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#41
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Pentode gm wired as a triode
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#43
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Pentode gm wired as a triode
Ian said
Er, I thought it was about how gm is split in the ratio of the plate and screen currents - so its about current AND gm. But gm is *not* always split in the same ratio as that of screen currents. That is an approximation that becomes less true as you approach the knee, AFAICS from the curves. Anyway, an interesting question is *why* the 6AU6 has so much screen current. What is it about the geometry of the valve that determines the ratio of screen and anode currents and gms? Yes it is odd isn't it. They even have (had) a patent on the technique. There is a totally classic page of English Engineer's AVO-speak that explains a constant used to correct for the ensuing error. You might expect a conversion from rms to average, and a factor of 2 because of the half-wave operation, but the number they use isn't quite what you would expect, AFAIR. cheers, Ian1 |
#44
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Pentode gm wired as a triode
Ian Iveson wrote:
Ian said Er, I thought it was about how gm is split in the ratio of the plate and screen currents - so its about current AND gm. But gm is *not* always split in the same ratio as that of screen currents. That is an approximation that becomes less true as you approach the knee, AFAICS from the curves. Indeed, but I don't intend to work in that region because gm is lower and distortion higher. Anyway, an interesting question is *why* the 6AU6 has so much screen current. What is it about the geometry of the valve that determines the ratio of screen and anode currents and gms? That is an intersting question. Do you know the answer? Yes it is odd isn't it. They even have (had) a patent on the technique. There is a totally classic page of English Engineer's AVO-speak that explains a constant used to correct for the ensuing error. You might expect a conversion from rms to average, and a factor of 2 because of the half-wave operation, but the number they use isn't quite what you would expect, AFAIR. AVO speak is exactly right. The manual I have says: "It was eventually found that a complete co-relation between the two sets of condition was held when the grid voltage took the form of a sinusoidal waveform with the positive half cycle suppressed (in other words rectified but completely unsmoothed AC) and the following relationships were maintained: Va (rms) = 1.1 Va indicated dc Vg2(rms) = 1.1 Vg2 indicated dc Vg1(mean unsmoothed) = 0.51 Vg1 indicated dc Ia(mean dc) = 0.5 indicated Ia" No explanation whatsoever as to why these particular values hold. Ian cheers, Ian1 |
#45
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Pentode gm wired as a triode
Ian said:
AVO speak is exactly right. The manual I have says: "It was eventually found that a complete co-relation between the two sets of condition was held when the grid voltage took the form of a sinusoidal waveform with the positive half cycle suppressed (in other words rectified but completely unsmoothed AC) and the following relationships were maintained: Va (rms) = 1.1 Va indicated dc Vg2(rms) = 1.1 Vg2 indicated dc Vg1(mean unsmoothed) = 0.51 Vg1 indicated dc Ia(mean dc) = 0.5 indicated Ia" No explanation whatsoever as to why these particular values hold. Right, thanks, that's the page. It reads like an explanation, but isn't. But we might be missing something obvious. Just in case anyone is following this and might have an answer...the "rectification" of the screen and anode supplies is done by the valve under test...ie the valve is subjected to the whole AC cycle. The only rectifier in the tester is used for the grid and bridge circuit. Hence it avoids the need for a high power rectifier. That's how it is with the CT160 anyway. Indeed, but I don't intend to work in that region because gm is lower and distortion higher. Wise decision. But the 80V test was quite close. What is it about the geometry of the valve that determines the ratio of screen and anode currents and gms? That is an intersting question. Do you know the answer? No. I almost don't dare ask :-) The knee is at a remarkably low voltage, and high current, and there's no kink. A power valve like that would be an instant candidate for pentode operation with cathode feedback, in my amps perhaps, to replace the much gentler 6CH6. It would be interesting to try pp 6AU6 as a driver or headphone amp with a transformer: perhaps 30% cathode winding and cross-coupled screens to 30% taps to maintain constant Vks. But perhaps there is a necessary link between the knee position and the high screen current. Perhaps the screen is relatively close to the grid. That might also explain why small differences in the details of construction between different examples lead to relatively large variations in characteristics. I seriously don't dare ask why sharp cut-off pentodes are, er, sharp cut-off, and in what particular circumstances it is advantageous for them to be so. I'm tired of getting my ankles savaged. cheers, Ian1 |
#46
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Pentode gm wired as a triode
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#47
Posted to rec.audio.tubes
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Pentode gm wired as a triode
Ian Iveson wrote:
Ian said: AVO speak is exactly right. The manual I have says: "It was eventually found that a complete co-relation between the two sets of condition was held when the grid voltage took the form of a sinusoidal waveform with the positive half cycle suppressed (in other words rectified but completely unsmoothed AC) and the following relationships were maintained: Va (rms) = 1.1 Va indicated dc Vg2(rms) = 1.1 Vg2 indicated dc Vg1(mean unsmoothed) = 0.51 Vg1 indicated dc Ia(mean dc) = 0.5 indicated Ia" No explanation whatsoever as to why these particular values hold. Right, thanks, that's the page. It reads like an explanation, but isn't. But we might be missing something obvious. Just in case anyone is following this and might have an answer...the "rectification" of the screen and anode supplies is done by the valve under test...ie the valve is subjected to the whole AC cycle. The only rectifier in the tester is used for the grid and bridge circuit. Hence it avoids the need for a high power rectifier. That's how it is with the CT160 anyway. According to the circuit I have there is a rectifier in the screen connection too. Indeed, but I don't intend to work in that region because gm is lower and distortion higher. Wise decision. But the 80V test was quite close. What is it about the geometry of the valve that determines the ratio of screen and anode currents and gms? That is an intersting question. Do you know the answer? No. I almost don't dare ask :-) The knee is at a remarkably low voltage, and high current, and there's no kink. A power valve like that would be an instant candidate for pentode operation with cathode feedback, in my amps perhaps, to replace the much gentler 6CH6. It would be interesting to try pp 6AU6 as a driver or headphone amp with a transformer: perhaps 30% cathode winding and cross-coupled screens to 30% taps to maintain constant Vks. I think this has already been done. I have done lots of Googling for 6AU6 and I am pretty sure I came across one guitar amp and one headphones amp based on the 6AU6. But perhaps there is a necessary link between the knee position and the high screen current. Perhaps the screen is relatively close to the grid. That might also explain why small differences in the details of construction between different examples lead to relatively large variations in characteristics. As far as my rudimentray knowledge goes, gm of a particular tube depends primarily on two things. First the relative distances of the cathode, grid and anode and, secondly, the spacing of the turns on the grid. I have recently purchased over 50 6AU6 tubes from various sources and checked them all on my AVO tester under the same conditions. I know there can be considerable variation between tubes of the same type even from the same manufacturer but this lot of 50 seem to fall into three groups. The largest group ( about 35) show a gm of about 3.9mA/V. The second group (about 12) shows a gm of over 5mA/V. The smallest group (of 3) are probably worn out and give a gm reading of 2mA/V or less. Ignoring the duffers there are clearly two typical gm readings. I have looked at the construction and as yet cannot draw any conclusion as to group and construction other than the higher gm tubes are mainly about a quarter of an inch taller than the others. I seriously don't dare ask why sharp cut-off pentodes are, er, sharp cut-off, and in what particular circumstances it is advantageous for them to be so. I'm tired of getting my ankles savaged. They are sharp cut off because the grid turns are equally spaced so the grid volts at which the electron stream gets cut off is the same everywhere so the cut off occurs quite sharply. Variable mu or remote cut off tubes have a grid wound with variable spacing between turns. So the closer turns reach cut off first but only shut of part of the electron stream. Wider spaced turns cut off at higher voltages. The overall effect is a much more gradual cut off. Ian (TB) |
#48
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Pentode gm wired as a triode
Patrick Turner wrote:
wrote: Patrick Turner wrote: Ian Iveson wrote: Other Ian said I don't see that but maybe I am looking at the wrong part of the data sheet. The part I see under 'Characteristics and Typical Operation' has a column with screen and plate volts both 100V and a gm of 3.9mA/V. These are the voltages my AVO tester is set up with for meauding the 6AU6 in pentode mode. The plate current is 5mA and the screen current is 2.1mA, about 30% of cathode current - OK that's higher than my 20% figure but not hugely. This gives a triode equivalent gm of 3.9 x 7.1/5 = 5.5mA/V, still rather lower than the 7 I was measuring. I can see in the right hand column where the plate is at 250V and the screen at 150 that the gm rises to 5.2. The triode equivalent under these conditions works out at about 7.2mA/V which is much closer to what I measured but with rather different voltages. Firstly, 30% is much greater than 20% in my book. 50% greater, in fact. My data book for 6AU6 has the following data, Ig2 = 4.3mA, Eg2 = 150V, when Ia = 10.6mA, Ea 250V, gm = 5.2mA/V, And Ig2 = 2.1mA, Eg2 = 100mA when Ia = 5.0mA, Ea = 100V, gm = 3.9mA/V So over a broad range of voltages, Ig2 = 40% of Ia. However, if you go to the RCA sheets with 6AU6 TRIODE curves, at Ia = 10mA and Ea = 250V the µ = 38.6 and Ra = 9.1k, and gm = µ / Ra = 38.6 / 9,100 = 4.24 mA/V Who was it who told me that triode gm was HIGHER than pentode gm???????? Actually is was page 34 of RDH4. In theory this is right. RDH4 says the gm is shared with respect to the proportion of Ig2 and Ia. But if we consider the real world use of pentodes like the 6AU6, then we'd expect gm in triode to be 40% due to screen, and 60% due to anode, if that's the ratio of Ig2 to Ia, and when in triode, the Ea = Eg2. So you'd think it could be compared with a pentode set up where Ea = Eg2 in the idle condition. And one would expect that the pentode gm would be a lot less the the triode connection, but anode voltage moves and screen voltage does not, and just exactly what you will get depends on someone doing a test on a 6AU6 so that we don't just have to take a tiny little paragraph from RDH4 as gospel. That will be me. I just got emails to say the last of the parts I ordered have been despatched. So patrick, what is the simplest, reasonably accurate way for me to measure the 6AU6 gm in triode mode? At the top of page 36 RDH4 mentions the 6AU6, and in pentode you get Ra = 1.5M, gm = 4.45mA/V. µ = gm x Ra = 0.00445 x 1,500,000 = 6,675. Triode µ is given as 36, and THEREFORE screen µ = 6,675 / 36 = 185. OK so far, and that's all RDH4 says about 6AU6 in this part of the book. If therefore we say that gm of the screen = µ / Ra, then gm g2 = 185 / 1,500,000 = 0.123mA/V, but this sounds way too low. So what is the Ra of the 6AU6 when driven as a triode with fixed g1 voltage and drive to g2??? RDH4 doesn't fill in the gaps AT ALL. MUCH more could be said but isn't. So to know what you really get you have to set up a tube and measure its gain at low output voltages for negligibe distortions, say 10Vrms will do, and use an accurate volt meter for input and output voltage, and use two RLs with one twice the value of the other, and finally you can get two gain equations to work out Ra, µ and hence gm for any tube, triode or pentode or g2 drive with g1 at say 0V. OK, you just answered my earlier question. So if I use 47K as one anode resistor and a pair in series for the other, keep the cathode resistor (bypassed) the same in both cases and measure the output with my VTVM we should be good to go. I have a good quality stable oscillator so I can easily set the input voltage to avoid distortion at the output. To understand the tubes this way you have to be just as clever and disciplined as the authors of RDH4. A tall order. It isn't very difficult, but it just takes time, and you have to build a decent test rig. Yup, I am. Ian |
#49
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Pentode gm wired as a triode
wrote: Patrick Turner wrote: wrote: Patrick Turner wrote: Ian Iveson wrote: Other Ian said I don't see that but maybe I am looking at the wrong part of the data sheet. The part I see under 'Characteristics and Typical Operation' has a column with screen and plate volts both 100V and a gm of 3.9mA/V. These are the voltages my AVO tester is set up with for meauding the 6AU6 in pentode mode. The plate current is 5mA and the screen current is 2.1mA, about 30% of cathode current - OK that's higher than my 20% figure but not hugely. This gives a triode equivalent gm of 3.9 x 7.1/5 = 5.5mA/V, still rather lower than the 7 I was measuring. I can see in the right hand column where the plate is at 250V and the screen at 150 that the gm rises to 5.2. The triode equivalent under these conditions works out at about 7.2mA/V which is much closer to what I measured but with rather different voltages. Firstly, 30% is much greater than 20% in my book. 50% greater, in fact. My data book for 6AU6 has the following data, Ig2 = 4.3mA, Eg2 = 150V, when Ia = 10.6mA, Ea 250V, gm = 5.2mA/V, And Ig2 = 2.1mA, Eg2 = 100mA when Ia = 5.0mA, Ea = 100V, gm = 3.9mA/V So over a broad range of voltages, Ig2 = 40% of Ia. However, if you go to the RCA sheets with 6AU6 TRIODE curves, at Ia = 10mA and Ea = 250V the µ = 38.6 and Ra = 9.1k, and gm = µ / Ra = 38.6 / 9,100 = 4.24 mA/V Who was it who told me that triode gm was HIGHER than pentode gm???????? Actually is was page 34 of RDH4. In theory this is right. RDH4 says the gm is shared with respect to the proportion of Ig2 and Ia. But if we consider the real world use of pentodes like the 6AU6, then we'd expect gm in triode to be 40% due to screen, and 60% due to anode, if that's the ratio of Ig2 to Ia, and when in triode, the Ea = Eg2. So you'd think it could be compared with a pentode set up where Ea = Eg2 in the idle condition. And one would expect that the pentode gm would be a lot less the the triode connection, but anode voltage moves and screen voltage does not, and just exactly what you will get depends on someone doing a test on a 6AU6 so that we don't just have to take a tiny little paragraph from RDH4 as gospel. That will be me. I just got emails to say the last of the parts I ordered have been despatched. So patrick, what is the simplest, reasonably accurate way for me to measure the 6AU6 gm in triode mode? Just set one up in common cathode with 39k load from a B+ of 250V and adjust the bypassed Rk so that Ea = 100V so that Ia will be 3.8mA. Have a 470k grid bias R and RCA socket to accept a signal from your sig gene. Have 0.47uF from the anode to another RCA socket to take the anode signal out to a CRO, and to a point where you can measure the signal voltage. The total RL load can easily be halved from the 39k carrying dc to 19.5k by simply adding another 39k from the earthy side of the 0.47uF and 0V. Apply a 1kHz sine wave, adjust initially for 5Vrms, THD should be under 1% for all tests. Keep the input level constant at whatever is needed for 5Vrms output and measure the grid signal with only 39k present as the dc load. Gain, A = Vout / Vin. Without changing input signal, add the second 39k and measure the Vout, and find the gain for 19.5k. The rest you can work out because you will have two equations for A = µ x RL / ( RL + Ra ), one for each RL, and you know A and RL in both cases, and from agebraic substituition you can find µ and Ra easily. gm = µ / Ra. At the top of page 36 RDH4 mentions the 6AU6, and in pentode you get Ra = 1.5M, gm = 4.45mA/V. µ = gm x Ra = 0.00445 x 1,500,000 = 6,675. Triode µ is given as 36, and THEREFORE screen µ = 6,675 / 36 = 185. OK so far, and that's all RDH4 says about 6AU6 in this part of the book. If therefore we say that gm of the screen = µ / Ra, then gm g2 = 185 / 1,500,000 = 0.123mA/V, but this sounds way too low. So what is the Ra of the 6AU6 when driven as a triode with fixed g1 voltage and drive to g2??? RDH4 doesn't fill in the gaps AT ALL. MUCH more could be said but isn't. So to know what you really get you have to set up a tube and measure its gain at low output voltages for negligibe distortions, say 10Vrms will do, and use an accurate volt meter for input and output voltage, and use two RLs with one twice the value of the other, and finally you can get two gain equations to work out Ra, µ and hence gm for any tube, triode or pentode or g2 drive with g1 at say 0V. OK, you just answered my earlier question. So if I use 47K as one anode resistor and a pair in series for the other, keep the cathode resistor (bypassed) the same in both cases and measure the output with my VTVM we should be good to go. I have a good quality stable oscillator so I can easily set the input voltage to avoid distortion at the output. For keeping the Ea constant while changing RL, use the method of a constant dc RL and make the change to RL via the cap coupled load. Its simple and easy. To understand the tubes this way you have to be just as clever and disciplined as the authors of RDH4. A tall order. No, the crew of guys who wrote RDH4 were just ordinary guys with very ordinary technical training. The RDH4 seems lofty, but was written for the average technical person, not for the post graduate honours student at uni. It isn't very difficult, but it just takes time, and you have to build a decent test rig. Yup, I am. Good, Patrick Turner. Ian |
#50
Posted to rec.audio.tubes
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Pentode gm wired as a triode
Patrick Turner wrote:
Just set one up in common cathode with 39k load from a B+ of 250V and adjust the bypassed Rk so that Ea = 100V so that Ia will be 3.8mA. Have a 470k grid bias R and RCA socket to accept a signal from your sig gene. Have 0.47uF from the anode to another RCA socket to take the anode signal out to a CRO, and to a point where you can measure the signal voltage. The total RL load can easily be halved from the 39k carrying dc to 19.5k by simply adding another 39k from the earthy side of the 0.47uF and 0V. Apply a 1kHz sine wave, adjust initially for 5Vrms, THD should be under 1% for all tests. Keep the input level constant at whatever is needed for 5Vrms output and measure the grid signal with only 39k present as the dc load. Gain, A = Vout / Vin. Without changing input signal, add the second 39k and measure the Vout, and find the gain for 19.5k. The rest you can work out because you will have two equations for A = µ x RL / ( RL + Ra ), one for each RL, and you know A and RL in both cases, and from agebraic substituition you can find µ and Ra easily. gm = µ / Ra. Thanks Patrick. I think I'll add a switch to include/exclude the additional 39K resistor. My supply is currently 350V so I might use 47K which give Ia of 5.3mA with Ea of 100V OR I might add a dropper and some extra smoothing to bring Ebb down to 250V. That would be handy later when making noise measurements. So a 27K series resistor and 47uF smoothing should do it. Many thanks for your help so far. Cheers Ian (TB) |
#51
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Pentode gm wired as a triode
Ian wrote:
According to the circuit I have there is a rectifier in the screen connection too. Right, thanks, should have looked...it's been ages since I used it. Just half wave for the screen, presumably, or it would get a battering during the half cycle when the anode is -ve. The knee is at a remarkably low voltage, and high current, and there's no kink. A power valve like that would be an instant candidate for pentode operation with cathode feedback, in my amps perhaps, to replace the much gentler 6CH6. It would be interesting to try pp 6AU6 as a driver or headphone amp with a transformer: perhaps 30% cathode winding and cross-coupled screens to 30% taps to maintain constant Vks. I think this has already been done. I have done lots of Googling for 6AU6 and I am pretty sure I came across one guitar amp and one headphones amp based on the 6AU6. Pentode with cathode feedback? But perhaps there is a necessary link between the knee position and the high screen current. Perhaps the screen is relatively close to the grid. That might also explain why small differences in the details of construction between different examples lead to relatively large variations in characteristics. As far as my rudimentray knowledge goes, gm of a particular tube depends primarily on two things. First the relative distances of the cathode, grid and anode and, secondly, the spacing of the turns on the grid. I have recently purchased over 50 6AU6 tubes from various sources and checked them all on my AVO tester under the same conditions. I know there can be considerable variation between tubes of the same type even from the same manufacturer but this lot of 50 seem to fall into three groups. The largest group ( about 35) show a gm of about 3.9mA/V. The second group (about 12) shows a gm of over 5mA/V. The smallest group (of 3) are probably worn out and give a gm reading of 2mA/V or less. Ignoring the duffers there are clearly two typical gm readings. I have looked at the construction and as yet cannot draw any conclusion as to group and construction other than the higher gm tubes are mainly about a quarter of an inch taller than the others. Right. And cathode area, material, and temperature, presumably, considering that gm changes with age. But I was particularly wondering about the current or gm split between screen and anode. Do the taller valves have longer innards? I wonder if, in middle age, these valves become closer? Is it better to design for particular examples of valves, at particular points in their lives, or for the species? If the latter, then it might make sense to use the published data rather than actual tested values. I seriously don't dare ask why sharp cut-off pentodes are, er, sharp cut-off, and in what particular circumstances it is advantageous for them to be so. I'm tired of getting my ankles savaged. They are sharp cut off because the grid turns are equally spaced so the grid volts at which the electron stream gets cut off is the same everywhere so the cut off occurs quite sharply. Variable mu or remote cut off tubes have a grid wound with variable spacing between turns. So the closer turns reach cut off first but only shut of part of the electron stream. Wider spaced turns cut off at higher voltages. The overall effect is a much more gradual cut off. OK, thanks. How is sharpness desirable in some circuits, but not others, I wonder? cheers, Ian1 |
#52
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Pentode gm wired as a triode
Ian Iveson wrote:
Ian wrote: According to the circuit I have there is a rectifier in the screen connection too. Right, thanks, should have looked...it's been ages since I used it. Just half wave for the screen, presumably, or it would get a battering during the half cycle when the anode is -ve. The knee is at a remarkably low voltage, and high current, and there's no kink. A power valve like that would be an instant candidate for pentode operation with cathode feedback, in my amps perhaps, to replace the much gentler 6CH6. It would be interesting to try pp 6AU6 as a driver or headphone amp with a transformer: perhaps 30% cathode winding and cross-coupled screens to 30% taps to maintain constant Vks. I think this has already been done. I have done lots of Googling for 6AU6 and I am pretty sure I came across one guitar amp and one headphones amp based on the 6AU6. Pentode with cathode feedback? Not sure about the guitar amp but I copied the headphone amp article. I used a pentode connected 6AU6 with unbypassed cathode as a SE output with global NFB to the input stage which was one half of a 12AU7 But perhaps there is a necessary link between the knee position and the high screen current. Perhaps the screen is relatively close to the grid. That might also explain why small differences in the details of construction between different examples lead to relatively large variations in characteristics. As far as my rudimentray knowledge goes, gm of a particular tube depends primarily on two things. First the relative distances of the cathode, grid and anode and, secondly, the spacing of the turns on the grid. I have recently purchased over 50 6AU6 tubes from various sources and checked them all on my AVO tester under the same conditions. I know there can be considerable variation between tubes of the same type even from the same manufacturer but this lot of 50 seem to fall into three groups. The largest group ( about 35) show a gm of about 3.9mA/V. The second group (about 12) shows a gm of over 5mA/V. The smallest group (of 3) are probably worn out and give a gm reading of 2mA/V or less. Ignoring the duffers there are clearly two typical gm readings. I have looked at the construction and as yet cannot draw any conclusion as to group and construction other than the higher gm tubes are mainly about a quarter of an inch taller than the others. Right. And cathode area, material, and temperature, presumably, considering that gm changes with age. But I was particularly wondering about the current or gm split between screen and anode. Do the taller valves have longer innards? No they are pretty much the same, they just have an extra bit of metal like a brolly over the top of the whole assembly. I wonder if, in middle age, these valves become closer? Is it better to design for particular examples of valves, at particular points in their lives, or for the species? If the latter, then it might make sense to use the published data rather than actual tested values. Depends on the performance you want to achieve. For some applications it seems it is necessary or a least a good idea to select tubes e.g first stage of preamps, but elsewhere almost any example will suffice. I seriously don't dare ask why sharp cut-off pentodes are, er, sharp cut-off, and in what particular circumstances it is advantageous for them to be so. I'm tired of getting my ankles savaged. They are sharp cut off because the grid turns are equally spaced so the grid volts at which the electron stream gets cut off is the same everywhere so the cut off occurs quite sharply. Variable mu or remote cut off tubes have a grid wound with variable spacing between turns. So the closer turns reach cut off first but only shut of part of the electron stream. Wider spaced turns cut off at higher voltages. The overall effect is a much more gradual cut off. OK, thanks. How is sharpness desirable in some circuits, but not others, I wonder? I suspect winding regular spaced grids is easiest and that was how it was done at first. Later, when someone wanted an AGC circuit, a variable spacing technique was devised. Ian (TB) |
#53
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Pentode gm wired as a triode
On Sat, 30 Jun 2007 14:40:11 GMT, "Ian Iveson"
wrote: OK, thanks. How is sharpness desirable in some circuits, but not others, I wonder? Sharpness implies lower second harmonic distortion. Maybe suprisingly, this isn't something always desirable in tuned RF amplifiers. The simultaneous gain reduction and input signal handling increase with a simple change of bias of "remote cutoff" valves is a handy combination for linear amplifiers operating in a low dynamic range environment with a large dynamic range of possible inputs. Third harmonic distortion *is* important for RF amplifiers though, because it causes crossmodulation. Variable-mu valves can be made to have low third and high second harmonic distortion. Thanks, as always, Chris Hornbeck "Money doesn't buy happiness. But happiness isn't everything." - Jean Seberg |
#54
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Pentode gm wired as a triode
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#55
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Pentode gm wired as a triode
Chris explained:
OK, thanks. How is sharpness desirable in some circuits, but not others, I wonder? Sharpness implies lower second harmonic distortion. Maybe suprisingly, this isn't something always desirable in tuned RF amplifiers. The simultaneous gain reduction and input signal handling increase with a simple change of bias of "remote cutoff" valves is a handy combination for linear amplifiers operating in a low dynamic range environment with a large dynamic range of possible inputs. Third harmonic distortion *is* important for RF amplifiers though, because it causes crossmodulation. Variable-mu valves can be made to have low third and high second harmonic distortion. Aha. Interesting. Thanks for the clarity. cheers, Ian1 |
#56
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Pentode gm wired as a triode
On Sat, 30 Jun 2007 03:24:55 -0500, flipper wrote:
As I said, the canonical system has only one input. Look at a tube datasheet and, in particular, Plate V and grid V. That's 2. Pay close attention, folks. Here is where the argument takes its big twist. The canonical model has an input that arrives at a summing point, from which a forward function leads to the output. From the output, a feedback function leads to the same single summing point. Notice there can be one input only. Not two, or three, or any other multiplicity. Just one. Guess we'll have to throw away all those opamps with a + and - input. Mathematically, opamps have a single, differential, input with two terminals. The interesting (apparently only to me) bit is that your model has an output which is also an input (anode). This element differs from the classic model, and so IMO should be the next point of discussion. Thanks, as always, Chris Hornbeck "Money doesn't buy happiness. But happiness isn't everything." - Jean Seberg |
#57
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Pentode gm wired as a triode
Chris Hornbeck wrote: On Sat, 30 Jun 2007 03:24:55 -0500, flipper wrote: As I said, the canonical system has only one input. Look at a tube datasheet and, in particular, Plate V and grid V. That's 2. Pay close attention, folks. Here is where the argument takes its big twist. The canonical model has an input that arrives at a summing point, from which a forward function leads to the output. From the output, a feedback function leads to the same single summing point. Notice there can be one input only. Not two, or three, or any other multiplicity. Just one. Guess we'll have to throw away all those opamps with a + and - input. Mathematically, opamps have a single, differential, input with two terminals. The interesting (apparently only to me) bit is that your model has an output which is also an input (anode). This element differs from the classic model, and so IMO should be the next point of discussion. Suppose you have a shunt NFB network between an anode and a grid with a pair of resistors, and with cathode grounded. Two of the conventionally considered terminals, grid and cathode, are input terminals. The anode is the conventional output terminal but in a shunt NFB array it sends a signal back to the grid and thus contributes input to the grid. Thus indirectly, it is an input terminal. In a cathode follower, output is from a cathode, usually a low impedance input terminal. ALL the output signal is fed back so cathode inputs signal relative to the grid signal. Similarly a follower opamp does a similar job to the CF. Classical Model? this is 2007, not Greece in 500BC. But there always has to be two input terminals, at least a ground and active. A "classical" application of a differential amp is in nearly every tube amp with global NFB where the cathode has say a 1V sample of the output signal applied as NFB at low Z and the and the grid has 1.2V applied at high Z as input signal. Only the difference between the two, 0.2V, is amplified. We need to differentiate between being too sloppy with terminology, and to rigidly classical. Diferrentiation is part of a balanced mind and ideas. Patrick Turner. Thanks, as always, Chris Hornbeck "Money doesn't buy happiness. But happiness isn't everything." - Jean Seberg |
#58
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Pentode gm wired as a triode
...Two voltage inputs, grid and plate, with a current output... As I said, the canonical system has only one input. Look at a tube datasheet and, in particular, Plate V and grid V. That's 2. And the amount of resulting plate current is the output. That's legit. Now use a resistor on the plate to B+. The current via Ohm's law will cause a voltage drop to happen on the plate, thus reducing the plate voltage. Which in turn will affect the output current. Feedback! (ducking, running for cover) :-) |
#59
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Pentode gm wired as a triode
On Tue, 03 Jul 2007 07:56:41 GMT, Patrick Turner
wrote: Suppose you have a shunt NFB network between an anode and a grid with a pair of resistors, and with cathode grounded. Two of the conventionally considered terminals, grid and cathode, are input terminals. The anode is the conventional output terminal but in a shunt NFB array it sends a signal back to the grid and thus contributes input to the grid. Thus indirectly, it is an input terminal. In a cathode follower, output is from a cathode, usually a low impedance input terminal. ALL the output signal is fed back so cathode inputs signal relative to the grid signal. But what does any of this have to do with the operation of a vacuum valve? Similarly a follower opamp does a similar job to the CF. Therefore opamps have internal feedback? Sorry, but I still find available arguments silly. Thanks, as always, Chris Hornbeck "Money doesn't buy happiness. But happiness isn't everything." - Jean Seberg |
#60
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Pentode gm wired as a triode
On Tue, 03 Jul 2007 17:48:00 -0500, flipper wrote:
The answer is yes and no, but not quite. Functionally, the plate 'output' is current while the plate 'input' (forgive the 'canonical confusion', because we aren't to that point yet) is voltage. So the 'no' part is they're different signals. That the two are on 'the same pin' (the 'yes' part) is a physical aspect of the device but 'more than one' signal, and more than one direction, on a single wire, or pin, is not something unique to the triode as engineers often intentionally design such things. I.E. multiplexing, which also includes time division (but not applicable to this case).. Point is, it is, as you rightly contended in the opamp case, the 'mathematical' that determines, not the physical. (nor that ST uses the word 'input' on two pins). That both plate current output and plate voltage input are on the 'same pin', by nature of the device, leads to the argument its 'internal', a claim I take partial dispute with because there's a 'missing piece' to finish the 'loop': something to convert the current output to a voltage representation which is, then, applied to the plate: I.E. the plate load. And that is external. I'd wager that most engineers would have no trouble at all with an integrated circuit constructed in a similar manner, I.E.pin multiplexing with an 'internal' connection forming the feedback loop and I imagine that's because they'd be happy envisioning 'lumped circuits' inside the thing and, perhaps, as someone amusingly postulated, 'confused' by 'magical field effects' but microwave designers have no such mystification about designing, or comprehending, distributed parameters and they are no less 'real' than lumped models of the real field effects. And then, lastly, we have the trivial case that no one seems to have any problem whatsoever recognizing "cathode feedback" even though the 'output' and (feedback) 'input' (for that local loop) are on 'the same pin', nor that the 'voltage' is created by an external load converting output current to voltage, that the cathode is the summing junction, and that the whole thing works because of 'mystical field effects' just like all electronic devices do. But what does any of this have to do with the operation of a vacuum valve? And contrary to the argument it 'adds nothing' I find it eminently useful to, among other things, illuminating why a pentode 'mysteriously' behaves like a triode when so connected, as opposed to what I consider un illuminating arm waving that it behaves like a triode because it now is one and, well, that's just 'how triodes behave' vs. 'how pentodes behave." To me, that is like asking why gray clouds often produce rain while white ones generally don't where the answer given is because that's what gray and white clouds do. I hope none of that sounds 'ungrateful'. I'm just trying to explain how I see it and why. I'm sorry, but I still find the available arguments silly. Thanks, as always, Chris Hornbeck "Money doesn't buy happiness. But happiness isn't everything." - Jean Seberg |
#61
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Pentode gm wired as a triode
On Tue, 03 Jul 2007 23:07:08 -0500, flipper wrote:
But what does any of this have to do with the operation of a vacuum valve? Frankly, I don't know how to answer what the operation of a vacuum valve has to do with the operation of a vacuum valve as this seems rather self evident. Maybe if you could explain how you come to the conclusion that cathode feedback in a triode has nothing to do with how a triode operates I could get a better idea of what you're trying to ask. Cathode feedback, or any other external characteristic, has *nothing* to do with the operation of the triode. Nothing. Do the triode's characteristics differ in *any* way when operated as a cathode follower, or indeed in *any* external configuration? Are it's plate curves changed? Is there any way that you could observe, while viewing only its three (yeah, yeah, but you know what I mean) terminals that the triode was being used as a cathode follower, or in any other external configuration? If your model of internal feedback is to be convincing, it will need to free itself of external (and, frankly, shakey) props. I'm sorry, but I still find the available arguments silly. I'm sorry but just declaring something 'silly', and nothing else, isn't illuminating.. "Feedback" is a word with meaning established by folks smarter than us and back before we were born. Anybody wishing to make a special case for their pet theory will have to get used to a certain level of scepticism. The word has an established meaning in engineering and attempts to redefine it will get short shrift. Can you make a convincing case for your model that doesn't include any special exceptions? Anything less just wouldn't be the Cowboy Way. Thanks, as always, Chris Hornbeck "Money doesn't buy happiness. But happiness isn't everything." - Jean Seberg |
#62
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Pentode gm wired as a triode
Chris Hornbeck wrote: On Tue, 03 Jul 2007 07:56:41 GMT, Patrick Turner wrote: Suppose you have a shunt NFB network between an anode and a grid with a pair of resistors, and with cathode grounded. Two of the conventionally considered terminals, grid and cathode, are input terminals. The anode is the conventional output terminal but in a shunt NFB array it sends a signal back to the grid and thus contributes input to the grid. Thus indirectly, it is an input terminal. In a cathode follower, output is from a cathode, usually a low impedance input terminal. ALL the output signal is fed back so cathode inputs signal relative to the grid signal. But what does any of this have to do with the operation of a vacuum valve? Lots, if you happened to notice the repeated descriptions of vacuum valves ( tubes ) and ways of hooking them up.... Similarly a follower opamp does a similar job to the CF. Therefore opamps have internal feedback? Some indeed to have internal FB, but its not like that of a triode. Sorry, but I still find available arguments silly. Ah, so the unavailable arguments are the wise ones... Patrick Turner. Thanks, as always, Chris Hornbeck "Money doesn't buy happiness. But happiness isn't everything." - Jean Seberg |
#63
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Pentode gm wired as a triode
Chris Hornbeck wrote: On Tue, 03 Jul 2007 23:07:08 -0500, flipper wrote: But what does any of this have to do with the operation of a vacuum valve? Frankly, I don't know how to answer what the operation of a vacuum valve has to do with the operation of a vacuum valve as this seems rather self evident. Maybe if you could explain how you come to the conclusion that cathode feedback in a triode has nothing to do with how a triode operates I could get a better idea of what you're trying to ask. Cathode feedback, or any other external characteristic, has *nothing* to do with the operation of the triode. Nothing. Its true that a tube or valve has no consciousness of the external circuit around itself. We do, and a distinction does need to be made with regard to internal NFB and external. It is by external observation that those of us with sufficient consciousness will conclude that triodes have internal NFB and are not just a resistor. Do the triode's characteristics differ in *any* way when operated as a cathode follower, or indeed in *any* external configuration? Are it's plate curves changed? Is there any way that you could observe, while viewing only its three (yeah, yeah, but you know what I mean) terminals that the triode was being used as a cathode follower, or in any other external configuration? If your model of internal feedback is to be convincing, it will need to free itself of external (and, frankly, shakey) props. Plenty of models have bee drawn in RDH4 of the changed Ra curves for a CF or a tube with an unbypassed Rk, but this is to illustrate the result of exernal NFB connection. No matter how you connect a triode, the internal FB remains, and the equivalent basic circuit of low Z generator and series R representing Ra all remain the same. I'm sorry, but I still find the available arguments silly. I'm sorry but just declaring something 'silly', and nothing else, isn't illuminating.. "Feedback" is a word with meaning established by folks smarter than us and back before we were born. NFB wasn't known well at first by our ancestors. Although very intelligent, none were much different to us and we are as silly or wise as they ever were. But NFB became something usable, I recall a Mr Black began publishing aspects of NFB useage in 1928. The first tubes were all triodes, and NFB wasn't a term thought much about until thery tried to amplify higher and higher frequencies, and then the C between anode and grid meant the input impedance to a triode became less as F rose, limiting frequencies much above a MHz. AFAIK, the boffins of 1920 had a fair idea of the summed joint actions of electrostatic fields due to grid and anode voltage changes. I suggest you bury your head in the shelves of books available for study from the time vacuum tubes were first developed. Tetrodes were invented to INTERRUPT the NFB electrostatic field effect of the anode upon its own electron flow and to reduce the Cga capacitance, ie, reduce the Miller effect. Anybody wishing to make a special case for their pet theory will have to get used to a certain level of scepticism. The word has an established meaning in engineering and attempts to redefine it will get short shrift. Can you make a convincing case for your model that doesn't include any special exceptions? Anything less just wouldn't be the Cowboy Way. I for one don't need to make exceptions for the triode. I take it for granted it has an internal NFB loop whether we like it or not. Internal triode NFB gives it very different properties to a pentode, and the difference can be dramatically seen when running a 6AU6 in either triode or pentode mode. Even when running a triode such as 6AU6 as a triode in a CF, the internal NFB operates to maintain that Vk / Vgk = open loop gain = µ x RL ( RL + Ra ) where Ra and µ as as previously discussed in posts about measuring them to find gm, and where RL = Rk and in fact = total dc and ac loads. Were we to use the 6AU6 as a pentode CF with the screen also changing its signal just as the cathode, then the open loop gain will be much higher than the triode case. But regardless of triode or pentode, the output resistance of either pentode or trioded 6AU6 = 1/gm approximately. A model of any type of circuit including a triode can be made to include the triode NFB but most people don't ever bother; they just consider the simple generator model with series Ra and this model IS the result of the tube including its internal NFB. A circuit with a pentode can be drawn similarly, only that Ra is a lot higher, and µ a lot higher. The accedemic importance of the internal NFB of triodes, or lack of it in pentodes fades in the presence of the importance given to what we might do with such devices, ie, in ways to build amplifiers. I find myself so busy each day with a soldering iron that I scarcely give internal NFB in triodes the slightest thought. I am outcomes oriented, and do need to please people, so I don't have some undefined vague quest to investigate the mysterious behaviour of tubes any more than I have already. Patrick Turner. Thanks, as always, Chris Hornbeck "Money doesn't buy happiness. But happiness isn't everything." - Jean Seberg |
#64
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Pentode gm wired as a triode
On Wed, 04 Jul 2007 09:15:37 GMT, Patrick Turner
wrote: Cathode feedback, or any other external characteristic, has *nothing* to do with the operation of the triode. Nothing. Its true that a tube or valve has no consciousness of the external circuit around itself. We do, and a distinction does need to be made with regard to internal NFB and external. Excellent. If you could somehow explain this to "flipper", maybe we could move on. I've tried twice to start a post, but have found myself overwhelmed. Seems an obvious point, but maybe not... It is by external observation that those of us with sufficient consciousness will conclude that triodes have internal NFB and are not just a resistor. This, of course, remains to be proven. We're arguing about applicability of models, so sweeping generalizations are somewhere short of convincing. Yet. Thanks, as always, Chris Hornbeck "The air is always curved if you choose to see it." -Steve McMullen reviewing the first Curved Air album |
#65
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Pentode gm wired as a triode
Chris Hornbeck wrote: On Wed, 04 Jul 2007 09:15:37 GMT, Patrick Turner wrote: Cathode feedback, or any other external characteristic, has *nothing* to do with the operation of the triode. Nothing. Its true that a tube or valve has no consciousness of the external circuit around itself. We do, and a distinction does need to be made with regard to internal NFB and external. Excellent. If you could somehow explain this to "flipper", maybe we could move on. I've tried twice to start a post, but have found myself overwhelmed. Seems an obvious point, but maybe not... Try to be friendly. Don't worry, be happy. Having established this state in yourself, consider that ones intellectual adversaries need prolonged persuasions, not prolonged revenges. Dis-establish your anger which if acted upon as a first response will yield to you a vale of tears for all concerned. Treat even your adversaries with respect, and not as they do unto you, and then hopefully they'll explore a journey of learning with you, because there is something in it for them. But if they tell you to eat ****, and get stuffed, and keep repeating their recalcitrance, let em have it both barrels to illustrate the idiocy of they determined negative efforts. They'll take even longer to heal, but it would not be because you didn't give them a huge long chance to mend their ways. Show your eventual forgiveness and show that you'll talk again later if they manage something interesting to say to the group. Most ppl come around, or at least leave you alone...... Meanwhile valves, or tubes have only spirits, and no brains. The God Of Triodes meant it to be this way. On at least 50,000 other planets in the very tiny part of the universe which we are aware of, beings who have evolved to farnarkle around with electronics have been allowed devices which presently could be assumed to be very different to what the GOT allowed us to findoutabout. The 3 prong devices we have thus far managed to make act in ways which can give very different circuit results yet they remain themselves, and should we shrink ourselves down in size to be able to walk inside a triode or a tiny transistor, and set up camp, we would see electrons coming and going along pathways of vacuum or substrate materials, and marvel at it all, and notice the changes in voltages promoting the currents, and not be aware of where the heck the little device is located, perhaps in someone's preamp, or ad converter, or microwave oven. Of course if one was equipped with a suitable small amp and speaker while standing on the riverbanks where electrons flow past, one might dip a wire in, and hear Motzart coming from the stream, and yes, we are probably in someone's preamp. If it sounded dirty, maybe its a driver transistor on a solid state amp, and the error signal is present to cancel the the output stage distortions. If you just hear a stream of clicks, its a lousy campsite; you are inside digital gear :-( The nicest site to camp would be in a triode with no external NFB loop :-) There we'd sample the signal and find it clean enough, almost always we'd find it to be music, and it'd be much more spacious, and the weather would be nice and warm. Having shrunk ourselves down in size to allow such observations poses the greater problem of what to do with the excessive mass and bulk we have dispensed with, E = mc squared comes to mind, but I don't like violence. Patrick Turner. It is by external observation that those of us with sufficient consciousness will conclude that triodes have internal NFB and are not just a resistor. This, of course, remains to be proven. We're arguing about applicability of models, so sweeping generalizations are somewhere short of convincing. Yet. Thanks, as always, Chris Hornbeck "The air is always curved if you choose to see it." -Steve McMullen reviewing the first Curved Air album |
#66
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Pentode gm wired as a triode
On Thu, 05 Jul 2007 08:38:00 GMT, Patrick Turner
wrote: Try to be friendly. Don't worry, be happy. Lots of other interesting and surprisingly well edited stuff snipped for bandwidth but largely recommended for content Dunno why anyone would consider my comments as in any way unfriendly or antagonistic. We're discussing models, not religion. It's perfectly OK to change one's mind, just as it's perfectly OK to be wrong. Personally, I'm wrong *lot's*; any real world gig includes it as a daily regimen. Only the President of the United States is never wrong, and look how well that works. My comments are just my comments. To sum up my current opinion about the best (that I've heard to date) thoughts arguing for the inherent-feedback-in- vacuum-valve-triodes(-but-not-in-any-other-active-device...): 1) Electrostatic field summing effects upon a virtual grid: Why is there no measurable external effect? This is a ghost- in-the-machine argument of the type that went out of use in real science shortly after Newton. And even before that, Occar's Razor should have been a warning sign. 2) Any argument relying on some analogy from some external circuit configuration: How can *any* external circuit configuration have *any* bearing on the internal operation of a device when it's impossible to measure *anything* about the external circuit configuration when observing only the terminals of the device? IOW, only the device itself can be observed, or measured. If this is difficult to imagine as true, one need only make some actual measurements. Thanks, as always, Chris Hornbeck "The air is always curved if you choose to see it." -Steve McMullen reviewing the first Curved Air album |
#67
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Pentode gm wired as a triode
On Thu, 05 Jul 2007 07:59:21 -0500, flipper wrote:
"flipper", "Flipper" doesn't have any problem with it. If "flipper" is your real name, I'm sorry to have enclosed it in parenthesis. I post with my real name, but others feel otherwise. That's their call. Quite unusual name; what's the origin? maybe we could move on. I've tried twice to start a post, but have found myself overwhelmed. Seems an obvious point, but maybe not... Maybe if you'd be a bit more precise and clear as to what you are agreeing with and what you're disputing because, as it is, you appear to jump between 'feedback' and 'internal' and cathode and plate as if they're all synonyms. The (hijacked) thread topic is the old one of inherent-feedback- in-triode-vacuum-valves. You've posted elaborate models pro-INFBiTVV that rely on analogy to external circuits. Arguments by analogy aren't automagically suspect; rather, the reliance on an *external* analogy is fatally flawed. A powerful way to think about the flaw is to imagine observing a triode (or any other active device) with fast voltmeters on all active terminals. Can we, using these fast voltmeters (or oscilloscopes, or, in fact, *ANY* measuring device, observe the triode acting any differently in *ANY* external circuit? Because we cannot, any external circuit is "trivial". It cannot contribute anything interesting to the internal model of the triode, because it doesn't contribute anything *at all* to the operation of the triode. So, the actual internal operation of the triode remains to be completely described. Not that surprising; the universe is bazillions of years old and models get better but are still somewhat iffy. Dark matter, anyone? Thanks, as always, Chris Hornbeck "The air is always curved if you choose to see it." -Steve McMullen reviewing the first Curved Air album |
#68
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Pentode gm wired as a triode
Chris Hornbeck wrote: On Thu, 05 Jul 2007 08:38:00 GMT, Patrick Turner wrote: Try to be friendly. Don't worry, be happy. Lots of other interesting and surprisingly well edited stuff snipped for bandwidth but largely recommended for content Dunno why anyone would consider my comments as in any way unfriendly or antagonistic. We're discussing models, not religion. It's perfectly OK to change one's mind, just as it's perfectly OK to be wrong. Personally, I'm wrong *lot's*; any real world gig includes it as a daily regimen. Only the President of the United States is never wrong, and look how well that works. My comments are just my comments. To sum up my current opinion about the best (that I've heard to date) thoughts arguing for the inherent-feedback-in- vacuum-valve-triodes(-but-not-in-any-other-active-device...): 1) Electrostatic field summing effects upon a virtual grid: Why is there no measurable external effect? This is a ghost- in-the-machine argument of the type that went out of use in real science shortly after Newton. And even before that, Occar's Razor should have been a warning sign. 2) Any argument relying on some analogy from some external circuit configuration: How can *any* external circuit configuration have *any* bearing on the internal operation of a device when it's impossible to measure *anything* about the external circuit configuration when observing only the terminals of the device? IOW, only the device itself can be observed, or measured. If this is difficult to imagine as true, one need only make some actual measurements. Thanks, as always, Chris Hornbeck "The air is always curved if you choose to see it." -Steve McMullen reviewing the first Curved Air album There are things in physics where if we measure, we spoil what is being measured, and nothing is certain. Quantum physics is like this, and the more they try to unravel the atom to find out what particles are responsible for say, mass and what exact particles are responsible for say time, then the more difficult finding the truth seems to be. But a if you were small enough, and had the right gear, you could measure summed fields in a triode and deduce there is a NFB network present. Patrick Turner. |
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