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#1
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6SN7 et al mu follower distortion
I have just about completed my distortion tests of 6SN7, 7N7 and 6CG7 mu
followers using tubes kindly loaned or donated by group members (Thanks Peter and Matthew). Here is a summary of the results. Basic set up is similar to Morgan Jones with 8mA standing current, the bottom triode biassed to about Vg=-3V and the top pentode CCS replaced by a normal mu follower CF set to Vg=-1.3V, with 10K between the tubes and a 320V supply and the output via a 0.1uF into 100K load. I tested distortion at a variety of levels at 200Hz, 2KHz and 20KHz. As the distortion was identical at all three frequencies the majority of tests were done only at 2KHz. I tested 4x6CG7 (Matsu****a), 2x7N7 (Sylvania) and 8x6SN7 (RCA) at output voltages between 1 and 50V rms. All the tubes produced remarkably similar results. The variation between types was not greater than the variation within a type although the best tube of all was a 6SN7. Typical THD readings we 2V rms 0.04% 10V rms 0.2% 20V rms 0.4% 50V rms 1.0% At 50V rms we are pretty close to grid current but since the oscillator used has a low output impedance no grid current distortion was observed. A subsequent test feeding the oscillator via a 120K series resistor showed no sign of grid current up to 25V rms output (as far as the oscillator output would go). I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. Cheers Ian |
#2
Posted to rec.audio.tubes
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6SN7 et al mu follower distortion
On Dec 30, 9:37*pm, Ian Bell wrote:
I have just about completed my distortion tests of 6SN7, 7N7 and 6CG7 mu followers using tubes kindly loaned or donated by group members (Thanks Peter and Matthew). Here is a summary of the results. Basic set up is similar to Morgan Jones with 8mA standing current, the bottom triode biassed to about Vg=-3V and the top pentode CCS replaced by a normal mu follower *CF set to Vg=-1.3V, with 10K between the tubes and a 320V supply and the output via a 0.1uF into 100K load. I tested distortion at a variety of levels at 200Hz, 2KHz and 20KHz. As the distortion was identical at all three frequencies the majority of tests were done only at 2KHz. I tested 4x6CG7 (Matsu****a), 2x7N7 (Sylvania) and 8x6SN7 (RCA) *at output voltages between 1 and 50V rms. All the tubes produced remarkably similar results. The variation between types was not greater than the variation within a type although the best tube of all was a 6SN7. *Typical THD readings we 2V *rms 0.04% 10V rms 0.2% 20V rms 0.4% 50V rms 1.0% At 50V rms we are pretty close to grid current but since the oscillator used has a low output impedance no grid current distortion was observed. A subsequent test feeding the oscillator via a 120K series resistor showed no sign of grid current up to 25V rms output (as far as the oscillator output would go). I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. Cheers Ian Nice work, Ian. There's a sense in which it is a mistake to work with the 6SN7 and its workalikes because it is such a silent tube that the differentials will also be small and undramatic, but on the other hand it gives you a real feeling for what a good tube responds like, and how subtle changes can have a big impact on results, which is probably the most important thing to learn in playing along the loadline and transfer curves. You should also check out the tests with the 6S?7 done by Lynn Olson and his pal Matt Kamna (a measurement expert), and by the ever-excellent Steve Bench. Andre Jute Visit Jute on Amps at http://members.lycos.co.uk/fiultra/ "wonderfully well written and reasoned information for the tube audio constructor" John Broskie TubeCAD & GlassWare "an unbelievably comprehensive web site containing vital gems of wisdom" Stuart Perry Hi-Fi News & Record Review |
#3
Posted to rec.audio.tubes
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6SN7 et al mu follower distortion
Andre Jute wrote:
On Dec 30, 9:37 pm, Ian Bell wrote: I have just about completed my distortion tests of 6SN7, 7N7 and 6CG7 mu followers using tubes kindly loaned or donated by group members (Thanks Peter and Matthew). Here is a summary of the results. Basic set up is similar to Morgan Jones with 8mA standing current, the bottom triode biassed to about Vg=-3V and the top pentode CCS replaced by a normal mu follower CF set to Vg=-1.3V, with 10K between the tubes and a 320V supply and the output via a 0.1uF into 100K load. I tested distortion at a variety of levels at 200Hz, 2KHz and 20KHz. As the distortion was identical at all three frequencies the majority of tests were done only at 2KHz. I tested 4x6CG7 (Matsu****a), 2x7N7 (Sylvania) and 8x6SN7 (RCA) at output voltages between 1 and 50V rms. All the tubes produced remarkably similar results. The variation between types was not greater than the variation within a type although the best tube of all was a 6SN7. Typical THD readings we 2V rms 0.04% 10V rms 0.2% 20V rms 0.4% 50V rms 1.0% At 50V rms we are pretty close to grid current but since the oscillator used has a low output impedance no grid current distortion was observed. A subsequent test feeding the oscillator via a 120K series resistor showed no sign of grid current up to 25V rms output (as far as the oscillator output would go). I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. Cheers Ian Nice work, Ian. There's a sense in which it is a mistake to work with the 6SN7 and its workalikes because it is such a silent tube that the differentials will also be small and undramatic, but on the other hand it gives you a real feeling for what a good tube responds like, and how subtle changes can have a big impact on results, which is probably the most important thing to learn in playing along the loadline and transfer curves. Yes, the 6SN7 and derivatives do seem to deserve their reputation. I get the feeling later tubes tended to go for high mu, smaller (cheaper) construction, sod the distortion and use loads of global NFB. You should also check out the tests with the 6S?7 done by Lynn Olson and his pal Matt Kamna (a measurement expert), I don't think I have seen those. I'll try find them. and by the ever-excellent Steve Bench. Yes, I am well aware of Steve's excellent work. Cheers Ian |
#4
Posted to rec.audio.tubes
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6SN7 et al mu follower distortion
On Dec 30, 11:16*pm, Ian Bell wrote:
Andre Jute wrote: On Dec 30, 9:37 pm, Ian Bell wrote: I have just about completed my distortion tests of 6SN7, 7N7 and 6CG7 mu followers using tubes kindly loaned or donated by group members (Thanks Peter and Matthew). Here is a summary of the results. Basic set up is similar to Morgan Jones with 8mA standing current, the bottom triode biassed to about Vg=-3V and the top pentode CCS replaced by a normal mu follower *CF set to Vg=-1.3V, with 10K between the tubes and a 320V supply and the output via a 0.1uF into 100K load. I tested distortion at a variety of levels at 200Hz, 2KHz and 20KHz. As the distortion was identical at all three frequencies the majority of tests were done only at 2KHz. I tested 4x6CG7 (Matsu****a), 2x7N7 (Sylvania) and 8x6SN7 (RCA) *at output voltages between 1 and 50V rms. All the tubes produced remarkably similar results. The variation between types was not greater than the variation within a type although the best tube of all was a 6SN7. *Typical THD readings we 2V *rms 0.04% 10V rms 0.2% 20V rms 0.4% 50V rms 1.0% At 50V rms we are pretty close to grid current but since the oscillator used has a low output impedance no grid current distortion was observed. A subsequent test feeding the oscillator via a 120K series resistor showed no sign of grid current up to 25V rms output (as far as the oscillator output would go). I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. Cheers Ian Nice work, Ian. There's a sense in which it is a mistake to work with the 6SN7 and its workalikes because it is such a silent tube that the differentials will also be small and undramatic, but on the other hand it gives you a real feeling for what a good tube responds like, and how subtle changes can have a big impact on results, which is probably the most important thing to learn in playing along the loadline and transfer curves. Yes, the 6SN7 and derivatives do seem to deserve their reputation. I get the feeling later tubes tended to go for high mu, smaller (cheaper) construction, sod the distortion and use loads of global NFB. You should also check out the tests with the 6S?7 done by Lynn Olson and his pal Matt Kamna (a measurement expert), I don't think I have seen those. I'll try find them. http://www.nutshellhifi.com/library/FindingCG.html will take you directly to the tests. Worth reading around the Nutshell site. I used to know LynnO on the Joenet in decades gone by, when it seemed everyone interested in tube hi-fi knew everyone else. -- AJ * and by the ever-excellent Steve Bench. Yes, I am well aware of Steve's excellent work. Cheers Ian |
#5
Posted to rec.audio.tubes
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6SN7 et al mu follower distortion
Andre Jute wrote:
On Dec 30, 11:16 pm, Ian Bell wrote: Andre Jute wrote: On Dec 30, 9:37 pm, Ian Bell wrote: I have just about completed my distortion tests of 6SN7, 7N7 and 6CG7 mu followers using tubes kindly loaned or donated by group members (Thanks Peter and Matthew). Here is a summary of the results. Basic set up is similar to Morgan Jones with 8mA standing current, the bottom triode biassed to about Vg=-3V and the top pentode CCS replaced by a normal mu follower CF set to Vg=-1.3V, with 10K between the tubes and a 320V supply and the output via a 0.1uF into 100K load. I tested distortion at a variety of levels at 200Hz, 2KHz and 20KHz. As the distortion was identical at all three frequencies the majority of tests were done only at 2KHz. I tested 4x6CG7 (Matsu****a), 2x7N7 (Sylvania) and 8x6SN7 (RCA) at output voltages between 1 and 50V rms. All the tubes produced remarkably similar results. The variation between types was not greater than the variation within a type although the best tube of all was a 6SN7. Typical THD readings we 2V rms 0.04% 10V rms 0.2% 20V rms 0.4% 50V rms 1.0% At 50V rms we are pretty close to grid current but since the oscillator used has a low output impedance no grid current distortion was observed. A subsequent test feeding the oscillator via a 120K series resistor showed no sign of grid current up to 25V rms output (as far as the oscillator output would go). I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. Cheers Ian Nice work, Ian. There's a sense in which it is a mistake to work with the 6SN7 and its workalikes because it is such a silent tube that the differentials will also be small and undramatic, but on the other hand it gives you a real feeling for what a good tube responds like, and how subtle changes can have a big impact on results, which is probably the most important thing to learn in playing along the loadline and transfer curves. Yes, the 6SN7 and derivatives do seem to deserve their reputation. I get the feeling later tubes tended to go for high mu, smaller (cheaper) construction, sod the distortion and use loads of global NFB. You should also check out the tests with the 6S?7 done by Lynn Olson and his pal Matt Kamna (a measurement expert), I don't think I have seen those. I'll try find them. http://www.nutshellhifi.com/library/FindingCG.html will take you directly to the tests. Thanks for that Andre. I had come across it before but not bookmarked it. It is a pity he did no test on his 6SN7 in a mu follower set up! He also makes the usual point about NFB altering distortion spectra but fails to back it up with measurements. Surprising since he had all that lovely kit available to do it with. Cheers Ian Worth reading around the Nutshell site. I used to know LynnO on the Joenet in decades gone by, when it seemed everyone interested in tube hi-fi knew everyone else. -- AJ and by the ever-excellent Steve Bench. Yes, I am well aware of Steve's excellent work. Cheers Ian |
#6
Posted to rec.audio.tubes
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6SN7 et al mu follower distortion
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#7
Posted to rec.audio.tubes
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6SN7 et al mu follower distortion
Ian Bell wrote: I have just about completed my distortion tests of 6SN7, 7N7 and 6CG7 mu followers using tubes kindly loaned or donated by group members (Thanks Peter and Matthew). Here is a summary of the results. Basic set up is similar to Morgan Jones with 8mA standing current, the bottom triode biassed to about Vg=-3V and the top pentode CCS replaced by a normal mu follower CF set to Vg=-1.3V, with 10K between the tubes and a 320V supply and the output via a 0.1uF into 100K load. I tested distortion at a variety of levels at 200Hz, 2KHz and 20KHz. As the distortion was identical at all three frequencies the majority of tests were done only at 2KHz. I tested 4x6CG7 (Matsu****a), 2x7N7 (Sylvania) and 8x6SN7 (RCA) at output voltages between 1 and 50V rms. All the tubes produced remarkably similar results. The variation between types was not greater than the variation within a type although the best tube of all was a 6SN7. Typical THD readings we 2V rms 0.04% 10V rms 0.2% 20V rms 0.4% 50V rms 1.0% At 50V rms we are pretty close to grid current but since the oscillator used has a low output impedance no grid current distortion was observed. A subsequent test feeding the oscillator via a 120K series resistor showed no sign of grid current up to 25V rms output (as far as the oscillator output would go). I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. Of course? * Duncan Amplfication Generic Triode Model (Spice 3F4 Implementation) * Copyright (C)1997-2002 Duncan Amplfication * Unauthorised Commercial use prohibited * Please refer to documentation at http://www.duncanamps.com ..SUBCKT NH6SN7GTB A G K * ANODE MODEL BLIM LI 0 V=(URAMP(V(A)-V(K))^ 1 )* 0.0037 BGG GG 0 V=V(G)-V(K)- 0 BRP1 RP1 0 V=URAMP(-V(GG)* 0.02 ) BRP2 RP2 0 V=V(RP1)-URAMP(V(RP1)-0.999) BRPF RP 0 V=(1-V(RP2)^ 2 )+URAMP(V(GG))* 0.002 BGR GR 0 V=URAMP(V(GG))-URAMP(-(V(GG)*(1+V(GG)* 0.006167 ))) BEM EM 0 V=URAMP(V(A)-V(K)+V(GR)* 19.2642 ) BEP EP 0 V=(V(EM)^ 1.4 )*V(RP)* 0.0000189 BEL1 EL1 0 V=URAMP(V(EP)) BEL EL 0 V=V(EL1)-URAMP(V(EL1)-V(LI)) BLD LD 0 V=URAMP(V(EP)-V(LI)) BAK A K I=V(EL) * GRID MODEL BGF GF 0 V=(URAMP(V(G)-V(K)- 0 )^1.5)* 0.000213 BG G K I=V(GF)+V(LD) * CAPS CAK A K 0.0000000000007 CGK G K 0.0000000000024 CGA G A 0.0000000000039 ..ENDS So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. Thanks, Ian. Nice to see actual measurements of real circuits. I wonder if you measured at any intermediate operating points, in between the two you mention? Just wondering if the change in distortion is a trend or a blip. Are you able to vary the HT voltage, to explore other parts of the safe operating area? Some idea of a trend would be good there too. How does simulated total distortion compare with your real measurements? cheers, and happy new year, Ian |
#8
Posted to rec.audio.tubes
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6SN7 et al mu follower distortion
Ian Iveson wrote:
Ian Bell wrote: I have just about completed my distortion tests of 6SN7, 7N7 and 6CG7 mu followers using tubes kindly loaned or donated by group members (Thanks Peter and Matthew). Here is a summary of the results. Basic set up is similar to Morgan Jones with 8mA standing current, the bottom triode biassed to about Vg=-3V and the top pentode CCS replaced by a normal mu follower CF set to Vg=-1.3V, with 10K between the tubes and a 320V supply and the output via a 0.1uF into 100K load. I tested distortion at a variety of levels at 200Hz, 2KHz and 20KHz. As the distortion was identical at all three frequencies the majority of tests were done only at 2KHz. I tested 4x6CG7 (Matsu****a), 2x7N7 (Sylvania) and 8x6SN7 (RCA) at output voltages between 1 and 50V rms. All the tubes produced remarkably similar results. The variation between types was not greater than the variation within a type although the best tube of all was a 6SN7. Typical THD readings we 2V rms 0.04% 10V rms 0.2% 20V rms 0.4% 50V rms 1.0% At 50V rms we are pretty close to grid current but since the oscillator used has a low output impedance no grid current distortion was observed. A subsequent test feeding the oscillator via a 120K series resistor showed no sign of grid current up to 25V rms output (as far as the oscillator output would go). I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. Of course? Ah, well, perhaps not. I have tried very many models and all but one fail to accurately model the normal operating region where the grid is negative wrt the cathode. The one that does model the normal operating region accurately (probably because it is based on The Audio Designers Tube Register curves which are the measured curves of actual tubes) does not include a grid model. Maybe I can make a hybrid and nick the grid model from the Duncan Amps models. * Duncan Amplfication Generic Triode Model (Spice 3F4 Implementation) * Copyright (C)1997-2002 Duncan Amplfication * Unauthorised Commercial use prohibited * Please refer to documentation at http://www.duncanamps.com .SUBCKT NH6SN7GTB A G K * ANODE MODEL BLIM LI 0 V=(URAMP(V(A)-V(K))^ 1 )* 0.0037 BGG GG 0 V=V(G)-V(K)- 0 BRP1 RP1 0 V=URAMP(-V(GG)* 0.02 ) BRP2 RP2 0 V=V(RP1)-URAMP(V(RP1)-0.999) BRPF RP 0 V=(1-V(RP2)^ 2 )+URAMP(V(GG))* 0.002 BGR GR 0 V=URAMP(V(GG))-URAMP(-(V(GG)*(1+V(GG)* 0.006167 ))) BEM EM 0 V=URAMP(V(A)-V(K)+V(GR)* 19.2642 ) BEP EP 0 V=(V(EM)^ 1.4 )*V(RP)* 0.0000189 BEL1 EL1 0 V=URAMP(V(EP)) BEL EL 0 V=V(EL1)-URAMP(V(EL1)-V(LI)) BLD LD 0 V=URAMP(V(EP)-V(LI)) BAK A K I=V(EL) * GRID MODEL BGF GF 0 V=(URAMP(V(G)-V(K)- 0 )^1.5)* 0.000213 BG G K I=V(GF)+V(LD) * CAPS CAK A K 0.0000000000007 CGK G K 0.0000000000024 CGA G A 0.0000000000039 .ENDS So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. Thanks, Ian. Nice to see actual measurements of real circuits. I wonder if you measured at any intermediate operating points, in between the two you mention? Just wondering if the change in distortion is a trend or a blip. Yes I did, I just gave a sample in the summary. I measured several tubes in 1V increments from 1V to 10V rms and then in 5V increments thereafter. As expected the distortion is very close to being directly proportional to signal level as expected until the onset of grid current when it rises steeply. Only the lower level measurements are suspect as they are approximately double the distortion of the oscillator itself. Are you able to vary the HT voltage, to explore other parts of the safe operating area? Some idea of a trend would be good there too. Not at the moment. 320V is as high as I can get with HT and that's just high enough for the circuit used. I suspect if I could raise the HT I could get even lower figures. However I was regularly achieving 0.4% THD at 20Vrms which is at -48dB relative to the fundamental. For most tubes, Morgan Jones only achieved -50dB second harmonic with a pentode CCS plate load at 19.5V rms so I do not expect there is much improvement to be gained by raising the HT. Personally, I was surprised I got distortion figures so close to Jones' with such a simple circuit which speaks volumes for the inherent linearity of the 6SN7 family. At present both tube halves sit comfortably within the SOAR. The bottom tube has a plate/cathode voltage around 140V which at 8mA is less than 1.2W dissipation and the top one has about 100V across it which is 800mW both of which are well within the SOAR of the 6CG7. I would be more concerned about exceeding heater/cathode voltages than SOAR. At present the heaters are raised to +75V (6CG7 has max +Ve dc of 100) adn the top cathode is at about 220V (6CG7 has max -ve dc of 200V which would allow cathode to go to 275V) so there's no much margin on either one. How does simulated total distortion compare with your real measurements? As I mentioned above, one model reproduces the distortion levels very accurately except for grid current. All the others I have tried are way out. cheers, and happy new year, Ian An A Happy New Year to you to. Cheers ian |
#9
Posted to rec.audio.tubes
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6SN7 et al mu follower distortion
Ian Bell wrote: Ian Iveson wrote: Ian Bell wrote: I have just about completed my distortion tests of 6SN7, 7N7 and 6CG7 mu followers using tubes kindly loaned or donated by group members (Thanks Peter and Matthew). Here is a summary of the results. Basic set up is similar to Morgan Jones with 8mA standing current, the bottom triode biassed to about Vg=-3V and the top pentode CCS replaced by a normal mu follower CF set to Vg=-1.3V, with 10K between the tubes and a 320V supply and the output via a 0.1uF into 100K load. I tested distortion at a variety of levels at 200Hz, 2KHz and 20KHz. As the distortion was identical at all three frequencies the majority of tests were done only at 2KHz. I tested 4x6CG7 (Matsu****a), 2x7N7 (Sylvania) and 8x6SN7 (RCA) at output voltages between 1 and 50V rms. All the tubes produced remarkably similar results. The variation between types was not greater than the variation within a type although the best tube of all was a 6SN7. Typical THD readings we 2V rms 0.04% 10V rms 0.2% 20V rms 0.4% 50V rms 1.0% At 50V rms we are pretty close to grid current but since the oscillator used has a low output impedance no grid current distortion was observed. A subsequent test feeding the oscillator via a 120K series resistor showed no sign of grid current up to 25V rms output (as far as the oscillator output would go). I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. Of course? Ah, well, perhaps not. I have tried very many models and all but one fail to accurately model the normal operating region where the grid is negative wrt the cathode. The one that does model the normal operating region accurately (probably because it is based on The Audio Designers Tube Register curves which are the measured curves of actual tubes) does not include a grid model. Maybe I can make a hybrid and nick the grid model from the Duncan Amps models. If you stick to building real circuits and measuring them you will always be able to tweak it up to get the widest V swing, symetrical clipping, and lowest THD widest BW and lowest Rout as all or some of these things might matter to you. The 10k between each 6sn7 triode section is just high enough so that the bottom tube sees a load of approximately the top tube open loop gain x 10k without output any load connected. In your case, if you need 1Vrms to exist between grid and cathode of top triode for a cathode output voltage of say 18V, then OLG = 18/1 = 18. Then the bottom tube sees a load = 18 x 10k, or 180k which is about 9Ra, and which is approaching the load for minimum THD. The triode anode load is a CCS, and were you to use a CCS instead of the 10k, then THD would maybe drop 6dB, but because your figures are fairly good so far, the complexity if an added CCS isn't neccessary. I like to operate the top tube with a fixed bias via 1M from a bias supply, then cap couple the top grid to the bottom anode. The grid bias voltage you should have in your case should **ALWAYS** be well above thet -1.3V you've been quoting. Bias for to and bottom triodes should be close to about -5V which implies that if you have Ia = 5mA, then there's 50V across the 10k, and Ea = 135V for both triodes. A quick draw of a load line on anode curves with a load slope of 180k will soon tell you what sort of swing you'll get and how little the swing will be if the Ia is higher, and Eg lower. The other thing is that loading the top tube cathode does slightly make the circuit work as a SRPP, but the effect is minor. Try measuring the THD with and without a load, and with a real world range of loads lower than the one you tried. * Duncan Amplfication Generic Triode Model (Spice 3F4 Implementation) * Copyright (C)1997-2002 Duncan Amplfication * Unauthorised Commercial use prohibited * Please refer to documentation at http://www.duncanamps.com .SUBCKT NH6SN7GTB A G K * ANODE MODEL BLIM LI 0 V=(URAMP(V(A)-V(K))^ 1 )* 0.0037 BGG GG 0 V=V(G)-V(K)- 0 BRP1 RP1 0 V=URAMP(-V(GG)* 0.02 ) BRP2 RP2 0 V=V(RP1)-URAMP(V(RP1)-0.999) BRPF RP 0 V=(1-V(RP2)^ 2 )+URAMP(V(GG))* 0.002 BGR GR 0 V=URAMP(V(GG))-URAMP(-(V(GG)*(1+V(GG)* 0.006167 ))) BEM EM 0 V=URAMP(V(A)-V(K)+V(GR)* 19.2642 ) BEP EP 0 V=(V(EM)^ 1.4 )*V(RP)* 0.0000189 BEL1 EL1 0 V=URAMP(V(EP)) BEL EL 0 V=V(EL1)-URAMP(V(EL1)-V(LI)) BLD LD 0 V=URAMP(V(EP)-V(LI)) BAK A K I=V(EL) * GRID MODEL BGF GF 0 V=(URAMP(V(G)-V(K)- 0 )^1.5)* 0.000213 BG G K I=V(GF)+V(LD) * CAPS CAK A K 0.0000000000007 CGK G K 0.0000000000024 CGA G A 0.0000000000039 .ENDS So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. To get truer THD results, THD of your oscillator always should be 1/10 of the minimum THD you will expect to be able to measure. Therefore say you wish to measure down to 0.01%, then the oscillator THD should be 0.001%. If the oscillator THD is high, and is 2H, then it either adds to the THD of the triodes which is predominantly 2H, or it cancels, so you won't ever get an accurate idea of THD as the level is tested at lower and lower voltages where you may be using the amplifier. I got mightily fed up with poor measurements and wrong measurements when I started so I built a simplr Wien bridge oscillator for 1kHz using opamps and a "grain of wheat" light bulb, and trimmed it to operate only at 1kHz with about 20 cycles of F adjustment with both fine and course adjust pots. It makes about 0.2% THD at 1kHz, and once the F is set, the F is very stable. Some oscillators have an anoyingly variable F which means you'll have difficulty nulling the F out when you test the output of the DUT. I then build a discrete component amp with BJTs to raise the 1V of the oscillator to make 4 voltage ranges, 0-1, 0-2, 0-4, 0-8V, so that on the lowest range, only small fraction of the oscillator is amplified to make 1V of output. The voltage amp is set up to act as a bandpass filter with a very simple R&C NFB network which gives gain = 8x at 1kHz, but much less at 2kHz and 500Hz and further away from the 1kHz. The result is that the THD at the amp output in the 1V range is 0.001%. I have an output level potentionmeter of 5k which produces less THD than the preceeding gear. I also made a bridged T tunable null filter, and another 1.6Khz to 10kHz bandpass amp to amplify the output from the bridged T to see really low levels of THD. I also build a high input impedance buffer to accept signals from the types of circuit you are doing so the load of the nulling filter didn't affect what i was trying to measure. I also fitted switchable hum filters. I think I spent about 6 weeks building and rebuilding that oscillator and tweaking it. But it taught me all about analog measuring, and building low noise circuits, and about just how difficult it is the get right. The result is that I don't have to estimate what THD might be while I measure. What I measure IS THE REAL DEAL within 10%, ie, if I measure 0.01%, it indicates THD is between 0.009% and 0.011%, and the measurement is good enough. I don't have a schematic online of my seld designed test gear but I am sure anyone here could build their own THD measuring gear after reading old Wireless World mags or searching around like I did 14 years ago. And BTW, its easier to get good results buidling such gear by just using opamps rather than attempting anything with tubes. I also made a passive LC filter using air cored L and C carefully set up with two LC parallel resonant sections fed by 4.7k R, so that although the insertion loss is 12dB, the output signal has THD another 20dB lower than the oscillator produces, so THD 0.001%. But wherever you have coils in THD measuring gear, you cannot use iron cores because you'll get iron core distortion much larger than what you are trying to measure. Even an iron box to shield the coils causes THD. So you have to keep the LC filter away from iron, and well away from any mains wiring. The oscillator and following amps need to be fed by a remote PS with good rail filtering. Thanks, Ian. Nice to see actual measurements of real circuits. I wonder if you measured at any intermediate operating points, in between the two you mention? Just wondering if the change in distortion is a trend or a blip. Yes I did, I just gave a sample in the summary. I measured several tubes in 1V increments from 1V to 10V rms and then in 5V increments thereafter. As expected the distortion is very close to being directly proportional to signal level as expected until the onset of grid current when it rises steeply. Only the lower level measurements are suspect as they are approximately double the distortion of the oscillator itself. ! Are you able to vary the HT voltage, to explore other parts of the safe operating area? Some idea of a trend would be good there too. Not at the moment. 320V is as high as I can get with HT and that's just high enough for the circuit used. I suspect if I could raise the HT I could get even lower figures. However I was regularly achieving 0.4% THD at 20Vrms which is at -48dB relative to the fundamental. For most tubes, Morgan Jones only achieved -50dB second harmonic with a pentode CCS plate load at 19.5V rms so I do not expect there is much improvement to be gained by raising the HT. Personally, I was surprised I got distortion figures so close to Jones' with such a simple circuit which speaks volumes for the inherent linearity of the 6SN7 family. 0.4% at 20Vrms isn't too bad, but you should get about 1/2 that, or no more than 0.1% at 10V. This is about the limit for SE triode signal amps without any loop NFB. At present both tube halves sit comfortably within the SOAR. The bottom tube has a plate/cathode voltage around 140V which at 8mA is less than 1.2W dissipation and the top one has about 100V across it which is 800mW both of which are well within the SOAR of the 6CG7. This explains your 1.3V bias. I never use a single 1/2 6SN7 at 8 mA because I doubt there is any sonic benefit and you should get at least the same low THD with 4mA and the lower bias. When the anode load becomes high, it doesn't matter if the load line is down in the more curved regions of Ra because the load line is close to horizontal and intersects the Ra curves at about the same points in their curvatures. I would be more concerned about exceeding heater/cathode voltages than SOAR. At present the heaters are raised to +75V (6CG7 has max +Ve dc of 100) adn the top cathode is at about 220V (6CG7 has max -ve dc of 200V which would allow cathode to go to 275V) so there's no much margin on either one. You can get over the heater bias problem by using two single triodes, say 6J5, or 2 x 6SN7 tubes, and bias bottom heaters at 0V and the top heaters at close to whatever the cathode Ek is. How does simulated total distortion compare with your real measurements? As I mentioned above, one model reproduces the distortion levels very accurately except for grid current. All the others I have tried are way out. Hmm, I have never wasted time modelling or simulating things beyond what I can achieve in my head or with a ruler and set of Ra curves. When you actually build it, its the real thing. I did try to make an output stage for a phono amp using a 12AT7 for gain and loaded with a CCS, then a direct coupled 6CG7 as a normal output cathode follower. It measured well. But I preferred the sound of the 12AT7 when used as a bottom tube and 6CG7 at the top tube as a µ-follower. It also measured well. And is wasted less PSU power. See the "Rocket" schematic about 1/4 the way down the page at http://www.turneraudio.com.au/preamp...hono-2005.html Patrick Turner. PS, I find it difficult to be happier at Xmas or NY than at any other time of the year. But try not to do anything I would not enjoy..... cheers, and happy new year, Ian An A Happy New Year to you to. Cheers ian |
#10
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6SN7 et al mu follower distortion
On Jan 2, 11:46*am, Patrick Turner wrote:
The other thing is that loading the top tube cathode does slightly make the circuit work as a SRPP, but the effect is minor. Actually, any circuit which works even partially like an SRPP is a good circuit, especially if the advantage of the SRPP it mimics is a low impedance. I'm not always so sure that the mu-follower is truly worth the additional complication over an SRPP for anyone except the purist who can, as a consolation for the extra expense and complication, mutter to himself over and over, "Yes, but an SRPP isn't a real current source and a mu-follower is an exemplary current source... It's true, it's true!" An SRPP with a single extra resistor to arrange a voltage lift can in most instances be arranged to work for practical purposes as well, and probably more reliably, than a mu- follower. Okay, I don't want to sound like a cost accountant, but someone must be realistic. Quite a bit of other good stuff, some of it at least arguable on grounds of taste, some of it indubitably right, snipped in the interest of bandwidth. Andre Jute The tubes tend to make people believe in a god, and SS leads them to the devil. -- Patrick Turner |
#11
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6SN7 et al mu follower distortion
Patrick Turner wrote:
Ian Bell wrote: Ian Iveson wrote: Ian Bell wrote: I have just about completed my distortion tests of 6SN7, 7N7 and 6CG7 mu followers using tubes kindly loaned or donated by group members (Thanks Peter and Matthew). Here is a summary of the results. Basic set up is similar to Morgan Jones with 8mA standing current, the bottom triode biassed to about Vg=-3V and the top pentode CCS replaced by a normal mu follower CF set to Vg=-1.3V, with 10K between the tubes and a 320V supply and the output via a 0.1uF into 100K load. I tested distortion at a variety of levels at 200Hz, 2KHz and 20KHz. As the distortion was identical at all three frequencies the majority of tests were done only at 2KHz. I tested 4x6CG7 (Matsu****a), 2x7N7 (Sylvania) and 8x6SN7 (RCA) at output voltages between 1 and 50V rms. All the tubes produced remarkably similar results. The variation between types was not greater than the variation within a type although the best tube of all was a 6SN7. Typical THD readings we 2V rms 0.04% 10V rms 0.2% 20V rms 0.4% 50V rms 1.0% At 50V rms we are pretty close to grid current but since the oscillator used has a low output impedance no grid current distortion was observed. A subsequent test feeding the oscillator via a 120K series resistor showed no sign of grid current up to 25V rms output (as far as the oscillator output would go). I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. Of course? Ah, well, perhaps not. I have tried very many models and all but one fail to accurately model the normal operating region where the grid is negative wrt the cathode. The one that does model the normal operating region accurately (probably because it is based on The Audio Designers Tube Register curves which are the measured curves of actual tubes) does not include a grid model. Maybe I can make a hybrid and nick the grid model from the Duncan Amps models. If you stick to building real circuits and measuring them you will always be able to tweak it up to get the widest V swing, symetrical clipping, and lowest THD widest BW and lowest Rout as all or some of these things might matter to you. Tweaking may be necessary but it is not sufficient IMHO. It is effective only with that particular set of components. If you build another another one just the same it won't perform just the same and you will be need to tweak it again to achieve similar performance. To me that's poor design practice. Better to understand what is going one so a circuit with repeatable performance can be built without tweaking. The 10k between each 6sn7 triode section is just high enough so that the bottom tube sees a load of approximately the top tube open loop gain x 10k without output any load connected. In your case, if you need 1Vrms to exist between grid and cathode of top triode for a cathode output voltage of say 18V, then OLG = 18/1 = 18. Then the bottom tube sees a load = 18 x 10k, or 180k which is about 9Ra, and which is approaching the load for minimum THD. The triode anode load is a CCS, and were you to use a CCS instead of the 10k, then THD would maybe drop 6dB, but because your figures are fairly good so far, the complexity if an added CCS isn't neccessary. Jones achieved only 2dB better figures with a CCS so I would be doubtful that a 6dB improvement could be had. I like to operate the top tube with a fixed bias via 1M from a bias supply, then cap couple the top grid to the bottom anode. The grid bias voltage you should have in your case should **ALWAYS** be well above thet -1.3V you've been quoting. For the bottom tube probably yes but it does not seem to affect the top triode performance. Bias for top and bottom triodes should be close to about -5V Why 5V ?? which implies that if you have Ia = 5mA, then there's 50V across the 10k, and Ea = 135V for both triodes. A quick draw of a load line on anode curves with a load slope of 180k will soon tell you what sort of swing you'll get and how little the swing will be if the Ia is higher, and Eg lower. I don't think so. The curves are much more bunched together when Ia is 5mA. My tests at lower values of Ia gave higher distortion levels. The other thing is that loading the top tube cathode does slightly make the circuit work as a SRPP, but the effect is minor. Try measuring the THD with and without a load, and with a real world range of loads lower than the one you tried. I have but I did not mention them in the summary. Distortion at 20V rms 2KHz into various loads was: 100K load 0.41% 25K load 0.46% 9.9K load 0.52% By measuring loaded and unloaded output levels, the output impedance was calculated as 830 ohms. * Duncan Amplfication Generic Triode Model (Spice 3F4 Implementation) * Copyright (C)1997-2002 Duncan Amplfication * Unauthorised Commercial use prohibited * Please refer to documentation at http://www.duncanamps.com .SUBCKT NH6SN7GTB A G K * ANODE MODEL BLIM LI 0 V=(URAMP(V(A)-V(K))^ 1 )* 0.0037 BGG GG 0 V=V(G)-V(K)- 0 BRP1 RP1 0 V=URAMP(-V(GG)* 0.02 ) BRP2 RP2 0 V=V(RP1)-URAMP(V(RP1)-0.999) BRPF RP 0 V=(1-V(RP2)^ 2 )+URAMP(V(GG))* 0.002 BGR GR 0 V=URAMP(V(GG))-URAMP(-(V(GG)*(1+V(GG)* 0.006167 ))) BEM EM 0 V=URAMP(V(A)-V(K)+V(GR)* 19.2642 ) BEP EP 0 V=(V(EM)^ 1.4 )*V(RP)* 0.0000189 BEL1 EL1 0 V=URAMP(V(EP)) BEL EL 0 V=V(EL1)-URAMP(V(EL1)-V(LI)) BLD LD 0 V=URAMP(V(EP)-V(LI)) BAK A K I=V(EL) * GRID MODEL BGF GF 0 V=(URAMP(V(G)-V(K)- 0 )^1.5)* 0.000213 BG G K I=V(GF)+V(LD) * CAPS CAK A K 0.0000000000007 CGK G K 0.0000000000024 CGA G A 0.0000000000039 .ENDS So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. To get truer THD results, THD of your oscillator always should be 1/10 of the minimum THD you will expect to be able to measure. Therefore say you wish to measure down to 0.01%, then the oscillator THD should be 0.001%. Agreed. If the oscillator THD is high, and is 2H, then it either adds to the THD of the triodes which is predominantly 2H, or it cancels, so you won't ever get an accurate idea of THD as the level is tested at lower and lower voltages where you may be using the amplifier. Agreed. I got mightily fed up with poor measurements and wrong measurements when I started so I built a simplr Wien bridge oscillator for 1kHz using opamps and a "grain of wheat" light bulb, and trimmed it to operate only at 1kHz with about 20 cycles of F adjustment with both fine and course adjust pots. It makes about 0.2% THD at 1kHz, and once the F is set, the F is very stable. Some oscillators have an anoyingly variable F which means you'll have difficulty nulling the F out when you test the output of the DUT. I then build a discrete component amp with BJTs to raise the 1V of the oscillator to make 4 voltage ranges, 0-1, 0-2, 0-4, 0-8V, so that on the lowest range, only small fraction of the oscillator is amplified to make 1V of output. The voltage amp is set up to act as a bandpass filter with a very simple R&C NFB network which gives gain = 8x at 1kHz, but much less at 2kHz and 500Hz and further away from the 1kHz. The result is that the THD at the amp output in the 1V range is 0.001%. I have an output level potentionmeter of 5k which produces less THD than the preceeding gear. I also made a bridged T tunable null filter, and another 1.6Khz to 10kHz bandpass amp to amplify the output from the bridged T to see really low levels of THD. I also build a high input impedance buffer to accept signals from the types of circuit you are doing so the load of the nulling filter didn't affect what i was trying to measure. I also fitted switchable hum filters. I think I spent about 6 weeks building and rebuilding that oscillator and tweaking it. But it taught me all about analog measuring, and building low noise circuits, and about just how difficult it is the get right. The result is that I don't have to estimate what THD might be while I measure. What I measure IS THE REAL DEAL within 10%, ie, if I measure 0.01%, it indicates THD is between 0.009% and 0.011%, and the measurement is good enough. I don't have a schematic online of my seld designed test gear but I am sure anyone here could build their own THD measuring gear after reading old Wireless World mags or searching around like I did 14 years ago. And BTW, its easier to get good results buidling such gear by just using opamps rather than attempting anything with tubes. I also made a passive LC filter using air cored L and C carefully set up with two LC parallel resonant sections fed by 4.7k R, so that although the insertion loss is 12dB, the output signal has THD another 20dB lower than the oscillator produces, so THD 0.001%. But wherever you have coils in THD measuring gear, you cannot use iron cores because you'll get iron core distortion much larger than what you are trying to measure. Even an iron box to shield the coils causes THD. So you have to keep the LC filter away from iron, and well away from any mains wiring. The oscillator and following amps need to be fed by a remote PS with good rail filtering. Thanks, Ian. Nice to see actual measurements of real circuits. I wonder if you measured at any intermediate operating points, in between the two you mention? Just wondering if the change in distortion is a trend or a blip. Yes I did, I just gave a sample in the summary. I measured several tubes in 1V increments from 1V to 10V rms and then in 5V increments thereafter. As expected the distortion is very close to being directly proportional to signal level as expected until the onset of grid current when it rises steeply. Only the lower level measurements are suspect as they are approximately double the distortion of the oscillator itself. ! Are you able to vary the HT voltage, to explore other parts of the safe operating area? Some idea of a trend would be good there too. Not at the moment. 320V is as high as I can get with HT and that's just high enough for the circuit used. I suspect if I could raise the HT I could get even lower figures. However I was regularly achieving 0.4% THD at 20Vrms which is at -48dB relative to the fundamental. For most tubes, Morgan Jones only achieved -50dB second harmonic with a pentode CCS plate load at 19.5V rms so I do not expect there is much improvement to be gained by raising the HT. Personally, I was surprised I got distortion figures so close to Jones' with such a simple circuit which speaks volumes for the inherent linearity of the 6SN7 family. 0.4% at 20Vrms isn't too bad, but you should get about 1/2 that, or no more than 0.1% at 10V. Er, half of 0.4% is 0.2% or did I miss something? This is about the limit for SE triode signal amps without any loop NFB. At present both tube halves sit comfortably within the SOAR. The bottom tube has a plate/cathode voltage around 140V which at 8mA is less than 1.2W dissipation and the top one has about 100V across it which is 800mW both of which are well within the SOAR of the 6CG7. This explains your 1.3V bias. I never use a single 1/2 6SN7 at 8 mA because I doubt there is any sonic benefit and you should get at least the same low THD with 4mA and the lower bias. I disagree. My 8mA bias is with the bottom tube at just over -3V so grid current is not an issue. THD is definitely measurably lower at 8mA Ia than at lower currents. When the anode load becomes high, it doesn't matter if the load line is down in the more curved regions of Ra because the load line is close to horizontal and intersects the Ra curves at about the same points in their curvatures. I would be more concerned about exceeding heater/cathode voltages than SOAR. At present the heaters are raised to +75V (6CG7 has max +Ve dc of 100) adn the top cathode is at about 220V (6CG7 has max -ve dc of 200V which would allow cathode to go to 275V) so there's no much margin on either one. You can get over the heater bias problem by using two single triodes, say 6J5, or 2 x 6SN7 tubes, and bias bottom heaters at 0V and the top heaters at close to whatever the cathode Ek is. Yes, I plan to use them in pairs and had considered having both bottom triodes in one 6SN7 and the top pair in another with separate heater supplies. How does simulated total distortion compare with your real measurements? As I mentioned above, one model reproduces the distortion levels very accurately except for grid current. All the others I have tried are way out. Hmm, I have never wasted time modelling or simulating things beyond what I can achieve in my head or with a ruler and set of Ra curves. When you actually build it, its the real thing. Unfortunately Ra curves are not a very good indicator of what real tubes actually do. Manufacturers data sheets are a long way from what the tubes actually available today actually do. The most reliable set of curves I have found are those created from measurements of the tubes of today as found in the Audio Designers Tube Register. The spice models based on these same sets of curves give simulated results very close to those I measured. Other spice models I have found to give very inaccurate results. Equally, manufactures data sheets are only suitable for making the crudest of estimations of THD in low level preamplifiers. I did try to make an output stage for a phono amp using a 12AT7 for gain and loaded with a CCS, then a direct coupled 6CG7 as a normal output cathode follower. It measured well. But I preferred the sound of the 12AT7 when used as a bottom tube and 6CG7 at the top tube as a µ-follower. It also measured well. You say they 'measured well'; care to put some actual figures to that statement? Cheers ian And is wasted less PSU power. See the "Rocket" schematic about 1/4 the way down the page at http://www.turneraudio.com.au/preamp...hono-2005.html Patrick Turner. PS, I find it difficult to be happier at Xmas or NY than at any other time of the year. But try not to do anything I would not enjoy..... cheers, and happy new year, Ian An A Happy New Year to you to. Cheers ian |
#12
Posted to rec.audio.tubes
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6SN7 et al mu follower distortion
Andre Jute wrote:
On Jan 2, 11:46 am, Patrick Turner wrote: The other thing is that loading the top tube cathode does slightly make the circuit work as a SRPP, but the effect is minor. Actually, any circuit which works even partially like an SRPP is a good circuit, especially if the advantage of the SRPP it mimics is a low impedance. I'm not always so sure that the mu-follower is truly worth the additional complication over an SRPP If you want low distortion then the mu follower is much better than the SRPP; definitely worth the extra complexity. for anyone except the purist who can, as a consolation for the extra expense and complication, mutter to himself over and over, "Yes, but an SRPP isn't a real current source and a mu-follower is an exemplary current source... It's true, it's true!" An SRPP with a single extra resistor to arrange a voltage lift can in most instances be arranged to work for practical purposes as well, and probably more reliably, than a mu- follower. OK, then show me an SRPP design with a 6SN7 that does significantly better than 0.4% at 20Vrms out into 10K at 2KHz. Okay, I don't want to sound like a cost accountant, but someone must be realistic. Indeed, and if you want to save a few pennies at the expense of a whole lot of distortion then SRPP is the way to go. Quite a bit of other good stuff, some of it at least arguable on grounds of taste, some of it indubitably right, snipped in the interest of bandwidth. LOL Cheers Ian Andre Jute The tubes tend to make people believe in a god, and SS leads them to the devil. -- Patrick Turner |
#13
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6SN7 et al mu follower distortion
On Jan 2, 10:45*pm, Ian Bell wrote:
Andre Jute wrote: On Jan 2, 11:46 am, Patrick Turner wrote: The other thing is that loading the top tube cathode does slightly make the circuit work as a SRPP, but the effect is minor. Actually, any circuit which works even partially like an SRPP is a good circuit, especially if the advantage of the SRPP it mimics is a low impedance. I'm not always so sure that the mu-follower is truly worth the additional complication over an SRPP If you want low distortion then the mu follower is much better than the SRPP; definitely worth the extra complexity. for anyone except the purist who can, as a consolation for the extra expense and complication, mutter to himself over and over, "Yes, but an SRPP isn't a real current source and a mu-follower is an exemplary current source... It's true, it's true!" An SRPP with a single extra resistor to arrange a voltage lift can in most instances be arranged to work for practical purposes as well, and probably more reliably, than a mu- follower. OK, then show me an SRPP design with a 6SN7 that does significantly better than 0.4% at 20Vrms out into 10K at 2KHz. Okay, I don't want to sound like a cost accountant, but someone must be realistic. Indeed, and if you want to save a few pennies at the expense of a whole lot of distortion then SRPP is the way to go. Quite a bit of other good stuff, some of it at least arguable on grounds of taste, some of it indubitably right, snipped in the interest of bandwidth. LOL Cheers Ian Andre Jute *The tubes tend to make people believe in a god, and SS leads them to the devil. -- Patrick Turner You're missing my point, Ian, probably unsurprisingly. You're interested in pre-amps and therefore need to extract the maximum of silence as well as a big gain. I use CD and integrated two stage amps (mostly); I start with 2Vrms of clean signal. Virtually any 6SN7 circuit that is competently built is silent enough for me, though in fact I use high-mu 417A/5842 almost exclusively these days. And I have many other trade-offs if I want addtional silence, as I usually do. I can trade tube longevity by running the tube at very high voltage and current. I can and do move the quiescent operating point well away from any possibility of grid current. I can and do flatten the loadline along which the signal swings by very high loads so that a real accountant would tear out his hair at the "waste" -- like building a 10 litre V16 engine for a car and then choking it back with the silencers to a whisper and a miserable 200bhp when it is capable of 600bhp. Everyone else makes a 300B give 8W; mine deliver 3.8W... Andre Jute Refinement beyond price |
#14
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6SN7 et al mu follower distortion
Ian Bell wrote
I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. Of course? Ah, well, perhaps not. I have tried very many models and all but one fail to accurately model the normal operating region where the grid is negative wrt the cathode. The one that does model the normal operating region accurately (probably because it is based on The Audio Designers Tube Register curves which are the measured curves of actual tubes) does not include a grid model. Maybe I can make a hybrid and nick the grid model from the Duncan Amps models. That would probably be OK but watch out for "LD" that links the grid model to the anode model. You may need to rename it, or add it's derivation into your anode model. I say probably because in reality grid current has a relationship to anode current, probably, because I guess it decreases perveance (less free electrons available so anode current should fall such that total cathode current remains the same, all other things being equal). But it should be a negligible error where grid current is small. * Duncan Amplfication Generic Triode Model (Spice 3F4 Implementation) * Copyright (C)1997-2002 Duncan Amplfication * Unauthorised Commercial use prohibited * Please refer to documentation at http://www.duncanamps.com .SUBCKT NH6SN7GTB A G K * ANODE MODEL BLIM LI 0 V=(URAMP(V(A)-V(K))^ 1 )* 0.0037 BGG GG 0 V=V(G)-V(K)- 0 BRP1 RP1 0 V=URAMP(-V(GG)* 0.02 ) BRP2 RP2 0 V=V(RP1)-URAMP(V(RP1)-0.999) BRPF RP 0 V=(1-V(RP2)^ 2 )+URAMP(V(GG))* 0.002 BGR GR 0 V=URAMP(V(GG))-URAMP(-(V(GG)*(1+V(GG)* 0.006167 ))) BEM EM 0 V=URAMP(V(A)-V(K)+V(GR)* 19.2642 ) BEP EP 0 V=(V(EM)^ 1.4 )*V(RP)* 0.0000189 BEL1 EL1 0 V=URAMP(V(EP)) BEL EL 0 V=V(EL1)-URAMP(V(EL1)-V(LI)) BLD LD 0 V=URAMP(V(EP)-V(LI)) BAK A K I=V(EL) * GRID MODEL BGF GF 0 V=(URAMP(V(G)-V(K)- 0 )^1.5)* 0.000213 BG G K I=V(GF)+V(LD) * CAPS CAK A K 0.0000000000007 CGK G K 0.0000000000024 CGA G A 0.0000000000039 .ENDS So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. Thanks, Ian. Nice to see actual measurements of real circuits. I wonder if you measured at any intermediate operating points, in between the two you mention? Just wondering if the change in distortion is a trend or a blip. Yes I did, I just gave a sample in the summary. I measured several tubes in 1V increments from 1V to 10V rms and then in 5V increments thereafter. As expected the distortion is very close to being directly proportional to signal level as expected until the onset of grid current when it rises steeply. Only the lower level measurements are suspect as they are approximately double the distortion of the oscillator itself. Right, I got that bit, and I can see why you expected what you got. What I was wondering, though, was whether you had tried a range of bias currents in between the two you mention, to establish the relationship between standing current and distortion for the same signal. I got the impression that you were asserting that lower standing current is related to lower distortion, but it's not very clear...perhaps I'm missing something. The question is slightly ambiguous, because of variation in mu and therefore in gain for different standing currents, but that shouldn't be so great as to prevent reasonable comparison. There is also the problem of ensuring that the bottom anode remains set at half the supply voltage. Two points aren't enough to establish a trend. I ask partly because it seemed to be the question that you set out with: why did MJ bias at such a level? I realise that standing current is also related to maximum signal capability, but still wonder if I want to amplify a small signal, where a low standing current would be sufficient to stay far enough clear of grid current, will I always get greater distortion by raising the standing current? Or have I got it the wrong way round? Whichever, I'm still wondering about sweet spots. Are you able to vary the HT voltage, to explore other parts of the safe operating area? Some idea of a trend would be good there too. Not at the moment. 320V is as high as I can get with HT and that's just high enough for the circuit used. I suspect if I could raise the HT I could get even lower figures. However I was regularly achieving 0.4% THD at 20Vrms which is at -48dB relative to the fundamental. For most tubes, Morgan Jones only achieved -50dB second harmonic with a pentode CCS plate load at 19.5V rms so I do not expect there is much improvement to be gained by raising the HT. Personally, I was surprised I got distortion figures so close to Jones' with such a simple circuit which speaks volumes for the inherent linearity of the 6SN7 family. At present both tube halves sit comfortably within the SOAR. The bottom tube has a plate/cathode voltage around 140V which at 8mA is less than 1.2W dissipation and the top one has about 100V across it which is 800mW both of which are well within the SOAR of the 6CG7. I would be more concerned about exceeding heater/cathode voltages than SOAR. At present the heaters are raised to +75V (6CG7 has max +Ve dc of 100) adn the top cathode is at about 220V (6CG7 has max -ve dc of 200V which would allow cathode to go to 275V) so there's no much margin on either one. How does simulated total distortion compare with your real measurements? As I mentioned above, one model reproduces the distortion levels very accurately except for grid current. All the others I have tried are way out. This comes as a pretty big suprise. Do you mean that you have measured simulated distortion and compared it to your real measurements? This makes a sweep of distortion level v standing current even more interesting. Could you post a copy of your model, please? AFAIK, the generic DM model underlying the 6SN7 I posted is the most accurate and sophisticated available, with one possible exception that you probably couldn't use on your simulator. Considering the DM triode comes with a grid model that had been omitted from previous, inferior triode models, and that DM is the originator of nearly all available models and I would guess *all* in common use, I surmise that *all* of the improvement in the model you have used is due to the more appropriate data used to generate it. It is therefore not unlikely that fitting the most recent DM model to that data would result in a model more accurate than the one you have, particularly in the BRH quarter of the SOAR, IIRC. Which is why I would appreciate a copy of the model text. If it is similar to the DM generic model, it may be possible to transfer common parameters, and leave any extra ones as they are. If not, then the data the model was fitted to would be useful, although a complete refit is a tedious business. Ian |
#15
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6SN7 et al mu follower distortion
Ian Iveson wrote:
Ian Bell wrote I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. Of course? Ah, well, perhaps not. I have tried very many models and all but one fail to accurately model the normal operating region where the grid is negative wrt the cathode. The one that does model the normal operating region accurately (probably because it is based on The Audio Designers Tube Register curves which are the measured curves of actual tubes) does not include a grid model. Maybe I can make a hybrid and nick the grid model from the Duncan Amps models. That would probably be OK but watch out for "LD" that links the grid model to the anode model. You may need to rename it, or add it's derivation into your anode model. Thanks for the tip. I'll watch out for that. I say probably because in reality grid current has a relationship to anode current, probably, because I guess it decreases perveance (less free electrons available so anode current should fall such that total cathode current remains the same, all other things being equal). But it should be a negligible error where grid current is small. Yes, and re-reading RDH4 on grid current there seems to be several contributory factors and I doubt the spice model includes them all. * Duncan Amplfication Generic Triode Model (Spice 3F4 Implementation) * Copyright (C)1997-2002 Duncan Amplfication * Unauthorised Commercial use prohibited * Please refer to documentation at http://www.duncanamps.com .SUBCKT NH6SN7GTB A G K * ANODE MODEL BLIM LI 0 V=(URAMP(V(A)-V(K))^ 1 )* 0.0037 BGG GG 0 V=V(G)-V(K)- 0 BRP1 RP1 0 V=URAMP(-V(GG)* 0.02 ) BRP2 RP2 0 V=V(RP1)-URAMP(V(RP1)-0.999) BRPF RP 0 V=(1-V(RP2)^ 2 )+URAMP(V(GG))* 0.002 BGR GR 0 V=URAMP(V(GG))-URAMP(-(V(GG)*(1+V(GG)* 0.006167 ))) BEM EM 0 V=URAMP(V(A)-V(K)+V(GR)* 19.2642 ) BEP EP 0 V=(V(EM)^ 1.4 )*V(RP)* 0.0000189 BEL1 EL1 0 V=URAMP(V(EP)) BEL EL 0 V=V(EL1)-URAMP(V(EL1)-V(LI)) BLD LD 0 V=URAMP(V(EP)-V(LI)) BAK A K I=V(EL) * GRID MODEL BGF GF 0 V=(URAMP(V(G)-V(K)- 0 )^1.5)* 0.000213 BG G K I=V(GF)+V(LD) * CAPS CAK A K 0.0000000000007 CGK G K 0.0000000000024 CGA G A 0.0000000000039 .ENDS So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. Thanks, Ian. Nice to see actual measurements of real circuits. I wonder if you measured at any intermediate operating points, in between the two you mention? Just wondering if the change in distortion is a trend or a blip. Yes I did, I just gave a sample in the summary. I measured several tubes in 1V increments from 1V to 10V rms and then in 5V increments thereafter. As expected the distortion is very close to being directly proportional to signal level as expected until the onset of grid current when it rises steeply. Only the lower level measurements are suspect as they are approximately double the distortion of the oscillator itself. Right, I got that bit, and I can see why you expected what you got. What I was wondering, though, was whether you had tried a range of bias currents in between the two you mention, to establish the relationship between standing current and distortion for the same signal. I got the impression that you were asserting that lower standing current is related to lower distortion, but it's not very clear...perhaps I'm missing something. The question is slightly ambiguous, because of variation in mu and therefore in gain for different standing currents, but that shouldn't be so great as to prevent reasonable comparison. There is also the problem of ensuring that the bottom anode remains set at half the supply voltage. Two points aren't enough to establish a trend. I ask partly because it seemed to be the question that you set out with: why did MJ bias at such a level? What I did do was: 1. Try a lot of tubes at 8mA and -3.4V (bottom triode) and 8mA 1.3V (top triode) and got consistent results between tube types and also got distortion ruoghly proportional to output level. 2. Tried a few tubes at 8mA and 1.3V both top and bottom triodes and got lower distortion at lower levels but grid current at higher levels. This presumably means that a lower bias voltage means lower distortion at a given current though it is by no means conclusive. 3,. Following Patrick's comment I did a quick test yesterday with both triodes biassed at 5mA and -5V (actually turned out to be 4.55mA and -4.55V). I got exactly the same distortion figures at 2V and 20V rms as in the original experiment (1 above). Frankly I expected the distortion to go up at the lower current but it did not - this is however one test on one tube only. I realise that standing current is also related to maximum signal capability, but still wonder if I want to amplify a small signal, where a low standing current would be sufficient to stay far enough clear of grid current, will I always get greater distortion by raising the standing current? Or have I got it the wrong way round? I am not sure but I would have expected distortion to decrease as standing current increases simply because you move away from the region where the curves bunch. And if you look at the spacing between curves they seem to me to be less evenly spaced as the bias voltage gets more negative which does explain why test #2 gave lower distortion Whichever, I'm still wondering about sweet spots. Me too. Test 3 seems to show that nearly halving the standing current makes no difference to distortion and leads to a bias voltage a long way from grid current problems. Are you able to vary the HT voltage, to explore other parts of the safe operating area? Some idea of a trend would be good there too. Not at the moment. 320V is as high as I can get with HT and that's just high enough for the circuit used. I suspect if I could raise the HT I could get even lower figures. However I was regularly achieving 0.4% THD at 20Vrms which is at -48dB relative to the fundamental. For most tubes, Morgan Jones only achieved -50dB second harmonic with a pentode CCS plate load at 19.5V rms so I do not expect there is much improvement to be gained by raising the HT. Personally, I was surprised I got distortion figures so close to Jones' with such a simple circuit which speaks volumes for the inherent linearity of the 6SN7 family. At present both tube halves sit comfortably within the SOAR. The bottom tube has a plate/cathode voltage around 140V which at 8mA is less than 1.2W dissipation and the top one has about 100V across it which is 800mW both of which are well within the SOAR of the 6CG7. I would be more concerned about exceeding heater/cathode voltages than SOAR. At present the heaters are raised to +75V (6CG7 has max +Ve dc of 100) adn the top cathode is at about 220V (6CG7 has max -ve dc of 200V which would allow cathode to go to 275V) so there's no much margin on either one. How does simulated total distortion compare with your real measurements? As I mentioned above, one model reproduces the distortion levels very accurately except for grid current. All the others I have tried are way out. This comes as a pretty big surprise. Do you mean that you have measured simulated distortion and compared it to your real measurements? Yes, the spice model based on the The Audio Designers Tube Register curves gives THD results that are extremely close to the measurements. This makes a sweep of distortion level v standing current even more interesting. Yes, that might be easier to achieve in a simulation than on the bench. Could you post a copy of your model, please? AFAIK, the generic DM model underlying the 6SN7 I posted is the most accurate and sophisticated available, with one possible exception that you probably couldn't use on your simulator. Considering the DM triode comes with a grid model that had been omitted from previous, inferior triode models, and that DM is the originator of nearly all available models and I would guess *all* in common use, I surmise that *all* of the improvement in the model you have used is due to the more appropriate data used to generate it. That would be my conclusion too. It is therefore not unlikely that fitting the most recent DM model to that data would result in a model more accurate than the one you have, particularly in the BRH quarter of the SOAR, IIRC. Possibly. The model I have apparently used quite a sophisticated curve fitting model so I am not sure why the DM model should be inherently better. Which is why I would appreciate a copy of the model text. If it is similar to the DM generic model, it may be possible to transfer common parameters, and leave any extra ones as they are. If not, then the data the model was fitted to would be useful, although a complete refit is a tedious business. No problem and I just realised I told a big fib. The model is apparently taken from the GE data sheet not the The Audio Designers Tube Register curves as I stated earlier - I wonder where I got that from?? - must check that out. Anyway, here is the 6SN7 portion of the file: *6SN7 LTSpice model from GE 6SN7 datasheet ..subckt 6sn7 P G K Bp P K I=(0.02003791851m)*uramp(V(P,K)*ln(1.0+(-0.07740549711)+exp((4.618036737)+(4.618036737)*((2 0.85288965)+(-110.4389272m)*V(G,K))*V(G,K)/sqrt((28.13407639)**2+(V(P,K)-(7.118597372))**2)))/(4.618036737))**(1.380047579) Cgp G P 4.0pF Cgk G K 2.6pF Cpk P K 0.7pF ..ends 6sn7 Note the preamble to the set of models is as follows: *Generated by Joel Tunnah using Curve Captor v0.9.1 *UPDATED 02/07/06 *Contents: *6SN7 *12AX7 *12AU7 *12AT7 *ECC88 *6C4Pi Cheers Ian Ian |
#16
Posted to rec.audio.tubes
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6SN7 et al mu follower distortion
"Ian Bell" wrote in message ... I have just about completed my distortion tests of 6SN7, 7N7 and 6CG7 mu followers using tubes kindly loaned or donated by group members (Thanks Peter and Matthew). Here is a summary of the results. Basic set up is similar to Morgan Jones with 8mA standing current, the bottom triode biassed to about Vg=-3V and the top pentode CCS replaced by a normal mu follower CF set to Vg=-1.3V, with 10K between the tubes and a 320V supply and the output via a 0.1uF into 100K load. I tested distortion at a variety of levels at 200Hz, 2KHz and 20KHz. As the distortion was identical at all three frequencies the majority of tests were done only at 2KHz. I tested 4x6CG7 (Matsu****a), 2x7N7 (Sylvania) and 8x6SN7 (RCA) at output voltages between 1 and 50V rms. All the tubes produced remarkably similar results. The variation between types was not greater than the variation within a type although the best tube of all was a 6SN7. Typical THD readings we 2V rms 0.04% 10V rms 0.2% 20V rms 0.4% 50V rms 1.0% At 50V rms we are pretty close to grid current but since the oscillator used has a low output impedance no grid current distortion was observed. A subsequent test feeding the oscillator via a 120K series resistor showed no sign of grid current up to 25V rms output (as far as the oscillator output would go). I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. Good work Ian. The THD figs you obtained match precisely those I have measured with 6CG7 in mu follower topology. I have built several such preamps. I found the most consistent tubes to be Siemens and Westinghouse both with 0.02% at 1kHz and 2Vrms output for a 125mV input. (24dB gain) You might like to take one step further now, and compare the distortion profile of tubes that have similar THD. Cheers Iain |
#17
Posted to rec.audio.tubes
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6SN7 et al mu follower distortion
Iain Churches wrote:
"Ian Bell" wrote in message ... I have just about completed my distortion tests of 6SN7, 7N7 and 6CG7 mu followers using tubes kindly loaned or donated by group members (Thanks Peter and Matthew). Here is a summary of the results. Basic set up is similar to Morgan Jones with 8mA standing current, the bottom triode biassed to about Vg=-3V and the top pentode CCS replaced by a normal mu follower CF set to Vg=-1.3V, with 10K between the tubes and a 320V supply and the output via a 0.1uF into 100K load. I tested distortion at a variety of levels at 200Hz, 2KHz and 20KHz. As the distortion was identical at all three frequencies the majority of tests were done only at 2KHz. I tested 4x6CG7 (Matsu****a), 2x7N7 (Sylvania) and 8x6SN7 (RCA) at output voltages between 1 and 50V rms. All the tubes produced remarkably similar results. The variation between types was not greater than the variation within a type although the best tube of all was a 6SN7. Typical THD readings we 2V rms 0.04% 10V rms 0.2% 20V rms 0.4% 50V rms 1.0% At 50V rms we are pretty close to grid current but since the oscillator used has a low output impedance no grid current distortion was observed. A subsequent test feeding the oscillator via a 120K series resistor showed no sign of grid current up to 25V rms output (as far as the oscillator output would go). I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. Good work Ian. The THD figs you obtained match precisely those I have measured with 6CG7 in mu follower topology. I have built several such preamps. I found the most consistent tubes to be Siemens and Westinghouse both with 0.02% at 1kHz and 2Vrms output for a 125mV input. (24dB gain) So far I have tried a couple of 7N7 kindly loaned by Matt, and a bunch of 6SN7s given to me by Peter. Their THD figures are remarkably consistent. I have also tried four Matsu****a 6CG7 tubes I got from Colomor and these are as good as the N7 tubes and quieter too - I think I'll be buying a few more of these. I am keeping an eye out for other makes of 6CG7 but so far they are rather few and far between and so far not a Siemens to be seen amongst them. You might like to take one step further now, and compare the distortion profile of tubes that have similar THD. Cheers Iain Thanks for the kind words. Yes, I would like to delve deeper and to that end I have just purchased an HP 3581A Wave Analyser which should let me take a closer look at the distortion spectra. Cheers Ian |
#18
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6SN7 et al mu follower distortion
Andre Jute wrote: On Jan 2, 11:46 am, Patrick Turner wrote: The other thing is that loading the top tube cathode does slightly make the circuit work as a SRPP, but the effect is minor. Actually, any circuit which works even partially like an SRPP is a good circuit, especially if the advantage of the SRPP it mimics is a low impedance. I'm not always so sure that the mu-follower is truly worth the additional complication over an SRPP for anyone except the purist who can, as a consolation for the extra expense and complication, mutter to himself over and over, "Yes, but an SRPP isn't a real current source and a mu-follower is an exemplary current source... It's true, it's true!" An SRPP with a single extra resistor to arrange a voltage lift can in most instances be arranged to work for practical purposes as well, and probably more reliably, than a mu- follower. The pure SRPP works OK for most folks if they like simplicity, but let's consider some facts. For SRPP, where Rk of the top and bottom triode are equal, Rout is less than the Ra of one triode, but several times that of a pure CF. If you move to µ-follower where the Rk between top and bottom triode equals at least half the Ra for one triode then the Rout plummets to be much closer to that of a CF. The penalty paid for the µ-foll is an extra Rk of at least Ra/2, and a bias resistor, and a capacitor to couple the bottom triode anode to the top triode grid. Maybe $2.00. With SRPP, one can tailor the Rk between the two tubes and the load to nearly equalize the ac anode current, but there will always be slightly more Iac in one tube than in the other when a load is connected so full series PP operation with complete cancelation of 2H never really happens. And instead, after mucking about with loads and Rk, usually you find the tubes not very well loaded, and the load each sees is not many times Ra, and you get considerable 3H instead of the more benign 2H. With µ-foll, you simply never have to worry about making the damn circuit work to produce minimal 2H cancelation. Its you have series PP where the top tube acts as a "bootstrapped follower", ie, quasi CF with low THD, and the bottom tube has a very high ac load many times Ra and many more times Ra than the SRPP could ever offer. Thus the bottom tube develops more gain and lower THD/IMD than most other arrangements and you get the saving of not having to waste dc power and generate distortion in cathode load resistors. Let's not try to escape the miniscule cost of a few R&C parts to get a µ-foll to work better than its primitive cousin the SRPP. In µ-foll there is mimimized load ac flow in the bottom gain triode, since it is buffered by the CF above it. I think the µ-foll sounds better. I have also used triode gain stages with CCS dc supply, and a following R load many times Ra and sonic results are the best. I really don't like resistances used for dc conveyance to a triode in critical circuits because they create unecessary distortion. One of my classic examples is this good sound preamp at http://www.turneraudio.com.au/line-preamp-2003.html There's a 1/2 12AU7 driving a 50k gain pot. But the anode is a CCS. If there was a dc load R the value would have been about 33k. With the 50k pot the load would have been about 20k, and only 2Ra at 4mAdc. But I have RL at 5Ra approx, and THD is low. It'd even be lower if the pot had been 100k, which may have been even better, as I have in my later Nemo line stage circuit at http://www.turneraudio.com.au/preamp...+psu-2005.html Okay, I don't want to sound like a cost accountant, but someone must be realistic. Sometimes it costs more to employ a bean counter than using a few more beans :-) Patrick Turner. Quite a bit of other good stuff, some of it at least arguable on grounds of taste, some of it indubitably right, snipped in the interest of bandwidth. Andre Jute The tubes tend to make people believe in a god, and SS leads them to the devil. -- Patrick Turner |
#19
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6SN7 et al mu follower distortion
Andre Jute wrote: On Jan 2, 10:45 pm, Ian Bell wrote: Andre Jute wrote: On Jan 2, 11:46 am, Patrick Turner wrote: The other thing is that loading the top tube cathode does slightly make the circuit work as a SRPP, but the effect is minor. Actually, any circuit which works even partially like an SRPP is a good circuit, especially if the advantage of the SRPP it mimics is a low impedance. I'm not always so sure that the mu-follower is truly worth the additional complication over an SRPP If you want low distortion then the mu follower is much better than the SRPP; definitely worth the extra complexity. for anyone except the purist who can, as a consolation for the extra expense and complication, mutter to himself over and over, "Yes, but an SRPP isn't a real current source and a mu-follower is an exemplary current source... It's true, it's true!" An SRPP with a single extra resistor to arrange a voltage lift can in most instances be arranged to work for practical purposes as well, and probably more reliably, than a mu- follower. OK, then show me an SRPP design with a 6SN7 that does significantly better than 0.4% at 20Vrms out into 10K at 2KHz. Okay, I don't want to sound like a cost accountant, but someone must be realistic. Indeed, and if you want to save a few pennies at the expense of a whole lot of distortion then SRPP is the way to go. Quite a bit of other good stuff, some of it at least arguable on grounds of taste, some of it indubitably right, snipped in the interest of bandwidth. LOL Cheers Ian Andre Jute The tubes tend to make people believe in a god, and SS leads them to the devil. -- Patrick Turner You're missing my point, Ian, probably unsurprisingly. You're interested in pre-amps and therefore need to extract the maximum of silence as well as a big gain. I use CD and integrated two stage amps (mostly); I start with 2Vrms of clean signal. Virtually any 6SN7 circuit that is competently built is silent enough for me, though in fact I use high-mu 417A/5842 almost exclusively these days. And I have many other trade-offs if I want addtional silence, as I usually do. I can trade tube longevity by running the tube at very high voltage and current. I can and do move the quiescent operating point well away from any possibility of grid current. I can and do flatten the loadline along which the signal swings by very high loads so that a real accountant would tear out his hair at the "waste" -- like building a 10 litre V16 engine for a car and then choking it back with the silencers to a whisper and a miserable 200bhp when it is capable of 600bhp. Everyone else makes a 300B give 8W; mine deliver 3.8W... Say you set up an SE 300B amp which can make 8W into 5 ohms, while dissipating no more than 30W at idle. Then you may find that maximum clipping power will decline with loads below 5 ohms and above 5 ohms, but it is an ideal way to set up a 300B to deal with speakers that are nominally 8 ohms, so tht when indeed the load is 8 ohms the PO might be much less than 8 ohms but fidelity and DF is higher. In fact, the 300B set up as I suggest will indeed put up with speakers nominally 4 ohms if the levels expected are low because one great feature of triodes is that they are fairly load tolerant. But where the host of the party is a triode, don't abuse his hospitality by inviting too many guests who will draw excessively from the limited barrel of amps available. If very few guests turn up and the load is deemed to be 16 ohms then decorum and convivialty will prevail and without the distortions of bad behaviours. The secret to partying well on one 300B for each channel is to have sensitive speakers! The more sensitive the speaker is, the less important is its impedance, and the sweeter the sound if you can make a sensitive speaker work properly. Patrick Turner. Andre Jute Refinement beyond price |
#20
Posted to rec.audio.tubes
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6SN7 et al mu follower distortion
Ian Bell wrote: [below]
Thanks very much, Ian, especially for the model. Sorry for the delay: I've been fully occupied with the crisis in Palestine. When time permits I'll compare the two models and try to produce a valid composite. In passing, that seems like an awful lot of decimal places! Cheers, Ian in message ... Ian Iveson wrote: Ian Bell wrote I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but with tubes with mu about 20 and with 20Vrms output you need to bias several volts -ve to be sure to be away from grid current. This does not mean that this bias point produces the lowest distortion. Simulation showed biassing the lower triode also at about 1.3V should give even lower distortion but of course the simulation does not simulate grid current. Of course? Ah, well, perhaps not. I have tried very many models and all but one fail to accurately model the normal operating region where the grid is negative wrt the cathode. The one that does model the normal operating region accurately (probably because it is based on The Audio Designers Tube Register curves which are the measured curves of actual tubes) does not include a grid model. Maybe I can make a hybrid and nick the grid model from the Duncan Amps models. That would probably be OK but watch out for "LD" that links the grid model to the anode model. You may need to rename it, or add it's derivation into your anode model. Thanks for the tip. I'll watch out for that. I say probably because in reality grid current has a relationship to anode current, probably, because I guess it decreases perveance (less free electrons available so anode current should fall such that total cathode current remains the same, all other things being equal). But it should be a negligible error where grid current is small. Yes, and re-reading RDH4 on grid current there seems to be several contributory factors and I doubt the spice model includes them all. * Duncan Amplfication Generic Triode Model (Spice 3F4 Implementation) * Copyright (C)1997-2002 Duncan Amplfication * Unauthorised Commercial use prohibited * Please refer to documentation at http://www.duncanamps.com .SUBCKT NH6SN7GTB A G K * ANODE MODEL BLIM LI 0 V=(URAMP(V(A)-V(K))^ 1 )* 0.0037 BGG GG 0 V=V(G)-V(K)- 0 BRP1 RP1 0 V=URAMP(-V(GG)* 0.02 ) BRP2 RP2 0 V=V(RP1)-URAMP(V(RP1)-0.999) BRPF RP 0 V=(1-V(RP2)^ 2 )+URAMP(V(GG))* 0.002 BGR GR 0 V=URAMP(V(GG))-URAMP(-(V(GG)*(1+V(GG)* 0.006167 ))) BEM EM 0 V=URAMP(V(A)-V(K)+V(GR)* 19.2642 ) BEP EP 0 V=(V(EM)^ 1.4 )*V(RP)* 0.0000189 BEL1 EL1 0 V=URAMP(V(EP)) BEL EL 0 V=V(EL1)-URAMP(V(EL1)-V(LI)) BLD LD 0 V=URAMP(V(EP)-V(LI)) BAK A K I=V(EL) * GRID MODEL BGF GF 0 V=(URAMP(V(G)-V(K)- 0 )^1.5)* 0.000213 BG G K I=V(GF)+V(LD) * CAPS CAK A K 0.0000000000007 CGK G K 0.0000000000024 CGA G A 0.0000000000039 .ENDS So I re-biassed the bottom tube to 1.3V and check the distortion. With a 120K in series with the oscillator, grid current distortion began at 6Vrms output. Below that the distortion was exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms was hard to measure as it expect it approaches the limit of both my distortion test set and the oscillator distortion. I would estimate it to be no more than 0.02% at 2Vrms. Thanks, Ian. Nice to see actual measurements of real circuits. I wonder if you measured at any intermediate operating points, in between the two you mention? Just wondering if the change in distortion is a trend or a blip. Yes I did, I just gave a sample in the summary. I measured several tubes in 1V increments from 1V to 10V rms and then in 5V increments thereafter. As expected the distortion is very close to being directly proportional to signal level as expected until the onset of grid current when it rises steeply. Only the lower level measurements are suspect as they are approximately double the distortion of the oscillator itself. Right, I got that bit, and I can see why you expected what you got. What I was wondering, though, was whether you had tried a range of bias currents in between the two you mention, to establish the relationship between standing current and distortion for the same signal. I got the impression that you were asserting that lower standing current is related to lower distortion, but it's not very clear...perhaps I'm missing something. The question is slightly ambiguous, because of variation in mu and therefore in gain for different standing currents, but that shouldn't be so great as to prevent reasonable comparison. There is also the problem of ensuring that the bottom anode remains set at half the supply voltage. Two points aren't enough to establish a trend. I ask partly because it seemed to be the question that you set out with: why did MJ bias at such a level? What I did do was: 1. Try a lot of tubes at 8mA and -3.4V (bottom triode) and 8mA 1.3V (top triode) and got consistent results between tube types and also got distortion ruoghly proportional to output level. 2. Tried a few tubes at 8mA and 1.3V both top and bottom triodes and got lower distortion at lower levels but grid current at higher levels. This presumably means that a lower bias voltage means lower distortion at a given current though it is by no means conclusive. 3,. Following Patrick's comment I did a quick test yesterday with both triodes biassed at 5mA and -5V (actually turned out to be 4.55mA and -4.55V). I got exactly the same distortion figures at 2V and 20V rms as in the original experiment (1 above). Frankly I expected the distortion to go up at the lower current but it did not - this is however one test on one tube only. I realise that standing current is also related to maximum signal capability, but still wonder if I want to amplify a small signal, where a low standing current would be sufficient to stay far enough clear of grid current, will I always get greater distortion by raising the standing current? Or have I got it the wrong way round? I am not sure but I would have expected distortion to decrease as standing current increases simply because you move away from the region where the curves bunch. And if you look at the spacing between curves they seem to me to be less evenly spaced as the bias voltage gets more negative which does explain why test #2 gave lower distortion Whichever, I'm still wondering about sweet spots. Me too. Test 3 seems to show that nearly halving the standing current makes no difference to distortion and leads to a bias voltage a long way from grid current problems. Are you able to vary the HT voltage, to explore other parts of the safe operating area? Some idea of a trend would be good there too. Not at the moment. 320V is as high as I can get with HT and that's just high enough for the circuit used. I suspect if I could raise the HT I could get even lower figures. However I was regularly achieving 0.4% THD at 20Vrms which is at -48dB relative to the fundamental. For most tubes, Morgan Jones only achieved -50dB second harmonic with a pentode CCS plate load at 19.5V rms so I do not expect there is much improvement to be gained by raising the HT. Personally, I was surprised I got distortion figures so close to Jones' with such a simple circuit which speaks volumes for the inherent linearity of the 6SN7 family. At present both tube halves sit comfortably within the SOAR. The bottom tube has a plate/cathode voltage around 140V which at 8mA is less than 1.2W dissipation and the top one has about 100V across it which is 800mW both of which are well within the SOAR of the 6CG7. I would be more concerned about exceeding heater/cathode voltages than SOAR. At present the heaters are raised to +75V (6CG7 has max +Ve dc of 100) adn the top cathode is at about 220V (6CG7 has max -ve dc of 200V which would allow cathode to go to 275V) so there's no much margin on either one. How does simulated total distortion compare with your real measurements? As I mentioned above, one model reproduces the distortion levels very accurately except for grid current. All the others I have tried are way out. This comes as a pretty big surprise. Do you mean that you have measured simulated distortion and compared it to your real measurements? Yes, the spice model based on the The Audio Designers Tube Register curves gives THD results that are extremely close to the measurements. This makes a sweep of distortion level v standing current even more interesting. Yes, that might be easier to achieve in a simulation than on the bench. Could you post a copy of your model, please? AFAIK, the generic DM model underlying the 6SN7 I posted is the most accurate and sophisticated available, with one possible exception that you probably couldn't use on your simulator. Considering the DM triode comes with a grid model that had been omitted from previous, inferior triode models, and that DM is the originator of nearly all available models and I would guess *all* in common use, I surmise that *all* of the improvement in the model you have used is due to the more appropriate data used to generate it. That would be my conclusion too. It is therefore not unlikely that fitting the most recent DM model to that data would result in a model more accurate than the one you have, particularly in the BRH quarter of the SOAR, IIRC. Possibly. The model I have apparently used quite a sophisticated curve fitting model so I am not sure why the DM model should be inherently better. Which is why I would appreciate a copy of the model text. If it is similar to the DM generic model, it may be possible to transfer common parameters, and leave any extra ones as they are. If not, then the data the model was fitted to would be useful, although a complete refit is a tedious business. No problem and I just realised I told a big fib. The model is apparently taken from the GE data sheet not the The Audio Designers Tube Register curves as I stated earlier - I wonder where I got that from?? - must check that out. Anyway, here is the 6SN7 portion of the file: *6SN7 LTSpice model from GE 6SN7 datasheet .subckt 6sn7 P G K Bp P K I=(0.02003791851m)*uramp(V(P,K)*ln(1.0+(-0.07740549711)+exp((4.618036737)+(4.618036737)*((2 0.85288965)+(-110.4389272m)*V(G,K))*V(G,K)/sqrt((28.13407639)**2+(V(P,K)-(7.118597372))**2)))/(4.618036737))**(1.380047579) Cgp G P 4.0pF Cgk G K 2.6pF Cpk P K 0.7pF .ends 6sn7 Note the preamble to the set of models is as follows: *Generated by Joel Tunnah using Curve Captor v0.9.1 *UPDATED 02/07/06 *Contents: *6SN7 *12AX7 *12AU7 *12AT7 *ECC88 *6C4Pi Cheers Ian Ian |
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