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"Engineer" wrote in message
... I have copied the original below, plus John's reply (also Bill's). Hope it all makes sense! A. How does the second output tube get it's grid-cathode (inverted) drive if the second grid floats? The first tube acts as a cathode follower, driving a small voltage across the cathode resistor. The other tube picks this up (ala grounded grid amp) and inverts it by its nature. The pitfall comes from loss of signal - the cathode would have signal voltage equal to input if the cathode resistor were a CCS, and since it's a very low resistance the signal output is low. Then, there's a similar loss going into the other cathode. B. If it somehow still gets driven, is it balanced? It has drive, but it is very unbalanced. Maybe if you measured the imbalance in a test circuit, then wound a transformer with an unbalanced primary, it would work a little better. C. Why not parallel the 470K grid resistor with, say, a 0.1 uF cap. to ensure the cathode space charge cannot move the grid at signal frequency and unbalance the drive? Extra part. D. Even if the second grid was properly decoupled, would just a common 330 ohm resistor provide a balanced output stage? Or would the second output tube always be chronically under driven? ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ Yes. So, there it is - I thought quite fascinating. I'd really like to hear what the RAT-ers have to say about that very We prefer the term RATs ) poor PP "design" in several commercial radios that prompted my first post, also John's neat idea. There's been many bad designs. Sometimes amazing that they actually made it to production, eh? Tim -- In the immortal words of Ned Flanders: "No foot longs!" Website @ http://webpages.charter.net/dawill/tmoranwms |
#3
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[Fwd: Old PP outputs stages]
Long post - apologies!
I first posted this to rec.antiques.radio+phono as that was the focus of my question (and I didn't want to cross-post.) However, I received a very interesting answer from John Byrns that I thought was worth sharing with the denizens of RAT. I have copied the original below, plus John's reply (also Bill's). Hope it all makes sense! Cheers, Roger -------- Original Message -------- Subject: Old PP outputs stages Date: Sun, 31 Aug 2003 15:32:14 -0700 From: Engineer Reply-To: Organization: Lorna & Roger at home Newsgroups: rec.antiques.radio+phono I've been perusing the schematics in Ed Kraushar's excellent "Vintage Canadian Radio Schematics" CD (OK, get a life, I know...!) and am a bit bemused by one of the phase splitter designs employed in a couple of the sets that have push-pull audio output stages. The no-brainer solution is, of course, the coupling transformer: primary in the audio amplifier plate circuit, secondary centre tap to ground (or output tube -ve bias) and the anti-phase secondary ends to the two output tube grids. Apart from quibbles over transformer frequency response, hardly a problem with AM radio, it's perfect, i.e. compact, economical (in quantity) and provides some free gain to boot. BTW, I would not use interstage coupling transformers in "modern" tube Hi-Fi designs even though, IIRC, Williamson and others did it for years, and there was at least one solid state amplifier in the late 1960-70's that use it (The Ravensbourne, I think it was, plus another in the same line, the Ravensbrook - I had one!) However, quite a few of the [old] radio receiver designs don't use coupling transformers. They use one of the following methods: 1. A triode phase splitter tube with equal cathode and plate load resistors - this is the classic design still used in some high end audio, so clearly OK for AM radio. 2. A separate phase inverter tube: the "in-phase" signal from the directly driven output tube grid is taken through an attenuator to the grid of a triode tube so that the "attenuation x gain" is equal to unity (we hope), then the output of this triode drives the second output tube grid. I guess it works when the design is done right, the gain is unity and the tube is new. But it's all open-loop so PP balanced drive is dependent on component values and tube gains remaining the same. This only gets a "B-" in my design book, and that's generous. 3. This is the odd one that prompted this posting. There's no interstage transformer, phase splitter or inverter tube, as such. Here, the audio amplifier output goes to only one output grid - via a coupling capacitor, of course. The two output tubes share an undecoupled cathode resistor (typically 330 ohms in the case of a pair of 6V6's.) So far so good - we presume the second O/P tube is cathode driven, providing the grid is grounded to signal by a large capacitor. But it's not! In at least two designs (Astra 1946-47 Model DR103-47 and Brunswick 1931-32 Model B-H) the second output tube grid goes to ground via 470K but without any decoupling capacitor. My questions on method (3), above, for the vintage radio experts a A. How does the second output tube get it's grid-cathode (inverted) drive if the second grid floats? B. If it somehow still gets driven, is it balanced? C. Why not parallel the 470K grid resistor with, say, a 0.1 uF cap. to ensure the cathode space charge cannot move the grid at signal frequency and unbalance the drive? D. Even if the second grid was properly decoupled, would just a common 330 ohm resistor provide a balanced output stage? Or would the second output tube always be chronically under driven? Cheers, Roger PS. Anyone [who] wants to see "state of the art" phase splitter design, see the Mullard "5-10" (also "5-20") schematics. Here's one source, the "5-10": http://www.bonavolta.ch/hobby/en/audio/el84_7.htm Note C3 to ground. But we're well away from vintage radio now! [BTW, I built two of these a long time ago, wish I still had then!] Bill Sheppard replied: In article , (Bill Sheppard) wrote: "John Byrns gave an excellent synopsis on the common-cathode, grounded grid scheme 3 or 4 years ago. Turns out the thing will always be lopsided because the grounded-grid tube will never 'see' as high a signal level between its G1 and cathode as the driven tube sees. This is with the grid directly grounded. Maybe John can elaborate further. John...?" John Byrn then replied as follows: "I had forgotten about posting on that circuit. Without looking it up, I think my main complaint was about the construction articles that were popular for a while, and featured this circuit. They basically just used an unbypassed common cathode resistor, whose value was essentially the same as normally used with a push pull pair of the same tubes. This doesn't really work at all well because the resistor is way too low in value, and the second tube receives very little drive. The circuit should work very well for class A operation if the cathode resistor is replaced with a current source though. Since a current source would be more complex than the phase inverter we were trying to eliminate, I did some simulations and found that the circuit worked fairly well if the cathode resistor were increased in value, to maybe 3 or 4 times the normal value, I forget exactly what value seemed to be a good compromise. The grid resistors of the two output tubes must then be connected to a tap on the cathode resistor to provide the correct bias, basically the upper part of the cathode resistor would be equal to the normal cathode resistor. "Someone in this group recently posted about a radio, I forget what it was, that had what I thought was the cleverest phase inverter for a radio that I have ever seen. I forget what tubes the actual radio used, but basically the detector and audio amplifier tube was something like a 12SQ7, with the normal plate load resistor split between the cathode and plate circuits the same way as in a split load phase inverter, with the two output stage grids connected to the plate and cathode as with the split load inverter. A normal split load phase inverter would provide no voltage gain, so an additional tube would normally be required to provide the needed gain. In this circuit the detector circuit and volume control are floated up from ground, and referenced to the cathode of the 12SQ7. This effectively drives the 12SQ7 between grid and cathode, providing normal gain, which is cut in half to each output grid by the effect of the split load. "The most obvious problem with this scheme is that the detector can no longer be used to provide the AGC voltage for the radio because the diode detector circuit is flapping around with the amplified audio at some positive potential. This problem is easily overcome by using a tube like the 12SF7, which includes a diode, as the IF amplifier. The diode can then serve as a shunt AGC rectifier feed from the primary of the IF transformer, as is often done with a second diode anyway. "This circuit is so clever I wonder why it wasn't commonly used? Does it have some subtle problem that is not obvious? Actually now that I think about it, I have also seen it used in some old British construction articles too. "Regards, "John Byrns" Me again... So, there it is - I thought quite fascinating. I'd really like to hear what the RAT-ers have to say about that very poor PP "design" in several commercial radios that prompted my first post, also John's neat idea. Cheers, Roger -- Roger Jones, P.Eng. Thornhill, Ontario, Canada. "Friends don't let friends vote Liberal" |
#4
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Engineer wrote: john stewart wrote: (snip) Hi Roger- I read your query along with John B's very good description of some of these phase splitters. Here are two more for you to ponder, as they were used in some 30's & 40's radios. In this scheme there is no seperate phase inverter tube or transformer. A resister of the proper value (about 2K) is inserted into the G2 lead of the upper pentode of a PP pair. The resulting signal on that G2 is capacitively coupled to the G1 of the lower member of the PP pair. Of course there is some imbalance, but that is partially corrected by the PP output transformer. This circuit was often seen using PP 6F6's but could be applied to other pentodes or beam tetrodes as well. John, the screen idea is what I came up with while trying to figure out how to phase split without an extra tube - the purpose of the original "kluge" - but I didn't calculate any values. Done properly, it just might just work better than a common cathode resistor - a good experiment for someone with an old PP 6V6 amp to play with (I don't have one or I'd try it!) Another notion I've intended to try out (and indeed will, one of these days) is to use the cathode coupling (short-tail pair) and augment the signal to the "off" phase with a voltage divider from plate of the "on" phase to grid. This would seem to give a less distorted signal than taking it from the screen grid. By making this variable, the amount of asymmetry can be varied to either side. This might actually be useful for blues harp or acoustic guitar players, since it appears that acoustic feedback can be reduced this way. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects: http://dogstar.dantimax.dk | +--------------------------------------------+ |
#5
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john stewart wrote:
(snip) Hi Roger- I read your query along with John B's very good description of some of these phase splitters. Here are two more for you to ponder, as they were used in some 30's & 40's radios. In this scheme there is no seperate phase inverter tube or transformer. A resister of the proper value (about 2K) is inserted into the G2 lead of the upper pentode of a PP pair. The resulting signal on that G2 is capacitively coupled to the G1 of the lower member of the PP pair. Of course there is some imbalance, but that is partially corrected by the PP output transformer. This circuit was often seen using PP 6F6's but could be applied to other pentodes or beam tetrodes as well. John, the screen idea is what I came up with while trying to figure out how to phase split without an extra tube - the purpose of the original "kluge" - but I didn't calculate any values. Done properly, it just might just work better than a common cathode resistor - a good experiment for someone with an old PP 6V6 amp to play with (I don't have one or I'd try it!) (snip) Cheers, John Stewart Cheers, Roger PS. Thanks to all for the great comments. -- Roger Jones, P.Eng. Thornhill, Ontario, Canada. "Friends don't let friends vote Liberal" |
#6
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"Fred Nachbaur" wrote
... Another notion I've intended to try out (and indeed will, one of these days) is to use the cathode coupling (short-tail pair) and augment the signal to the "off" phase with a voltage divider from plate of the "on" phase to grid. This would seem to give a less distorted signal than taking it from the screen grid. By making this variable, the amount of asymmetry can be varied to either side. This might actually be useful for blues harp or acoustic guitar players, since it appears that acoustic feedback can be reduced this way. Confused Ian again... you only identify two legs of the divider so your scheme seems ambiguous to me. Do you mean top to anode, bottom to same grid, and middle to opposite grid. Or top to anode, bottom to ground, and middle to opposite grid? Or maybe even top to anode, bottom to opposite anode, and middle to opposite grid? Interesting experiments, partly because of the problem getting a low enough source impedance for that opposite grid without causing distortion and/or wasting too much current through the divider or voltage across the current-sensing resistor. More interestingly because of the difference between the series and parallel-derived alternatives, when it comes to maintaining correct phase relationship across the whole bandwidth. Also wondering which sources of distortion will tend to cancel, and which will augment. For example with the series derivation from opposite screen, will the nonlinear blip in screen current at low Va result in a signal that will counter or reinforce the simultaneous drop in Ia? Dunno if that made sense...I know what I mean... cheers, Ian |
#7
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I'm listening to the radio with such an amp right now. My problem was this
VERY hot summer: I just couldn't run my usual 6L6GC PP poweramp the whole day without sweating just passing in front of it, therefore I put together a real poor-man's amp aimed at 2Wpower with less than 10W dissipation. As I said, it's truly for ambience listening. I took the schematics from Triode (http://www.triodeel.com/compact.html) but I replaced the EL84s with the two halves of a JJ/Tesla ECC99 (let's say roughly a 12BH7, a 3+3W double triode with a Ri=3000 Ohms). Instead of a 6C4 the driver is (obviously) an ECC82 used for both channels. The OPTs are readily available, "universal" Hammond mod. 125H, connected in order to get 10K A-A. The common cathode resistor is a 390R, 4W, metal oxide. Anode voltage is about 260V, with the cathodes sitting at about 10V above ground (A-K=250V). Power supply is guaranteed by a diodes bridge and a resistive Pi filter using 2x 47uF/450V electrolytics. I'm pretty satisfied with it, because with just 3 tubes I get the "musical wallpaper" I need all day long (I'm a professional engineer working mostly at home) without resorting to the sand state of matter. Now it works connected to a couple of self-made small loudpeakers using a single Fostex FE83 cone (2WRMS): it doesn't even sound so bad. I never measured any parameter because I didn't care, but if the fellow rodents are interested I can take some 'scope pictures. Ciao to all Fabio "Fred Nachbaur" ha scritto nel messaggio news:JZT4b.128124$K44.120827@edtnps84... Engineer wrote: john stewart wrote: (snip) Hi Roger- I read your query along with John B's very good description of some of these phase splitters. Here are two more for you to ponder, as they were used in some 30's & 40's radios. In this scheme there is no seperate phase inverter tube or transformer. A resister of the proper value (about 2K) is inserted into the G2 lead of the upper pentode of a PP pair. The resulting signal on that G2 is capacitively coupled to the G1 of the lower member of the PP pair. Of course there is some imbalance, but that is partially corrected by the PP output transformer. This circuit was often seen using PP 6F6's but could be applied to other pentodes or beam tetrodes as well. John, the screen idea is what I came up with while trying to figure out how to phase split without an extra tube - the purpose of the original "kluge" - but I didn't calculate any values. Done properly, it just might just work better than a common cathode resistor - a good experiment for someone with an old PP 6V6 amp to play with (I don't have one or I'd try it!) Another notion I've intended to try out (and indeed will, one of these days) is to use the cathode coupling (short-tail pair) and augment the signal to the "off" phase with a voltage divider from plate of the "on" phase to grid. This would seem to give a less distorted signal than taking it from the screen grid. By making this variable, the amount of asymmetry can be varied to either side. This might actually be useful for blues harp or acoustic guitar players, since it appears that acoustic feedback can be reduced this way. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects: http://dogstar.dantimax.dk | +--------------------------------------------+ |
#8
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Ian Iveson wrote: "Fred Nachbaur" wrote ... Another notion I've intended to try out (and indeed will, one of these days) is to use the cathode coupling (short-tail pair) and augment the signal to the "off" phase with a voltage divider from plate of the "on" phase to grid. This would seem to give a less distorted signal than taking it from the screen grid. By making this variable, the amount of asymmetry can be varied to either side. This might actually be useful for blues harp or acoustic guitar players, since it appears that acoustic feedback can be reduced this way. Confused Ian again... you only identify two legs of the divider so your scheme seems ambiguous to me. Do you mean top to anode, bottom to same grid, and middle to opposite grid. Or top to anode, bottom to ground, and middle to opposite grid? Or maybe even top to anode, bottom to opposite anode, and middle to opposite grid? None of the above. Top to "on" phase anode, middle to "off" phase grid, and bottom to ground. Interesting experiments, partly because of the problem getting a low enough source impedance for that opposite grid without causing distortion and/or wasting too much current through the divider or voltage across the current-sensing resistor. That's why it's nice (I think) to take it from the anode rather than the screen. The absolute value of the divider resistors can be made quite high, since the grid impedance will be very high in class A1. (Such approaches would probably bomb spectacularly if grid current is drawn.) The idea is merely to "grid-enhance" the cathode-coupled signal, much like the experimental "grid-enhanced cathode-coupled paraphase" did. http://www.dogstar.dantimax.dk/tubestuf/paraph.htm#tt42 More interestingly because of the difference between the series and parallel-derived alternatives, when it comes to maintaining correct phase relationship across the whole bandwidth. Also wondering which sources of distortion will tend to cancel, and which will augment. For example with the series derivation from opposite screen, will the nonlinear blip in screen current at low Va result in a signal that will counter or reinforce the simultaneous drop in Ia? All good questions. I can answer them all easily: "Don't know." However, I'm tempted to say that if such considerations are important, then it's unlikely that a "Muntzed" circuit would be a good starting point in the first place. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects, Vacuum Tubes & other stuff: | | http://www.dogstar.dantimax.dk | +--------------------------------------------+ |
#9
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In article ,
"Steve O'Neill" wrote: "There’s been a lot of discussion and speculation regarding the compact amp and it’s true mode of operation. I decided to build it so I could take some measurements. I built only the output stage using a decent vintage PP OPT measured at ~ 7800 ohm plate to plate with 4,8 and 16 ohm output taps. This was originally designed for a 6V6 amp. 10 ohm resistors were placed in series with the plate leads so DC and AC current could be measured. The cathode resistor was 125 ohm and a regulated PS was used at 260VDC. Output tubes were used/test good Sylvania 6BQ5s within a few mA at design conditions and within about 10% on gm. V1 is the top or “driven” tube and V2 is the bottom tube. Load =8ohm NI resistor. At idle V1Ip =37mA,V2Ip =38mA, V across cathode R = 10.1VDC . Max power out @ 1KHz (obvious clipping) V across 8 ohm = 7.4 VAC = 6.9W: Waveform was distorted, unsymmetrical but rounded Input to V1 betw grid and ground = 10.0 VAC, betw grid and cathode = 6.5VAC, input to V2 betw grid and cathode = 3.3VAC (also voltage across cathode resistor). AC plate current of V1 = 44mA: of V2 = 22mA. At 5VAC across 8 ohms @ 1KHz (3.1 watts or about –3dB below design output) Waveform was fairly clean with a little evidence of flattening on one side (2H?). Input to V1 betw grid and ground = 5.6VAC, betw grid and cathode = 3.7VAC, input to V2 betw grid and cathode = 1.95VAC (also voltage across cathode resistor). AC plate current of V1 = 30mA: of V2 = 16mA It’s been speculated that the lower tube was actually a CCS to counter the DC current of the upper tube thus allowing use of a PP OPT. A suggested test for this was to bypass the cathode resistor with a cap. The expected result if the lower tube was a CCS would be little change in output. Under the 3W output conditions when the cathode resistor was bypassed the RMS value of the output actually went up from 5.0 VAC to 5.1VAC. The output became very distorted with one side becoming quite flat as if one of the output tubes had been removed. Under this condition the AC plate current of V1 = 46mA: of V2 = 4mA. I made no distortion or frequency response measurements nor did I listen to the amp. Based on these measurements I conclude that the amp is truly a PP design with but with very poor balance. I also conclude that V2 isn’t a CCS at least as traditionally defined since V2 is driven and contributes to the output. I also observed that this design is very sensitive to output tube balance if anywhere near the “rated” 6W is going to be achieved. Any other interpretations??? " I would comment that the idea that the second tube is a constant current source would seem to be wrong based on the output transformer primary impedance used. If the second tube were only a CCS, then half the transformer primary should look like approximately 5,000 Ohms to the upper tube, this would imply a 20,000 Ohm center tapped primary. In reality with the commonly used cathode resistor values, the second tube does not contribute nearly as much power as it should, which makes me wonder if a transformer primary higher than 8,000 Ohms, but less than 20,000 Ohms might not give slightly more power output? The optimum value would need to be calculated. This would be like connecting a load somewhat higher than 8 Ohms to the 8 Ohm tap on the secondary of your transformer with the 7,800 Ohm primary. Would it be fair to use a transistor in the cathode tail circuit of this amplifier? A transistor hooked up as a constant current source would probably bump the power output up a notch. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#10
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Fred Nachbaur wrote:
(snip) Interesting experiments, partly because of the problem getting a low enough source impedance for that opposite grid without causing distortion and/or wasting too much current through the divider or voltage across the current-sensing resistor. My guess is that it would be just 2 Kohms, quite low enough to drive a properly biased G1. That's why it's nice (I think) to take it from the anode rather than the screen. The absolute value of the divider resistors can be made quite high, since the grid impedance will be very high in class A1. (Such approaches would probably bomb spectacularly if grid current is drawn.) Should work OK - I would not expect any G1 current on the second O/P tube. But I think the B+ would have to be decoupled to a "totally zero" impedance down to the lowest frequency of the O/P stage, even below or you might have an oscillator - I would not care to speculate on the frequency, somewhere between "motor-boat" and stuck-pig squeal perhaps! (snip) Cheers, Roger -- Roger Jones, P.Eng. Thornhill, Ontario, Canada. "Friends don't let friends vote Liberal" |
#11
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John- I have posted a reference to what I believe is the circuit you
have referred too on ABSE. Taken from RDH4 page 522. Cheers, John Stewart John Byrns wrote: In article , wrote: Me again... So, there it is - I thought quite fascinating. I'd really like to hear what the RAT-ers have to say about that very poor PP "design" in several commercial radios that prompted my first post, also John's neat idea. I'm not quite sure which "neat idea" you are speaking of, but assuming it was the floating diode detector in the AM radio, since I wrote that I have been wondering why the idea seemed so very familiar to me. I finally realized that it is because the RCA AM stereo radio described on my web pages uses a similar floating detector to drive a dematrixing circuit, not the same, but a similar application. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#12
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Steve O'Neill wrote:
Hi You stated at the end of your post: "Luckily for those radios that escaped the factory the bottom tube will not get in the way since it's plate reflects a CC load to the output transformer" Curious how you arrive at the conclusion that the bottom tube plate acts as a CC load to the OPT. I built the TRIODEEL compact amp output stage using recommended circuit values and measured the thing. By the measurements I made, the bottom tube did not behave as a CC load. The whole thing acted like a poorly balanced phase inverter: nothing more, nothing less. Did I measure something wrongly or are we speaking of different circuits here? -- Steve I finally got a look at that circuit by way of a link posted by Fabio Berutti. It is simple enough alright but doomed to poor performance. It may appeal to the SE crowd since it is bound to generate lots of even order harmonics with its very shot tail. I managed to find a drawing of the amp I built using this circuit. It is dated Feb 3, 1958. At the time I was trying to find a way of building a very low cost amp for hifi. I had come across a good supply of cheap power transformers with 170-0-170 HT, so bought several. The basic amps were various combos of PP pairs of 25L6, 35L6 or 50L6's, so the heaters could be run straight off the power line. I used a 6SJ7 to drive the PP amp with the short tail. I quickly came to the same conclusion that you did, although with less sophisticated test equipment. It looked terrible on the scope trace & did not sound much better. Why anyone would think this circuit would deliver hifi I'm not sure. We are told time & again that in this type of phase inverter that for balance the tail needs to be at least as large as the plate loads & preferable several times the plate load. This amp finished up with a 6SF5 split load phase inverter. Pentodes & Beam Tetrodes are inherentely CC devices when operated in their linear mode. If the short tail were bypassed then you would have a CC load applied to one end of the output transformer. However, in this instance, the lower of the PP pair is partially driven, so perhaps not as CC as when not driven. It would be interesting to get into the circuit to get some idea of what impedance we are talking about. However, who wants or needs to spend time it what amounts to trying to push a rope? BTW, I have posted both John B' s phase inverter & some other inverters (one referred too by Fred N), all from RDH4 on ABSE for everyones enjoyment. See Phase Inverters. John Stewart |
#13
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Curious how you arrive at the conclusion that the bottom tube plate acts as
a CC load to the OPT. I built the TRIODEEL compact amp output stage using recommended circuit values and measured the thing. By the measurements I made, the bottom tube did not behave as a CC load. The whole thing acted like a poorly balanced phase inverter: nothing more, nothing less. Did I measure something wrongly or are we speaking of different circuits here? Steve I forgot to mention in the previous post that it would seem the performance of the subject circuit is further compromised by the fact that the G2 current also runs thru the short tail. That would give rise to an even more interesting set of harmonics & some unbelievable IM products. Perhaps Fred N could sell that to one of his guitar pickin' buds!!! Cheers, John Stewart |
#14
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"john stewart" wrote in message ... snippp Perhaps Fred N could sell that to one of his guitar pickin' buds!!! Cheers, John Stewart FredN can't sell anything; he needs any bloody toob gadget only for his self he even can't sell any S**** gadget-he need that also for converting to tube gadget hehe -- .. Choky Prodanovic Aleksandar YU "don't use force, use a larger hammer" - ZM .. |
#15
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john stewart wrote: [...] I forgot to mention in the previous post that it would seem the performance of the subject circuit is further compromised by the fact that the G2 current also runs thru the short tail. That would give rise to an even more interesting set of harmonics & some unbelievable IM products. Perhaps Fred N could sell that to one of his guitar pickin' buds!!! Cheers, John Stewart Yes, that sort of thing is just the ticket for music producing amplifiers. ;-) Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects: http://dogstar.dantimax.dk | +--------------------------------------------+ |
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