''Some say'' is hardly a dogma and mostly a reference to what has been wrongly or rightly speculated. Sonic differences between very well implemented triode and UL modes are more down to amount and type of feedback IMHO.
Time to step in front of the whirling blades...
My feeling is that there are two separate issues here:
pentodes/tetrodes vs. triodes
and
UL vs. Pentode
In general terms, my ears tell me that pentodes and tetrodes seem to have a kind of "zing" added to the middle upper frequencies. A sort of "presence" rise, most noticeable with things like cymbals in particular.
I don't think the zing is natural nor in the original recordings.
Strapped pentodes run in triode operation seem to exhibit far less of this effect than do "real" triodes, but it is still slightly present.
What can we surmise is the cause of this? Well, clearly there must be some sort of interaction between the screen grid and the other elements. Imho, it may be entirely due to mechanical variation under load!! and not some purely electronic factor!! Regardless, this is my take on the "pentode sound".
The major reason that so many pentode amps have been made and sold is two fold:
- they make more power than triodes, and power sells
- no one made very many indirectly heated triodes, and DHTs are not as rugged, many more pentodes were made than triodes in general.
Nothing to do with quality, quality of sound or any other factors, imho.
There is a way to run two tubes together, as a "combined" output section wherein a triode and a pentode are both used in (p-p) parallel... that bears some investigation wrt the sound and vis-a-vis UL vs. Pentode...
Now, UL vs. Pentode...
I don't think you lose quite 50% power going from UL/Pentode to triode operation. In the case of a typical ST-70 you go from ~35 to >20watts... but I guess that's close to 50%. But 50% is just 3dB. So...
Many of the commercial amps of today run the screens into an over current situation on peaks... not good. Adds non-linear distortions.
Of course if you "tame them" and drop the screen current to an appropriate max level, then they don't make the full "marketing snot" that they claim anymore...
I tested a popular make (California mfr) amp that had "triode/pentode" switching some time ago. Removing the input circuit from the equation (ick!) and driving the grids in AB2 from a very high quality source, and doing all sorts of tests and changing of bias points, etc... (the output iron was quite surprisingly very good, btw - way better than the amp's circuit) I found that the tubes worked best and had the least non-linearity when run with a regulated screen supply that was optimized within a few volts for each particular tube pair! Either side of the optimized screen setting, not as good at all. Using the UL taps was rather less wonderful looking on the scope...
Of course, I've also worked with and on all the Dyna UL models, and most of the Acros, and many of the Heaths, and all sorts of other varied UL amps...
The best looking results I ever had from a pentode tube was with that test set up I mentioned above and the optimized screen voltage, regulated...
What I think is at play here is that the UL mode offers extra power and decreased quiescent current, plus a "cheaper build" given the fewer parts for that pesky screen supply, and has a certain harmonic structure that is inevitable (factoring out the inherent screen "zing") and different than the pure pentode.
Feedback, local or not does not cure the inherent tendencies of any given circuit's harmonic spectrum - it might reduce it or even alter the ratios, but the underlying part is still always there.
My personal experience says that wrapping feedback around an output transformer, or even an interstage transformer is a receipe for adding more problems than it usually will solve. Why? Because of two main things - phase shift that can't be undone, and the effect of most global feedback in tube amps is/was to flatten the frequency response mostly for the benefit of marketing specs over any sonic benefits.
If you want to make a tube amp flat, the way to do it is to start with really, really, really good transformers that extend out as close to (or beyond) 100kHz. as you can get, and have ample inductance to go low enough. Then if you have a clean, low distortion, well balanced driver and input stage, you've got a really good, low distortion amp to begin with.
Anyhow, I have yet to hear a UL amp that did not actually sound better with the screens tied into "triode" operation. I lay this off to the "screen zing" problem in the main.
So, the dislike of UL may stem from the above points, I would expect.
_-_-bear
My feeling is that there are two separate issues here:
pentodes/tetrodes vs. triodes
and
UL vs. Pentode
In general terms, my ears tell me that pentodes and tetrodes seem to have a kind of "zing" added to the middle upper frequencies. A sort of "presence" rise, most noticeable with things like cymbals in particular.
I don't think the zing is natural nor in the original recordings.
Strapped pentodes run in triode operation seem to exhibit far less of this effect than do "real" triodes, but it is still slightly present.
What can we surmise is the cause of this? Well, clearly there must be some sort of interaction between the screen grid and the other elements. Imho, it may be entirely due to mechanical variation under load!! and not some purely electronic factor!! Regardless, this is my take on the "pentode sound".
The major reason that so many pentode amps have been made and sold is two fold:
- they make more power than triodes, and power sells
- no one made very many indirectly heated triodes, and DHTs are not as rugged, many more pentodes were made than triodes in general.
Nothing to do with quality, quality of sound or any other factors, imho.
There is a way to run two tubes together, as a "combined" output section wherein a triode and a pentode are both used in (p-p) parallel... that bears some investigation wrt the sound and vis-a-vis UL vs. Pentode...
Now, UL vs. Pentode...
I don't think you lose quite 50% power going from UL/Pentode to triode operation. In the case of a typical ST-70 you go from ~35 to >20watts... but I guess that's close to 50%. But 50% is just 3dB. So...
Many of the commercial amps of today run the screens into an over current situation on peaks... not good. Adds non-linear distortions.
Of course if you "tame them" and drop the screen current to an appropriate max level, then they don't make the full "marketing snot" that they claim anymore...
I tested a popular make (California mfr) amp that had "triode/pentode" switching some time ago. Removing the input circuit from the equation (ick!) and driving the grids in AB2 from a very high quality source, and doing all sorts of tests and changing of bias points, etc... (the output iron was quite surprisingly very good, btw - way better than the amp's circuit) I found that the tubes worked best and had the least non-linearity when run with a regulated screen supply that was optimized within a few volts for each particular tube pair! Either side of the optimized screen setting, not as good at all. Using the UL taps was rather less wonderful looking on the scope...
Of course, I've also worked with and on all the Dyna UL models, and most of the Acros, and many of the Heaths, and all sorts of other varied UL amps...
The best looking results I ever had from a pentode tube was with that test set up I mentioned above and the optimized screen voltage, regulated...
What I think is at play here is that the UL mode offers extra power and decreased quiescent current, plus a "cheaper build" given the fewer parts for that pesky screen supply, and has a certain harmonic structure that is inevitable (factoring out the inherent screen "zing") and different than the pure pentode.
Feedback, local or not does not cure the inherent tendencies of any given circuit's harmonic spectrum - it might reduce it or even alter the ratios, but the underlying part is still always there.
My personal experience says that wrapping feedback around an output transformer, or even an interstage transformer is a receipe for adding more problems than it usually will solve. Why? Because of two main things - phase shift that can't be undone, and the effect of most global feedback in tube amps is/was to flatten the frequency response mostly for the benefit of marketing specs over any sonic benefits.
If you want to make a tube amp flat, the way to do it is to start with really, really, really good transformers that extend out as close to (or beyond) 100kHz. as you can get, and have ample inductance to go low enough. Then if you have a clean, low distortion, well balanced driver and input stage, you've got a really good, low distortion amp to begin with.
Anyhow, I have yet to hear a UL amp that did not actually sound better with the screens tied into "triode" operation. I lay this off to the "screen zing" problem in the main.
So, the dislike of UL may stem from the above points, I would expect.
_-_-bear
bear said:
It seems to me that another issue is missing : PSU regulation.
Penthode are relativly insensitive to plate voltage sag while triode are.
But penthodes ARE very sensitive to screen voltage, and - simply because they have higher Mu - most sensitive to bias shift than triodes.
Keeping screen and bias supply stable is easier than keeping plate supply stable.
Specially if PA stage is run far in AB1.
PSU is an integral part of the PA stage design.
So, my preferences go to screen and bias regulated penthodes.
UL then calls for separate screen windings and becomes less attractive !
Yves.
Hey Yvesm,
What pentodes are you speaking of with higher mu? I have a few data sheets in front of me ( for excellent audio valves ), that show mu g1g2 as low as 3. It of course extends right up to 211 levels for some, but a lot are less. The value spread is pretty comparable to power triodes.
cheers,
Douglas
What pentodes are you speaking of with higher mu? I have a few data sheets in front of me ( for excellent audio valves ), that show mu g1g2 as low as 3. It of course extends right up to 211 levels for some, but a lot are less. The value spread is pretty comparable to power triodes.
cheers,
Douglas
Yvesm said:
It would appear that I misunderstood your original post. Pentode gain is gm x Load. Mu I took as an intrinsic device property, and not one defined by, or dependant on other circuit details. For pentodes, this would be mu g1g2 and not the in-circuit gain.
Also it appears that the lower mu g1g2 valves exhibit a larger change in plate current( plate voltage and g1-k voltages held constant ) than the higher ones when g2 voltage is moved around.
cheers,
Douglas
Perhaps I'm missing something about the way UL works? 
The screen is set at some nominal voltage Vs, which then is modulated by the output signal, in phase with the plate signal. In effect, it is bootstrapped to the plate's AC variation. Yes?
So, on positive going peaks, the screen is driven HARD positive beyond the normal voltage limits of Pentode operation (or tries to be), no?
Is this feedback, or feedforward??
_-_-bear
The screen is set at some nominal voltage Vs, which then is modulated by the output signal, in phase with the plate signal. In effect, it is bootstrapped to the plate's AC variation. Yes?
So, on positive going peaks, the screen is driven HARD positive beyond the normal voltage limits of Pentode operation (or tries to be), no?
Is this feedback, or feedforward??
_-_-bear
For tapped anode winding U-L, plate V, g2 V and B+ are essentially the same. Run the plate up, and g2 goes up the tapped percentage. With a triode strapped pentode, the g2 exactly follows the anode, and g2 ratings are modified for this arrangement.
Turn the valve 'ON' all the way, and g2 is now above the anode voltage, at a point between anode V and B+ defined by the tap location.
Run U-L with a cathode FB and one can make g2 what ever one wants. Same as with a tertiary winding. Or, as in some Chicago and Plitron OPT's, one can tap g2 from the anode coil, and add the tertiary winding in the cathode to the U-L 'action'....
cheers,
Douglas
Turn the valve 'ON' all the way, and g2 is now above the anode voltage, at a point between anode V and B+ defined by the tap location.
Run U-L with a cathode FB and one can make g2 what ever one wants. Same as with a tertiary winding. Or, as in some Chicago and Plitron OPT's, one can tap g2 from the anode coil, and add the tertiary winding in the cathode to the U-L 'action'....
cheers,
Douglas
A jumper reference from another thread really, but absolutely relative since there is brief, easy to grasp, working modes explanation plus sonic discussion, between variations of pentode, UL, triode strapped, and SET using the same basic platform. Click square #10 to start. #13,14,15 are directly relative to our discussion.
It seems to me, from what has been said so far, that the separate winding cathode feedback approach may have more promise than the screen taps approach, if the results claimed for it are genuine. Williamson & Walker certainly thought so but they would say that, wouldn't they?
Cathode feedback allows the plate voltage to be much higher than the screen voltage, which might suit some tubes very well (e.g. 6L6, 807, even EL34). It also allows the screen to be regulated, which some see as critically important. A separate winding for the screens also gives screen voltage flexibility.
Transformers with separate windings, for either cathode or screen, are not rare but they're not cheap either. Perhaps that's just as well, though, assuming that the quality is reflected in the price, because the indication seems to be that low quality UL transformers give low quality results.
Cathode feedback allows the plate voltage to be much higher than the screen voltage, which might suit some tubes very well (e.g. 6L6, 807, even EL34). It also allows the screen to be regulated, which some see as critically important. A separate winding for the screens also gives screen voltage flexibility.
Transformers with separate windings, for either cathode or screen, are not rare but they're not cheap either. Perhaps that's just as well, though, assuming that the quality is reflected in the price, because the indication seems to be that low quality UL transformers give low quality results.
I do not know abour W&W what they would say, but back in 1960'th my dad had a tube tape recorder with SE 6P14P output that had a cathode winding. The recorder itself was so-so, but the amp was great and sounded gorgeous when I connected my DIY guitar to it.
I would try now such trick with GU-50 PP, but where to get output transformers? May be it is possible to order them somewhere relatively cheap?
What ratio is optimal for cathode windings?
I would try now such trick with GU-50 PP, but where to get output transformers? May be it is possible to order them somewhere relatively cheap?
What ratio is optimal for cathode windings?
The GU50 looked to me like it wanted a fairly high( numeric ) load, like ~10k including the cathode winding. Say 7k5 + 15% in the cathodes. Increasing the CFB makes driving it more difficult in terms of delivering the large swing.
I am using Chicago BOH-6's in a 6V6 amp. They sound brilliant. Can't be all that difficult to acquire another pair, or a single so it can be duplicated. The Acro TO-350 is a CFB output, and IIRC Sowter can wind a good copy. I had the big Dynaco A441 and it was not a spectacular output. I wound up selling mine instead of keeping them to copy at a later date if I wanted more.
cheers,
Douglas
I am using Chicago BOH-6's in a 6V6 amp. They sound brilliant. Can't be all that difficult to acquire another pair, or a single so it can be duplicated. The Acro TO-350 is a CFB output, and IIRC Sowter can wind a good copy. I had the big Dynaco A441 and it was not a spectacular output. I wound up selling mine instead of keeping them to copy at a later date if I wanted more.
cheers,
Douglas
I can only answer with a question: what do you think it should have?
Second, why not get a proper triode instead of strapping a pentode. There are *MANY* that will do well.
I have built a few CFB amps with about that same size tertiary windng. While maintaining class A operating points, the B+ begins to limit the deliverable voltage swing. Less would not give enough FB, and the amp would be sensitive to reactive behaviour as is usually seen in the bass frequencies. One could always pick a speaker that was better behaved...
cheers,
Douglas
Second, why not get a proper triode instead of strapping a pentode. There are *MANY* that will do well.
I have built a few CFB amps with about that same size tertiary windng. While maintaining class A operating points, the B+ begins to limit the deliverable voltage swing. Less would not give enough FB, and the amp would be sensitive to reactive behaviour as is usually seen in the bass frequencies. One could always pick a speaker that was better behaved...
cheers,
Douglas
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