perfect, the distortion within the envelope is probably 1%, and hence if I measured the THD of the AF after its been through the AM radio i would get THD of detected AF 1%, certainly at say 95% modulation. So to see how much THD is introduced by the radio set one may use a dual trace CRO to display both test signal at the radio input and the detected AF and overlay the waves to see how they compare. Using as much of the screen as possible, its not difficult to see if there is less than 1% of additional THD added by the radio set. All that takes time but its a very simple thing and I cannot understand why you would not have tried out all sorts of AM detector ideas like I have to see what works best for you. What the **** impedes your progress to your workshop and soldering iron? If you have a broken leg, or are impaired or disabled in some way, then let's hear about it. But even where one listens to short wave where AF detector output 0.1Vrms, the sound is good, although rather mauled by having travelled so far and being so riddles with noise and fading up and down. I have never ever seen any old radio or any old schematic of what was used for detecting AF for use in re-broadcasting. I found a schematic for an old Telefunken AM rebroadcast receiver, a.k.a. "Ballempfänger", on the web. *It was a rather complex radio although IIRC it used a straightforward vacuum diode detector. Nothing wrong with vacuum diode detectors if used wisely. They might be used instead of how I have used Ge diodes with CF. They all work best with low Z signal sources. I googled Telefunken AM rebroadcast receiver "Ballempfänger" Nothing to be seen. AFAIK, not one single commercial example exists of a radio with "infinite impedance detector" even though the Selsted & Smith example is given in RDH4, page 1,495, Figs 27.56, 27.57 RDH4 does not have anything that works as well as what I invented for myself 14 years ago. In the US J.W. Miller offered both commercial AM tuners and Radios using the infinite impedance detector, I have one of their AM Tuners. *Altec Lansing also offered an AM-FM tuner that used the infinite impedance detector in the AM section. *Sargent Rayment also offered a line of tuners and receivers that used the Selsted & Smith detector in the AM sections. *Ampex also built an AM-FM tuner that used a perverted variant of the Selsted & Smith detector, I also have one of these. I've never seen any of these things in Oz or any of their schematics. Infinite Z detector isn't bad, but gives very low output. It's hard to beat a carefully designed diode detector though. Depends. Most are distortion generators. Most bean counters justify their employment by being able to reduce the parts and labour needed to make something, and therefore increasing shareholder profits and most often reducing the sound quality and reliability in electronics produced by the company. The opposite approach, of adding components that serve no useful function is just as bad, actually worse. Each unto their own. everyone has a different way of doing things, and commercial competition stifles anyone who dares to use one more nut, bolt or resistor than the ****ing opposition brand. The result is that they all have competitions to seewho can dumb **** down the most while maintaining enough sales for the shareholders to dine well. If it makes you ill to use one more cathode follower than someone somewhere said could be superflous, then don't. But I will. In my vicinity there are no shareholders or bean counters, and my clients get a good deal without having to pay for someone's Cadillac or expensive lunches, neither of which contribute to sound quality. Patrick Turner. Regards, John Byrns Surf my web pages at, *http://fmamradios.com/- Hide quoted text - - Show quoted text -- Hide quoted text - - Show quoted text - |
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#69
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: On Jul 10, 1:09*pm, John Byrns wrote: In article , *Patrick Turner wrote: Don't worry, when, or if I become senile, i'll not recognise 5% ona CRO when its there. But most waves at that site showed slew distortions and cut off distortions and so forth, all of which can easily be avoided or minimised to be below 1% with my circuit over a wide AF range and if the output voltage 1 Vrms. Yes, there are plenty of distortions shown to illustrate the various problems that can occur in a poorly designed detector, and what causes the distortions. * There are also plenty of undistorted waves shown at that site, with distortions of less than 1%, to illustrate the good results that can be achieved with careful design. Because my home brew test signal of modulated RF waves isn't perfect, the distortion within the envelope is probably 1%, and hence if I measured the THD of the AF after its been through the AM radio i would get THD of detected AF 1%, certainly at say 95% modulation. So to see how much THD is introduced by the radio set one may use a dual trace CRO to display both test signal at the radio input and the detected AF and overlay the waves to see how they compare. Using as much of the screen as possible, its not difficult to see if there is less than 1% of additional THD added by the radio set. All that takes time but its a very simple thing Unfortunately it is also somewhat subjective. and I cannot understand why you would not have tried out all sorts of AM detector ideas like I have to see what works best for you. I already know that a well-designed diode detector works best for me; at least since I regressed to tubes, in my IC days I would have answered differently. You say that you ³have tried out all sorts of AM detector ideas², besides your spin on the diode detector, and your ill fated attempts at building a syncrodyne receiver, what have you tried? What the **** impedes your progress to your workshop and soldering iron? Two things, first the fact that AM detectors are of academic interest only, as there has been no AM broadcasting around this area, that needs anything better than the crudest AM detector, for at least 15 years or so. Second, I think I am more interested in putting my soldering iron to work on the John Stewart inspired ³improved 25L6² amplifier. There are three binary design decisions to be made before I purchase a chassis and begin punching it. If you have a broken leg, or are impaired or disabled in some way, then let's hear about it. Ah, you think you can drag me into an off topic discussion, it won't work, if you want to explore that kind of discussion, please take it to email. But even where one listens to short wave where AF detector output 0.1Vrms, the sound is good, although rather mauled by having travelled so far and being so riddles with noise and fading up and down. I have never ever seen any old radio or any old schematic of what was used for detecting AF for use in re-broadcasting. I found a schematic for an old Telefunken AM rebroadcast receiver, a.k.a. "Ballempfänger", on the web. *It was a rather complex radio although IIRC it used a straightforward vacuum diode detector. Nothing wrong with vacuum diode detectors if used wisely. They might be used instead of how I have used Ge diodes with CF. They all work best with low Z signal sources. I googled Telefunken AM rebroadcast receiver "Ballempfänger" Nothing to be seen. You weren't persistent enough, for a general description go he http://www.radiomuseum.org/r/telefun...faenger_b.html If you aren't a member, I'm not, you won't be able to see the schematic. The schematic is available from several other web sites though, you can find it in two parts he http://www.tsf-radio.org/schph.php?f...anger-E1-1.gif and he http://www.tsf-radio.org/schph.php?f...anger-E1-2.gif While the design looks like the ³bean counters² were kept at a safe distance, there still isn't a cathode follower in sight! AFAIK, not one single commercial example exists of a radio with "infinite impedance detector" even though the Selsted & Smith example is given in RDH4, page 1,495, Figs 27.56, 27.57 RDH4 does not have anything that works as well as what I invented for myself 14 years ago. In the US J.W. Miller offered both commercial AM tuners and Radios using the infinite impedance detector, I have one of their AM Tuners. *Altec Lansing also offered an AM-FM tuner that used the infinite impedance detector in the AM section. *Sargent Rayment also offered a line of tuners and receivers that used the Selsted & Smith detector in the AM sections. *Ampex also built an AM-FM tuner that used a perverted variant of the Selsted & Smith detector, I also have one of these. I've never seen any of these things in Oz or any of their schematics. Infinite Z detector isn't bad, but gives very low output. That's an interesting comment as the output level depends on the input just as with a diode detector. My experiments with the circuit suggest that it does exploit the characteristics of the low mu triodes to improve operation at low carrier levels. It's hard to beat a carefully designed diode detector though. Depends. Most are distortion generators. Those are not ³carefully² designed, it is easy to screw up a diode detector, it is equally easy to build a good one but it costs a little gain. This and the output transformer are the areas where the work of ³bean counters² can be most easily seen. Most bean counters justify their employment by being able to reduce the parts and labour needed to make something, and therefore increasing shareholder profits and most often reducing the sound quality and reliability in electronics produced by the company. The opposite approach, of adding components that serve no useful function is just as bad, actually worse. Each unto their own. everyone has a different way of doing things, and commercial competition stifles anyone who dares to use one more nut, bolt or resistor than the ****ing opposition brand. The result is that they all have competitions to seewho can dumb **** down the most while maintaining enough sales for the shareholders to dine well. Hardly, commercial products today are overflowing with every imaginable feature, each of which adds to the parts count. Competition today seems to be based more on features, with cost as a secondary issue, why make something more expensive than it needs to be to provide the required features and performance? If it makes you ill to use one more cathode follower than someone somewhere said could be superflous, then don't. But I will. In my vicinity there are no shareholders or bean counters, and my clients get a good deal without having to pay for someone's Cadillac or expensive lunches, neither of which contribute to sound quality. An unneeded cathode follower doesn't contribute to sound quality either. I won't bother to ask who pays for your lunches and transportation, least it take us off topic again, but I am reasonably sure they are accounted for in the price of the amplifiers you build. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#70
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
So to see how much THD is introduced by the radio set one may use a
dual trace CRO to display both test signal at the radio input and the detected AF and overlay the waves to see how they compare. Using as much of the screen as possible, its not difficult to see if there is less than 1% of additional THD added by the radio set. All that takes time but its a very simple thing Unfortunately it is also somewhat subjective. and I cannot understand why you would not have tried out all sorts of AM detector ideas like I have to see what works best for you. I already know that a well-designed diode detector works best for me; at least since I regressed to tubes, in my IC days I would have answered differently. With ICs you get a dc coupled amp and its possible to use an output diode pointed at an RC circuit with the audio voltage and Vdc fed back to the FB input port of the IC, and thus get what is supposed to make the most linear detector imaginable because there is MUCH more NFB being applied than when using a cathode follower like I do. The IC input impedance at IF frequency needs to be high though. I have circuits for doing all that but the CF I use is just fine. You say that you ³have tried out all sorts of AM detector ideas², besides your spin on the diode detector, and your ill fated attempts at building a syncrodyne receiver, what have you tried? I've tried a lot of variations on normal arrangements with IF coil feeding an anode in a vari-mu pentode IF amp and following RC circuit. The CF seems best to me. I tried to build a version of the DG Tucker Synchrodyne circuit published in Wireless World in about 1947. Maybe its now online some place, and what prevented progess was the making of a suitable balanced synchronous demodulator. Probably the best way is to use a toroidal transformer with the core µ just right for range of RF frequencies. But I never had time to explore how to make such a tranny. The tucker circuit uses lots more tubes than a superhet and needs much more practical expertise to get running without loud whistles while tuning. It was a potentially excellent thing but it was never to become popular because the superhet was king. Then tried making a synchrodyne based on a self oscillating 6BE6, and there are some simple circuits of those around, just RF input tube and 6BE6 needed only. That sort of worked a bit but monkey chatter and whistles and controlling oscillations just right were easier said than done so I abandoned the idea and proceeded with a good superhet. AGC is applied to RF vari-mu cascode triode input amp and 6AN7mixer, then 6BX6 sharp cut off IF amp without AGC and using some unbypassed Rk for local current FB. AF bandwidth is 10kHz, with minimum distance between IF coils, but even with max distance AF BW is about 6kHz. Tone control boosts and cuts F above 2kHz. 12AU7 CF detector used, and after comparing the sound of my kitchen AM radio to countless others I have repaired, my own design is very much better to listen to. I have a Marantz AM/FM tuned hooked up to allow switching to FM in mono, and when the same ABC news program is being broadcast at the same time on local ABC AM stations and FM stations the AM radio I have sounds clearer than the FM produced by the SS tuner. There is a subtle difference, and a pleasant one. The AM from the Marantz is ****ing awful. I try not to give AM radio much attention in my life because I have 1,001 projects to complete for customers and several amp projects fr myself in progress. But I get "trapped by AM radios" which ppl want fixed. The majority are collectors items with outrageously poor performance if they are restored to original condition, and because ppl use compact flourescent lamps and 101 other things which create noise, a ferrite rod antenna must be used and the MW RF input coild junked. Then inputs for tuner and CD must be put in with a switch to people are not stuck having to listen to the appalling programme material such as talk back and commercial garbage laced with adds. The speakers and amps need rebuilding to meet modern expectations. Thre are some who might junk all the tubes and put in a board full of SS parts but then that betrays tubes, and I don't do that trick. What the **** impedes your progress to your workshop and soldering iron? Two things, first the fact that AM detectors are of academic interest only, as there has been no AM broadcasting around this area, that needs anything better than the crudest AM detector, for at least 15 years or so. *Second, I think I am more interested in putting my soldering iron to work on the John Stewart inspired ³improved 25L6² amplifier. *There are three binary design decisions to be made before I purchase a chassis and begin punching it. So it seems unlikely you'll ever listen to the benefits of my ideas. Binary decisions eh. In a bind are you? If you have a broken leg, or are impaired or disabled in some way, then let's hear about it. Ah, you think you can drag me into an off topic discussion, it won't work, if you want to explore that kind of discussion, please take it to email. I guess you think some things are best kept out of the public gaze. Its remarkable how many people are secretive about themselves, I'm one who isn't. I have very little to feel ashamed of or embarrassed about. I googled * Telefunken AM rebroadcast receiver "Ballempfänger" Nothing to be seen. You weren't persistent enough, for a general description go he http://www.radiomuseum.org/r/telefun...faenger_b.html If you aren't a member, I'm not, you won't be able to see the schematic. *The schematic is available from several other web sites though, you can find it in two parts he http://www.tsf-radio.org/schph.php?f...Ball-Empfanger.... and he http://www.tsf-radio.org/schph.php?f...Ball-Empfanger.... While the design looks like the ³bean counters² were kept at a safe distance, there still isn't a cathode follower in sight! Those links open but the pictures won't open to reveal what you are talking about. I doubt I am missing anything. AFAIK, not one single commercial example exists of a radio with "infinite impedance detector" even though the Selsted & Smith example is given in RDH4, page 1,495, Figs 27.56, 27.57 RDH4 does not have anything that works as well as what I invented for myself 14 years ago. In the US J.W. Miller offered both commercial AM tuners and Radios using the infinite impedance detector, I have one of their AM Tuners. *Altec Lansing also offered an AM-FM tuner that used the infinite impedance detector in the AM section. *Sargent Rayment also offered a line of tuners and receivers that used the Selsted & Smith detector in the AM sections. *Ampex also built an AM-FM tuner that used a perverted variant of the Selsted & Smith detector, I also have one of these. I've never seen any of these things in Oz or any of their schematics. Infinite Z detector isn't bad, but gives very low output. That's an interesting comment as the output level depends on the input just as with a diode detector. *My experiments with the circuit suggest that it does exploit the characteristics of the low mu triodes to improve operation at low carrier levels. The infinite Z detector should work very well if a CF is used to buffer the IFT output to produce a low Z drive which might then be applied to a diode and then to resistance to then be able to extract the average voltage across the resistance, and without the diode having to charge a cap. Kinda like a half wave rectifier in a PSU where the aim is to have the AF signal linear to average voltage. One can use a choke and cap after the diode in the same way. I've not needed to explore all that because I find the CF driving the diode which charges the RC works just fine. The second CF can also be rigged as a low gain AF amp with RC coupllng but R is a bias R of say 1M5 taken to a tap on Rk, so it appears as maybe 5M0 to the detector circuit so cut off at high AM% is avoided. The anode then can easily give gain = 4. it does mean that where one has say 2Vrms of AF at the detector, you have 8Vrms at 12AU7 anode, and that's getting high, tempting high THD, so I prefer having two CF, with the second just to buffer the RC detector circuit and stop the vol control loading. some Gain for the poweramp is easily created in a tone control stage. It's hard to beat a carefully designed diode detector though. Depends. Most are distortion generators. Those are not ³carefully² designed, it is easy to screw up a diode detector, it is equally easy to build a good one but it costs a little gain. *This and the output transformer are the areas where the work of ³bean counters² can be most easily seen. Indeed. Most bean counters justify their employment by being able to reduce the parts and labour needed to make something, and therefore increasing shareholder profits and most often reducing the sound quality and reliability in electronics produced by the company. The opposite approach, of adding components that serve no useful function is just as bad, actually worse. Each unto their own. everyone has a different way of doing things, and commercial competition stifles anyone who dares to use one more nut, bolt or resistor than the ****ing opposition brand. The result is that they all have competitions to seewho can dumb **** down the most while maintaining enough sales for the shareholders to dine well. Hardly, commercial products today are overflowing with every imaginable feature, each of which adds to the parts count. *Competition today seems to be based more on features, with cost as a secondary issue, why make something more expensive than it needs to be to provide the required features and performance? I have a Marshall JCM2000 60W guitar amp pulled to bits on the bench. It has an enormous lot of features and 5 times the number of R&C used in 1960, and maybe 15 opamps and it is a complete PIA to work upon. Nobody much gives a **** about the concerns I have, such as it should be simple and easy. But some guitar amps are excellently configured with ultra simplicity, real class A, and hardly any features. The really good musos don't need features; they know how to entertain without needing gear features to cover shortcomings. If it makes you ill to use one more cathode follower than someone somewhere said could be superflous, then don't. But I will. In my vicinity there are no shareholders or bean counters, and my clients get a good deal without having to pay for someone's Cadillac or expensive lunches, neither of which contribute to sound quality. An unneeded cathode follower doesn't contribute to sound quality either. *I won't bother to ask who pays for your lunches and transportation, least it take us off topic again, but I am reasonably sure they are accounted for in the price of the amplifiers you build. I make all payments around here and I hear the benefits of my circuits as do my customers. I make very low wages with all I do because the general public never pays artisans very much. I like my work, so I do my work. Toilet cleaners get paid more. It worries me not, money ain't everything. Patrick Turner. |
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#71
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: I already know that a well-designed diode detector works best for me; at least since I regressed to tubes, in my IC days I would have answered differently. With ICs you get a dc coupled amp and its possible to use an output diode pointed at an RC circuit with the audio voltage and Vdc fed back to the FB input port of the IC, and thus get what is supposed to make the most linear detector imaginable because there is MUCH more NFB being applied than when using a cathode follower like I do. The IC input impedance at IF frequency needs to be high though. I have circuits for doing all that but the CF I use is just fine. HP had an interesting AM demodulator in their AM/FM modulation meter. The AM detector used transistors, not ICs. A rough description as I remember it is as follows. The transistors were configured in what was basically a darlington configuration. Feedback was taken from the collector through a capacitor to a pair of parallel diodes back to a summing junction at the base. The two diodes were connected in opposite directions, with a pair of parallel connected complex cognate networks in series with one diode at the end connected to the transistor base. The modulation output was taken across the network that has the form of a low pass filter IIRC. You say that you ³have tried out all sorts of AM detector ideas², besides your spin on the diode detector, and your ill fated attempts at building a syncrodyne receiver, what have you tried? I've tried a lot of variations on normal arrangements with IF coil feeding an anode in a vari-mu pentode IF amp and following RC circuit. The CF seems best to me. I tried to build a version of the DG Tucker Synchrodyne circuit published in Wireless World in about 1947. Maybe its now online some place, and what prevented progess was the making of a suitable balanced synchronous demodulator. Probably the best way is to use a toroidal transformer with the core µ just right for range of RF frequencies. But I never had time to explore how to make such a tranny. The tucker circuit uses lots more tubes than a superhet and needs much more practical expertise to get running without loud whistles while tuning. It was a potentially excellent thing but it was never to become popular because the superhet was king. Then tried making a synchrodyne based on a self oscillating 6BE6, and there are some simple circuits of those around, just RF input tube and 6BE6 needed only. That sort of worked a bit but monkey chatter and whistles and controlling oscillations just right were easier said than done so I abandoned the idea and proceeded with a good superhet. AGC is applied to RF vari-mu cascode triode input amp and 6AN7mixer, then 6BX6 sharp cut off IF amp without AGC and using some unbypassed Rk for local current FB. AF bandwidth is 10kHz, with minimum distance between IF coils, but even with max distance AF BW is about 6kHz. Tone control boosts and cuts F above 2kHz. 12AU7 CF detector used, and after comparing the sound of my kitchen AM radio to countless others I have repaired, my own design is very much better to listen to. If you are willing to use an IC you can get a whole synchronous detector circuit in a single IC in the form of one of the old CQUAM AM Stereo decoder IC chips. There are instructions on the web that describe how to do this. What the **** impedes your progress to your workshop and soldering iron? Two things, first the fact that AM detectors are of academic interest only, as there has been no AM broadcasting around this area, that needs anything better than the crudest AM detector, for at least 15 years or so. *Second, I think I am more interested in putting my soldering iron to work on the John Stewart inspired ³improved 25L6² amplifier. *There are three binary design decisions to be made before I purchase a chassis and begin punching it. So it seems unlikely you'll ever listen to the benefits of my ideas. Binary decisions eh. In a bind are you? Me thinks you misinterpreted what I meant by the term "Binary decisions". Here are the three decisions I have to make, perhaps I should flip a coin and be done with it. 1. The output stage will be of the distributed load type with 50% of the load in the cathode circuit, and 50% in the plate circuit. The decision yet to be taken is should I use the McIntosh output circuit, or an Acoustical style of output circuit? This decision will have some impact on the layout of the power supply. 2. I must settle on the output tubes to be used, a QUAD of 25L6GTs in push-pull parallel, or a pair of 7695s. Actually this is a ternary choice, the KT55 is another possible choice for the output tubes, while these are cool tubes, finding some would probably not be easy. The output tube choice has a major impact on the chassis size, and layout, so I have to settle it before I purchase a chassis and begin drilling and punching it. 3. The driver tube must also be specified, the binary choice was between the 6211, and the 12AU7, yesterday I made it a ternary choice by adding the 12AT7 to the list. These tubes all use the same socket, so they can all be tried with only a change of cathode resistor. I googled * Telefunken AM rebroadcast receiver "Ballempfänger" Nothing to be seen. You weren't persistent enough, for a general description go he http://www.radiomuseum.org/r/telefun...faenger_b.html If you aren't a member, I'm not, you won't be able to see the schematic. *The schematic is available from several other web sites though, you can find it in two parts he http://www.tsf-radio.org/schph.php?f...Ball-Empfanger... and he http://www.tsf-radio.org/schph.php?f...Ball-Empfanger... While the design looks like the ³bean counters² were kept at a safe distance, there still isn't a cathode follower in sight! Those links open but the pictures won't open to reveal what you are talking about. I doubt I am missing anything. I think you are just intimidated by the excellent German engineering in this receiver, to eliminate any possible excuse of your not being able to open it, I have posted a copy of the schematic on my web pages at this URL: http://fmamradios.com/stuff/E1.pdf -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#72
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jul 12, 12:42*pm, John Byrns wrote:
In article , *Patrick Turner wrote: I already know that a well-designed diode detector works best for me; at least since I regressed to tubes, in my IC days I would have answered differently. With ICs you get a dc coupled amp and its possible to use an output diode pointed at an RC circuit with the audio voltage and Vdc fed back to the FB input port of the IC, and thus get what is supposed to make the most linear detector imaginable because there is MUCH more NFB being applied than when using a cathode follower like I do. The IC input impedance at IF frequency needs to be high though. I have circuits for doing all that but the CF I use is just fine. HP had an interesting AM demodulator in their AM/FM modulation meter. *The AM detector used transistors, not ICs. *A rough description as I remember it is as follows. *The transistors were configured in what was basically a darlington configuration. *Feedback was taken from the collector through a capacitor to a pair of parallel diodes back to a summing junction at the base. *The two diodes were connected in opposite directions, with a pair of parallel connected complex cognate networks in series with one diode at the end connected to the transistor base. *The modulation output was taken across the network that has the form of a low pass filter IIRC. It'd be nice to have a schematic John. Then 1,000 words are told with 1 picture. You say that you ³have tried out all sorts of AM detector ideas², besides your spin on the diode detector, and your ill fated attempts at building a syncrodyne receiver, what have you tried? I've tried a lot of variations on normal arrangements with IF coil feeding an anode in a vari-mu pentode IF amp and following RC circuit. The CF seems best to me. I tried to build a version of the DG Tucker Synchrodyne circuit published in Wireless World in about 1947. Maybe its now online some place, and what prevented progess was the making of a suitable balanced synchronous demodulator. Probably the best way is to use a toroidal transformer with the core µ just right for range of RF frequencies. But I never had time to explore how to make such a tranny. The tucker circuit uses lots more tubes than a superhet and needs much more practical expertise to get running without loud whistles while tuning. It was a potentially excellent thing but it was never to become popular because the superhet was king. Then tried making a synchrodyne based on a self oscillating 6BE6, and there are some simple circuits of those around, just RF input tube and 6BE6 needed only. That sort of worked a bit but monkey chatter and whistles and controlling oscillations just right were easier said than done so I abandoned the idea and proceeded with a good superhet. AGC is applied to RF vari-mu cascode triode input amp and 6AN7mixer, then 6BX6 sharp cut off IF amp without AGC and using some unbypassed Rk for local current FB. AF bandwidth is 10kHz, with minimum distance between IF coils, but even with max distance AF BW is about 6kHz. Tone control boosts and cuts F above 2kHz. 12AU7 CF detector used, and after comparing the sound of my kitchen AM radio to countless others I have repaired, my own design is very much better to listen to. If you are willing to use an IC you can get a whole synchronous detector circuit in a single IC in the form of one of the old CQUAM AM Stereo decoder IC chips. * There are instructions on the web that describe how to do this. All sorts of things are possible, but the humble pair of CFs for me is the most appropriate in a tube radio. If I had the time I'd investigate further but I really have little need, and I'm perpetually busy fixing people's radios, amps and speakers. I dunno about you, but I must work or I starve, and find time to ride 200km a week on a bike. *There are three binary design decisions to be made before I purchase a chassis and begin punching it. So it seems unlikely you'll ever listen to the benefits of my ideas. Binary decisions eh. *In a bind are you? Me thinks you misinterpreted what I meant by the term "Binary decisions". Er, no, its just my Cents of Hugh Mer. Here are the three decisions I have to make, perhaps I should flip a coin and be done with it. 1. The output stage will be of the distributed load type with 50% of the load in the cathode circuit, and 50% in the plate circuit. *The decision yet to be taken is should I use the McIntosh output circuit, or an Acoustical style of output circuit? *This decision will have some impact on the layout of the power supply. So how long have you been wondering what the F to do? Who is t say you'll flip a coin tommorrow, then later un-decide what the coin said, and keep flipping until a better outcome appears. Could Flipper help? The McIntosh method gives the best measurements, and you don't need bifilar OPT windings. EAR did a quasi McI amp in the EAR 509 with some PL509 HORRIBLE output tubes, and it was to get 100W with high NFB and its sounded terrible, and using a pair of KT88 with UL or CFB of 20% and aiming to get 50W max with high % of class A1 working, say 15W max at least, will give excellent performance with one less gain stage. But during the next 15 years you'll have plenty of time to consider. 2. I must settle on the output tubes to be used, a QUAD of 25L6GTs in push-pull parallel, or a pair of 7695s. *Actually this is a ternary choice, the KT55 is another possible choice for the output tubes, while these are cool tubes, finding some would probably not be easy. *The output tube choice has a major impact on the chassis size, and layout, so I have to settle it before I purchase a chassis and begin drilling and punching it. Its often not what you use, but how you use it that counts. 3. The driver tube must also be specified, the binary choice was between the 6211, and the 12AU7, yesterday I made it a ternary choice by adding the 12AT7 to the list. *These tubes all use the same socket, so they can all be tried with only a change of cathode resistor. Good luck. I guess we may never know how your project ends up at the present rate of progress. I googled * Telefunken AM rebroadcast receiver "Ballempfänger" Nothing to be seen. You weren't persistent enough, for a general description go he http://www.radiomuseum.org/r/telefun...faenger_b.html If you aren't a member, I'm not, you won't be able to see the schematic. *The schematic is available from several other web sites though, you can find it in two parts he http://www.tsf-radio.org/schph.php?f...Ball-Empfanger... and he http://www.tsf-radio.org/schph.php?f...Ball-Empfanger... While the design looks like the ³bean counters² were kept at a safe distance, there still isn't a cathode follower in sight! Those links open but the pictures won't open to reveal what you are talking about. I doubt I am missing anything. I think you are just intimidated by the excellent German engineering in this receiver, to eliminate any possible excuse of your not being able to open it, I have posted a copy of the schematic on my web pages at this URL: http://fmamradios.com/stuff/E1.pdf Not a bad radio schematic, but I'm not going to adopt any of the German techniques. Too complex. This radio has UMPTEEN PARTS which don't need to be there just to get strong local AM stations with hi-fi quality. While you won't accept my use of one lousy cathode follower, you are putting a radio set under my nose which has maybe 200 parts I don't need to use to attain my simple goals. AM radios tend to waste huge chunks of my time with very little profit, and I just do what works for me, and this includes far more simplicity than what the Germans may have done. I have several communication radios and trancievers which all need re- builds. An old radio ham I knew died and his folks sold all they could, but then there was still a small mountain of collectable stuff left over in various states of dis-repair. So they rang me to collect what would have gone to a tip. Took me a week to collect and sort it then another week to extend my storage areas with shelving to keep the old junk. Going right over it all and all would take weeks before even one radio set could be declared to be "good at what they do", without all sorts if intermittencies and noises because of old switches and contacts and crap. But listening on SW is a bit boring, and even if one joins in discussions with radio amateurs after building one's own little old radio station, one ends talking to old guys about their latest surgery and health bothers and old age problems. Kinda puts me to sleep, listenin' to them droning on nerdishly, saying so many words, but trying to be un-personal, and not saying much at all. There are many who just like to broadcast, and their audience never really exists. They can't convey their creativity with pictures very well. Rather than put up with droners, I have my own life to do, and a living to be earned, and fitness to be maintained. Patrick Turner. |
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#73
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If you are willing to use an IC you can get a whole synchronous detector circuit
in a single IC in the form of one of the old CQUAM AM Stereo decoder IC chips. There are instructions on the web that describe how to do this. What the **** impedes your progress to your workshop and soldering iron? Two things, first the fact that AM detectors are of academic interest only, as there has been no AM broadcasting around this area, that needs anything better than the crudest AM detector, for at least 15 years or so. *Second, I think I am more interested in putting my soldering iron to work on the John Stewart inspired ³improved 25L6² amplifier. *There are three binary design decisions to be made before I purchase a chassis and begin punching it. So it seems unlikely you'll ever listen to the benefits of my ideas. Binary decisions eh. In a bind are you? Me thinks you misinterpreted what I meant by the term "Binary decisions". Here are the three decisions I have to make, perhaps I should flip a coin and be done with it. 1. The output stage will be of the distributed load type with 50% of the load in the cathode circuit, and 50% in the plate circuit. The decision yet to be taken is should I use the McIntosh output circuit, or an Acoustical style of output circuit? This decision will have some impact on the layout of the power supply. 2. I must settle on the output tubes to be used, a QUAD of 25L6GTs in push-pull parallel, or a pair of 7695s. Actually this is a ternary choice, the KT55 is another possible choice for the output tubes, while these are cool tubes, finding some would probably not be easy. The output tube choice has a major impact on the chassis size, and layout, so I have to settle it before I purchase a chassis and begin drilling and punching it. 3. The driver tube must also be specified, the binary choice was between the 6211, and the 12AU7, yesterday I made it a ternary choice by adding the 12AT7 to the list. These tubes all use the same socket, so they can all be tried with only a change of cathode resistor.http://fmamradios.com/stuff/E1.pdf -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/[/quote] Hey John, I just now checked at Antique Electronics. 7695s are cheap at $3.45. The 6.3 volt version is the 7754. You & I had discussed this cct at length in Fall 2004. I would have thought you would have moved forward with it by now. I ended up suggesting the 17EW8 as the driver into your choke coupled interstage transformer scheme since it has lower rp than some other drivers you had considered. The final schema with simulations is attached. The label on the OPTs should read 1:0.173, not as shown. Cheers to all, John |
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#74
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
snip
A drawback - a despised electrolytic as a shaping component. Btw, there's another means of achieving a modest 'LF shelf', using a concertina, and that's to boostrap the gain stage load off the concertina cathode. Gain drops at LF as the boostrap rolls off. This has been done in some Dynaco schematics which have the concertina cathode driving the top of a RLdc via an electro cap, therefore supplying positive FB to the input pentode by means of increasing the RL the pentode anode "sees" so that pentode gain goes much higher therefore making a given amount of GNFB much more effective. Basically while peter robs paul and you light a fire under both, they tend to become very active indeed. If the input tube is a lowly triode who's gain is determined by its lower µ the PFB effect is minimal, and whether there is a bootstrap or not won't change VLF gain very much, so the shelving networks seen throughout my website are perhaps the better way with triodes. It has the added advantage of boosting stage gain and theoretically lowering distortion from the (idealized) 'infinite impedance' of the bootstrap The bootstrap from concertina cathode could be taken from an additional triode cathode follower which is driven directly off the cathode so concertina R values remain unchanged, ie, anode RL = cathode RL. The CF can then be used to drive a two resistor R divider to V1 anode. One has to be careful that cut off distortion with the AC coupling does not happen. The concertina resistors need adjusting to re-balance the load, and that causes an offset toward B+, but that can be an advantage as well since direct coupled concertinas on modest B+ rails can end up with rather low anode voltage on the gain stage and the offset bumps that up. Yes, indeed. I would say I prefer the LTP driver with cathode CCS with say 6SN7/6CG7/12BH7/12AU7 or EL84. Then the LTP acts as a balanced pair with very low THD. Whatever the input tube is, pentode or triode, it has only to produce a low signal in SE mode so THD remains lower than if you have an input tube needing to make slightly more Va than is applied to each output tube grid. There's plenny of headroom for the function of the shelving network. The LTP does not need to be directly driven from input gain tube anode, so the LTP can have much more headroom than found in most samples of amps like Leak and Manley Labs et all. Patrick Turner. |
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#75
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jul 16, 3:07*pm, flipper wrote:
On Fri, 15 Jul 2011 02:03:07 -0700 (PDT), Patrick Turner wrote: snip A drawback - a despised electrolytic as a shaping component. Btw, there's another means of achieving a modest 'LF shelf', using a concertina, and that's to boostrap the gain stage load off the concertina cathode. Gain drops at LF as the boostrap rolls off. This has been done in some Dynaco schematics which have the concertina cathode driving the top of a RLdc via an electro cap, therefore supplying positive FB to the input pentode by means of increasing the RL the pentode anode "sees" so that pentode gain goes much higher therefore making a given amount of GNFB much more effective. Basically while peter robs paul and you light a fire under both, they tend to become very active indeed. Of course it's been done before. If the input tube is a lowly triode who's gain is determined by its lower µ the PFB effect is minimal, and whether there is a bootstrap or not won't change VLF gain very much, so the shelving networks seen throughout my website are perhaps the better way with triodes. Depends, but 6 dB isn't a bad rough cut rule of thumb. Sometimes 'just a little more' is all it takes and, at least with the ones I've done, the capacitor values are usually, or can be, small enough to use film and avoid electrolytics. My "Stealth AX" amp does it sort of 'in reverse'. The gain triode Rl is under the concertina splitter, for the 'infinite impedance' bootstrap, and the signal is cap coupled to the concertina grid. It has the added advantage of boosting stage gain and theoretically lowering distortion from the (idealized) 'infinite impedance' of the bootstrap The bootstrap from concertina cathode could be taken from an additional triode cathode follower which is driven directly off the cathode so concertina R values remain unchanged, ie, anode RL = cathode RL. The CF can then be used to drive a two resistor R divider to V1 anode. One has to be careful that cut off distortion with the AC coupling does not happen. It "could be" but it seems an unnecessary waste of a tube. The concertina resistors need adjusting to re-balance the load, and that causes an offset toward B+, but that can be an advantage as well since direct coupled concertinas on modest B+ rails can end up with rather low anode voltage on the gain stage and the offset bumps that up. Yes, indeed. I would say I prefer the LTP driver with cathode CCS with say 6SN7/6CG7/12BH7/12AU7 or EL84. Then the LTP acts as a balanced pair with very low THD. It isn't as low as a 100% NFB cathode follower, like the concertina, and it's more tubes. It depends. Have a look at http://www.turneraudio.com.au/300w-1...tput-jan06.htm Here I have an LTP which produces less than 0.5% THD at two phases of 85Vrms from each driver triode. To do a similar thing with concertina, you'd have a triode making 170Vrms Va-k, and one might do that using an EL84 in triode, and the drive voltage to the Concer grid would be 95Vrms. The previous stage has to make a shirt&trouser load sized signal, maybe with 5% THD. Now the concer stage OLG THD might be 10%, and with the CLG gain reduction to about 2, the 10% is reduced to 1.2%, but basically you end up with more 2H than I get with LTP which has natural cancelling of 2H. Now in a typical Williamson, the balanced amp needs to make only 2 phases of about say 32Vrms to power a couple of 6550 in triode. So the concer makes only two phases of 2V and input tube makes only 2.2V at clipping. Now with the Willy, the input concer stages usually have such LOW signals at normal listening levels, say less than 0.2Vrms, the THD is down at 0.05% and is reduced by the GNFB to utterly negligible levels. The same goes for the use of the LTP in my amps. So the discussion of "what is best" becomes of academic interest only. In small amps with UL output stages with EL84, the SE input triode with SE concer stage is all one needs and sounds/measured fine. If you use an extra triode and make an LTP, you might find the amp can be made more sensitive, and the THD will be almost indentical to the concer stage but with less 2H present. Both ideas work fine where signals are low, but in my high powered amps the concer drive stage is not so hot. McIntosh amps have balanced driver stages. Most makers use LTP or balanced with larger tubes. *Whatever the input tube is, pentode or triode, it has only to produce a low signal in SE mode so THD remains lower than if you have an input tube needing to make slightly more Va than is applied to each output tube grid. Distortion is also increased by the extra tubes. I happen to like the LTP too but that doesn't mean everything else is crap and, like all design tradeoffs, "it depends" (on everything else). There's more than one way to do things. A concertina is enough to drive a pair of 6BQ5s all by itself without interposing another set of tubes and while that may not be your cup of tea it makes for a perfectly fine little amp. I agree entirely. An LTP driven single ended has only half the gain of Williamson's concertina double driven short tail and while I know you think converting that to an LTP 'reduces distortion' it's at a place in the amp likely to be of little consequence since the vast majority of distortion is in the output stage. So far that's a "why not?" but the LPT does cause overload interaction on positive grid drive, as does a concertina *if* it's the thing doing the driving (but it's buffered in the Williamson). It might not seem like such a big deal but I've seen that cause HF instability on both the concertina and LPT and is the reason some add a series grid resistor between the concertina and output tubes when driving them directly. Usually HF instability is due to the OLG phase shift characteristic and once you have the correct gain shelving and zobels the HF becomes entirely stable. And of course the grid stoppers are always a good idea. There's plenny of headroom for the function of the shelving network. The LTP does not need to be directly driven from input gain tube anode, so the LTP can have much more headroom than found in most samples of amps like Leak and Manley Labs et all. What 'headroom'? The shelving network is zero loss in band. Or do you mean for your shelf's 'peaking', which a bootstrap roll off doesn't cause? All I said is there was "another means," and I don't know that I'd do it for 'just that', but it's an interesting aspect to consider if one also wants the added gain. Observation of the error signal at V1 anode output when feeding the amp a level input signal between 1Hz to 100Hz will show that the LF shelving network is OK. The reference 1kHz signal should produce half maximum l Vo. Then if you drop the input F and plot the response at all electrodes, you'll understand. And what will help matters is that you have passive input CR input filter with pole at 8 Hz. You just don't need to try to have the amp vainly try to make a big effort with a 3Hz signal. Patrick Turner. |
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#76
Posted to rec.audio.tubes
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HP AM Demodulator (was: VLF stability)
In article ,
Patrick Turner wrote: On Jul 12, 12:42*pm, John Byrns wrote: In article , *Patrick Turner wrote: I already know that a well-designed diode detector works best for me; at least since I regressed to tubes, in my IC days I would have answered differently. With ICs you get a dc coupled amp and its possible to use an output diode pointed at an RC circuit with the audio voltage and Vdc fed back to the FB input port of the IC, and thus get what is supposed to make the most linear detector imaginable because there is MUCH more NFB being applied than when using a cathode follower like I do. The IC input impedance at IF frequency needs to be high though. I have circuits for doing all that but the CF I use is just fine. HP had an interesting AM demodulator in their AM/FM modulation meter. The AM detector used transistors, not ICs. *A rough description as I remember it is as follows. *The transistors were configured in what was basically a darlington configuration. *Feedback was taken from the collector through a capacitor to a pair of parallel diodes back to a summing junction at the base. *The two diodes were connected in opposite directions, with a pair of parallel connected complex cognate networks in series with one diode at the end connected to the transistor base. *The modulation output was taken across the network that has the form of a low pass filter IIRC. It'd be nice to have a schematic John. Then 1,000 words are told with 1 picture. Hi Patrick, Sorry that I forgot about your schematic request, however sometimes other things in life take precedence. The following files contain a description and a simplified schematic of the HP AM Demodulator that I was talking about. http://www.fmamradios.com/HP_AM_Demo...979-Cover1.jpg www.fmamradios.com/HP_AM_Demod/HPJ-979-P12.jpg www.fmamradios.com/HP_AM_Demod/HPJ-979-P13.jpg www.fmamradios.com/HP_AM_Demod/HPJ-979-P14.jpg The AM distortion is claimed to be less than 0.1%, although this article doesn't say what modulation depth this is good to. IIRC the service manual contains those details. I believe that the ³audio² bandwidth of this detector is something on the order of 250 kHz. I have a copy of the relevant pages from the service manual somewhere around here, with a detailed schematic and parts list, however I am not going to try and dig it out, the simplified schematic should suffice to convey the basic idea. The specs as to modulation depth vs. distortion are probably also in the relevant HP catalog, I believe some of these old HP catalogs are available on line, although I don't have any links to hand. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#77
Posted to rec.audio.tubes
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HP AM Demodulator (was: VLF stability)
In article ,
John Byrns wrote: In article , Patrick Turner wrote: On Jul 12, 12:42*pm, John Byrns wrote: In article , *Patrick Turner wrote: I already know that a well-designed diode detector works best for me; at least since I regressed to tubes, in my IC days I would have answered differently. With ICs you get a dc coupled amp and its possible to use an output diode pointed at an RC circuit with the audio voltage and Vdc fed back to the FB input port of the IC, and thus get what is supposed to make the most linear detector imaginable because there is MUCH more NFB being applied than when using a cathode follower like I do. The IC input impedance at IF frequency needs to be high though. I have circuits for doing all that but the CF I use is just fine. HP had an interesting AM demodulator in their AM/FM modulation meter. The AM detector used transistors, not ICs. *A rough description as I remember it is as follows. *The transistors were configured in what was basically a darlington configuration. *Feedback was taken from the collector through a capacitor to a pair of parallel diodes back to a summing junction at the base. *The two diodes were connected in opposite directions, with a pair of parallel connected complex cognate networks in series with one diode at the end connected to the transistor base. *The modulation output was taken across the network that has the form of a low pass filter IIRC. It'd be nice to have a schematic John. Then 1,000 words are told with 1 picture. Hi Patrick, Sorry that I forgot about your schematic request, however sometimes other things in life take precedence. The following files contain a description and a simplified schematic of the HP AM Demodulator that I was talking about. http://www.fmamradios.com/HP_AM_Demo...979-Cover1.jpg www.fmamradios.com/HP_AM_Demod/HPJ-979-P12.jpg www.fmamradios.com/HP_AM_Demod/HPJ-979-P13.jpg www.fmamradios.com/HP_AM_Demod/HPJ-979-P14.jpg The AM distortion is claimed to be less than 0.1%, although this article doesn't say what modulation depth this is good to. IIRC the service manual contains those details. I believe that the ³audio² bandwidth of this detector is something on the order of 250 kHz. I have a copy of the relevant pages from the service manual somewhere around here, with a detailed schematic and parts list, however I am not going to try and dig it out, the simplified schematic should suffice to convey the basic idea. The specs as to modulation depth vs. distortion are probably also in the relevant HP catalog, I believe some of these old HP catalogs are available on line, although I don't have any links to hand. Oops sorry I screwed the links up, they should be as follows. http://www.fmamradios.com/HP_AM_Demo...979-Cover1.jpg http://www.fmamradios.com/HP_AM_Demod/HPJ-979-P12.jpg http://www.fmamradios.com/HP_AM_Demod/HPJ-979-P13.jpg http://www.fmamradios.com/HP_AM_Demod/HPJ-979-P14.jpg -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#78
Posted to rec.audio.tubes
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HP AM Demodulator (was: VLF stability)
Hi Patrick,
Sorry that I forgot about your schematic request, however sometimes other things in life take precedence. *The following files contain a description and a simplified schematic of the HP AM Demodulator that I was talking about. http://www.fmamradios.com/HP_AM_Demo...PJ-979-P14.jpg The AM distortion is claimed to be less than 0.1%, although this article doesn't say what modulation depth this is good to. *IIRC the service manual contains those details. *I believe that the ³audio² bandwidth of this detector is something on the order of 250 kHz. *I have a copy of the relevant pages from the service manual somewhere around here, with a detailed schematic and parts list, however I am not going to try and dig it out, the simplified schematic should suffice to convey the basic idea. *The specs as to modulation depth vs. distortion are probably also in the relevant HP catalog, I believe some of these old HP catalogs are available on line, although I don't have any links to hand. Thanks for all those links, but the schematics are not simply explained or detailed and use more parts than I do and don't use tubes and I'm not attarcted to trying out anthying they show. Most of it is about FM detection. Nothing is there that is specifically worked right out for a simple old superhet with 455kHz. My CF detector is looking real good to me. Patrick Turner. |
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