Quite so, PRR.
We are a little off-thread, but hopefully still interesting! I personally fancy the beam tube style because of the slightly higher efficiency, but mainly because I experienced with especially EL34s, that the g2 would occasionally glow in certain places where it was "unprotected" by a screening g1 winding. This often resulted in spurious oscillatory response, so I could not see any reason to stick to classic pentodes. There did not appear to be any "alignment" as such with G1, and in some tubes the relative placing would allow for several consecutive screen turns to be open to the cathode. (I am always collecting failed tubes and cutting them open.)
But back to UL vs triodes, if Kathodyne has not yet thrown up his hands in despair. I will be posting the promised figures shortly.
We are a little off-thread, but hopefully still interesting! I personally fancy the beam tube style because of the slightly higher efficiency, but mainly because I experienced with especially EL34s, that the g2 would occasionally glow in certain places where it was "unprotected" by a screening g1 winding. This often resulted in spurious oscillatory response, so I could not see any reason to stick to classic pentodes. There did not appear to be any "alignment" as such with G1, and in some tubes the relative placing would allow for several consecutive screen turns to be open to the cathode. (I am always collecting failed tubes and cutting them open.)
But back to UL vs triodes, if Kathodyne has not yet thrown up his hands in despair. I will be posting the promised figures shortly.
some of the low-price tubes like 50L6 may have been assembled on worn-out lathes to keep costs down.
That's not the problem. The 50L6 (and its 7-pin miniature cousin, the 50C5) were designed for use with the "suicide box" topology of directly rectifying the AC main without a power xfmr. That means low voltage, and the 50L6 (50C5) are rated at 110V(DC). It can get out a few watts, but the linearity is horrible.
I've seen some designs, done for S's 'n' G's, that attempt to improve the audio performance. What these do is bust the specs, and run them at 250V(DC), with the plate current reduced to keep the dissipation within ratings. Furthermore, these designs tend to resemble a solid state design: preamp -> VAS -> 50[CL][56], and use way more feedback than is normal for a VT design (40db+) to linearize those beasts. Still, the performance leaves a lot to be desired. Often offered as low power guitar amps since it's necessary to limit the bandwidth to prevent the OPT's from going funky with all that NFB.
That's not the problem. The 50L6 (and its 7-pin miniature cousin, the 50C5) were designed for use with the "suicide box" topology of directly rectifying the AC main without a power xfmr. That means low voltage, and the 50L6 (50C5) are rated at 110V(DC). It can get out a few watts, but the linearity is horrible.
I've seen some designs, done for S's 'n' G's, that attempt to improve the audio performance. What these do is bust the specs, and run them at 250V(DC), with the plate current reduced to keep the dissipation within ratings. Furthermore, these designs tend to resemble a solid state design: preamp -> VAS -> 50[CL][56], and use way more feedback than is normal for a VT design (40db+) to linearize those beasts. Still, the performance leaves a lot to be desired. Often offered as low power guitar amps since it's necessary to limit the bandwidth to prevent the OPT's from going funky with all that NFB.
How on earth did they get that round a (I suppose not the best quality) output transformer? I thought going past 26 dB was pushing it.
OK, I found the original web site, Here. As you can see from the schematic, there is one helluvalot of open loop gain there. Considering that a 50C5 (and cousins) typically work at a V(gk)= -7.5V (15Vp-p audio) you don't need that much open loop gain unless you're really going to pour on the NFB, which is what he does.
As for how to get that around an OPT, he limits the bandwidth at both ends so as to avoid the large phase shifts that occur at frequency extremes. This, in turn, makes the thing useless for almost everything but a guitar amp.
As I said before, it's mainly a "let's see if I can do it" type project. There are way better tubes for an amp of that power level (6AQ5A -- down-rated 6V6 essentially -- would be far better).
Schematic:
OK, I found the original web site, Here. As you can see from the schematic, there is one helluvalot of open loop gain there. Considering that a 50C5 (and cousins) typically work at a V(gk)= -7.5V (15Vp-p audio) you don't need that much open loop gain unless you're really going to pour on the NFB, which is what he does.
As for how to get that around an OPT, he limits the bandwidth at both ends so as to avoid the large phase shifts that occur at frequency extremes. This, in turn, makes the thing useless for almost everything but a guitar amp.
As I said before, it's mainly a "let's see if I can do it" type project. There are way better tubes for an amp of that power level (6AQ5A -- down-rated 6V6 essentially -- would be far better).
Schematic:
it'll stay a triode!
Hello,
Monday i went to Mattijs de Vries (www.machmat.com), he did some measurements on my amp.....
bandwidth -3b: @10hz-@80khz
output (triode PP): approx 12w @62,5ma 380v
distortion: 0,68% @70% output
-distortion is exactly symmetrical- many thanx to the longtailed pair splitter with current source (5965 @100v 5ma)
Some changes have been made to the amp:
OLD PSU = 2xpy500A - 47uf - 10H - 300uf - 1k5 - 82uf
NEW PSU= 2xpy500A - 220nF - 47uf -10H - 450uf - 220nf - 3k3 - 82uf - 220nf
the increase of capacitance for the output stage with the adding of little fast ones solved the bass issue...got plenty now!
the grid resistors of the 7027a from metalfilm to allen&bradley's
the current has been turned op to 70ma
and the coupling capacitors have been changed from auricap to PIO
Sorry for eventually subjective use of language; i find it very difficult to translate what i hear into words....here goes
these changes together make the amp sound really nice!! i'm listening to my music collection non-stop!! the PIO condensers were a little bit cloudy at first and things got a little unclear (although high freq backgroundsounds in music were like 3d immediately!!!) now after 2 days they're beginning to show their potency...it's gettin better!!
And a question....why feedback?? it's an error solving device...you send your signal back into the parts line, thus getting more 'parts sound'; stability problems; loss of sound...try not to make an error....then you don't have to correct it.
I'd like to see what you people think about feedback
-@prr@cerrem@johan potgieter : i very much appreciate your answers and the loads of information that i can look into because of them....
Unfortunately i just started with tubes about 2 years ago; with no technical/electrical study whatsoever...
Now i can calculate a common cathode -simple- schematic and i know what is going on in my amp and why things happen.
this is because i read and i read and i read and measure my amp and try to calculate these measurements...so i'm collecting bits of information and try to link these together with common sense and formulas i find...
the point? i try to understand but am not able to give an answer that makes sense
So, i wonder are there people who'd like to device some tube-setups which can be built at home by beginners....then they answer the questions given...which could be corrected and faults in thinking about the tubes would come above...a bit like a tube course?? in addition to the info for newbies (say moderates
)
Hello,
Monday i went to Mattijs de Vries (www.machmat.com), he did some measurements on my amp.....
bandwidth -3b: @10hz-@80khz
output (triode PP): approx 12w @62,5ma 380v
distortion: 0,68% @70% output
-distortion is exactly symmetrical- many thanx to the longtailed pair splitter with current source (5965 @100v 5ma)
Some changes have been made to the amp:
OLD PSU = 2xpy500A - 47uf - 10H - 300uf - 1k5 - 82uf
NEW PSU= 2xpy500A - 220nF - 47uf -10H - 450uf - 220nf - 3k3 - 82uf - 220nf
the increase of capacitance for the output stage with the adding of little fast ones solved the bass issue...got plenty now!
the grid resistors of the 7027a from metalfilm to allen&bradley's
the current has been turned op to 70ma
and the coupling capacitors have been changed from auricap to PIO
Sorry for eventually subjective use of language; i find it very difficult to translate what i hear into words....here goes
these changes together make the amp sound really nice!! i'm listening to my music collection non-stop!! the PIO condensers were a little bit cloudy at first and things got a little unclear (although high freq backgroundsounds in music were like 3d immediately!!!) now after 2 days they're beginning to show their potency...it's gettin better!!
And a question....why feedback?? it's an error solving device...you send your signal back into the parts line, thus getting more 'parts sound'; stability problems; loss of sound...try not to make an error....then you don't have to correct it.
I'd like to see what you people think about feedback
-@prr@cerrem@johan potgieter : i very much appreciate your answers and the loads of information that i can look into because of them....
Unfortunately i just started with tubes about 2 years ago; with no technical/electrical study whatsoever...
Now i can calculate a common cathode -simple- schematic and i know what is going on in my amp and why things happen.
this is because i read and i read and i read and measure my amp and try to calculate these measurements...so i'm collecting bits of information and try to link these together with common sense and formulas i find...
the point? i try to understand but am not able to give an answer that makes sense
So, i wonder are there people who'd like to device some tube-setups which can be built at home by beginners....then they answer the questions given...which could be corrected and faults in thinking about the tubes would come above...a bit like a tube course?? in addition to the info for newbies (say moderates
)
And a question....why feedback?? it's an error solving device...you send your signal back into the parts line, thus getting more 'parts sound'; stability problems; loss of sound...try not to make an error....then you don't have to correct it.
I'd like to see what you people think about feedback
Yes, feedback does correct for errors, amoung other things. Additional benefits include noise reduction, increased bandwidth, and reduced output impedance -- all important. However, it is not a panacea for all ills.
The A Number One problem with solid state is that transistors have incredible gains. Small signal triodes max out at amplification factors of about 100 (6AV6, 12AX7A, 6SF5). High gain pentodes max out at around u= 1100 (12BY7A, 6AG7). A common BJT, even run at emitter currents as low as 1.0mA, will have u= 3076. Run that current up to 50mA, and u= 153846. Having all that gain available is a great temptation to just slap a SS circuit together, and if the numbers come out looking awful, just pour on the feedback. (In the 1950s this was also a problem, though to a lesser extent, with VT designs as well.) Force the numbers to add up, but the sonics are uniformly terrible.
However, judicious use of feedback, after first making sure that the open loop operation has been made as good as practical, can work wonders. This corrects for inherent non-linearities that always exist. Tubes are good, but not perfect. It can also help linearize the otherwise non-linear behaviour of output matching xfmrs. For designs that use pentodes, feedback can certainly help with speaker damping by reducing impedance. (Corrects for exaggerated bass response)
So, i wonder are there people who'd like to device some tube-setups which can be built at home by beginners....then they answer the questions given...which could be corrected and faults in thinking about the tubes would come above...a bit like a tube course??
Not so easy as with transistors, which are real easy to prototype with on a solderless prototyping board. Furthermore, I don't think it's such a good idea given the voltages involved. Every "bread board" set-up I've seen on the 'Net looks like an accident waiting to happen.
Much better to build the circuit in place with tack soldering. Then you can do corrections/optimizations, and finally complete the circuit once you have it operating satisfactorily. That way, you won't be left wondering where the high voltage is. It's ceratinly safer than the rat's nests of wiring I often see with VT bread boards.
I'd like to see what you people think about feedback
Yes, feedback does correct for errors, amoung other things. Additional benefits include noise reduction, increased bandwidth, and reduced output impedance -- all important. However, it is not a panacea for all ills.
The A Number One problem with solid state is that transistors have incredible gains. Small signal triodes max out at amplification factors of about 100 (6AV6, 12AX7A, 6SF5). High gain pentodes max out at around u= 1100 (12BY7A, 6AG7). A common BJT, even run at emitter currents as low as 1.0mA, will have u= 3076. Run that current up to 50mA, and u= 153846. Having all that gain available is a great temptation to just slap a SS circuit together, and if the numbers come out looking awful, just pour on the feedback. (In the 1950s this was also a problem, though to a lesser extent, with VT designs as well.) Force the numbers to add up, but the sonics are uniformly terrible.
However, judicious use of feedback, after first making sure that the open loop operation has been made as good as practical, can work wonders. This corrects for inherent non-linearities that always exist. Tubes are good, but not perfect. It can also help linearize the otherwise non-linear behaviour of output matching xfmrs. For designs that use pentodes, feedback can certainly help with speaker damping by reducing impedance. (Corrects for exaggerated bass response)
So, i wonder are there people who'd like to device some tube-setups which can be built at home by beginners....then they answer the questions given...which could be corrected and faults in thinking about the tubes would come above...a bit like a tube course??
Not so easy as with transistors, which are real easy to prototype with on a solderless prototyping board. Furthermore, I don't think it's such a good idea given the voltages involved. Every "bread board" set-up I've seen on the 'Net looks like an accident waiting to happen.
Much better to build the circuit in place with tack soldering. Then you can do corrections/optimizations, and finally complete the circuit once you have it operating satisfactorily. That way, you won't be left wondering where the high voltage is. It's ceratinly safer than the rat's nests of wiring I often see with VT bread boards.
On Feedback
No need to feel "inadequate". Let me just say that I have been in design for 50 years (not bragging, lamenting!) and it would sure seem natural that one has picked up a few things along the way. Your turn will come.
There are whole chapters on this, thus a little difficult to explain in this space! Perhaps just a few warnings: Negative feedback is the most powerful tool available to improve an amp, and thus also the most abused. Be careful of some of the junk you will find expressed in this regard, especially on the internet. (I am referring to the web in general, not specifically to the DIY Audio site.) There is excellent advice, but also nonsense, and it may be difficult to notice the difference.
No amplifier is absolutely linear, and (negative) feedback simply linearises the design, thus decreasing distortion. Because it is self-controlling, it can never be perfect. There is the "feedback is bad" brigade. Feedback is only bad when too much is used with circuits that are inferior to begin with. But there can be stability problems at either end of the audio band, where the fed-back signal can eventually go in-phase because of phase shifts caused by mainly capacitors (also the output transformer in tube circuits). This will result in oscillation, or at least in spurious instability under certain signal conditions.
It is the job of the designer to see that response at band-ends is such that stability is always assured, and there are simple rules to achieve this. The basics are: The circuit should, before feedback, have negligible phase shift over the audio band. Then feedback is applied and one can perform the necessary tricks outside the audio band to keep things in order. This is relatively simple, and proven principles have been there since the beginning of electronic amplification. The distortion generated in any proper design should be low enough before feedback, that no more than 26 - 30 dB is needed to render an entirely acceptable product (that is a voltage factor of 20 - 30). I repeat that this is entirely feasible; it has been done in a multitude of successful designs, both tube and semiconductor, over many decades.
You mention putting the signal through the same imperfect components several times: Don't be worried about that. It is a very pious idea (one of many floating around) that since no component is perfect, every component must deteriorate the signal. Nonsense. Our ears are extremely sensitive to certain artifacts, but are mercifully still limited. The "imperfections" caused by good quality modern resistors and capacitors are orders of magnitude below this. E.g. the "extra" deterioration caused by the few extra components used to implement feedback may be say 0.1%, while the gain in quality is the feedback factor, say 20x. So there.
There are a few successful tube designs (I must watch the length of this) which I can pass on (perhaps off-site); practical construction will depend on available components, especially the output transformer. Semiconductor designs are freely available, e.g. the Douglas Self designs, but also others.
As said this topic cannot be covered here up to design level, but I trust these few basic remarks helped. I am certain there are other honourable members on-site that can comment.
No need to feel "inadequate". Let me just say that I have been in design for 50 years (not bragging, lamenting!) and it would sure seem natural that one has picked up a few things along the way. Your turn will come.
There are whole chapters on this, thus a little difficult to explain in this space! Perhaps just a few warnings: Negative feedback is the most powerful tool available to improve an amp, and thus also the most abused. Be careful of some of the junk you will find expressed in this regard, especially on the internet. (I am referring to the web in general, not specifically to the DIY Audio site.) There is excellent advice, but also nonsense, and it may be difficult to notice the difference.
No amplifier is absolutely linear, and (negative) feedback simply linearises the design, thus decreasing distortion. Because it is self-controlling, it can never be perfect. There is the "feedback is bad" brigade. Feedback is only bad when too much is used with circuits that are inferior to begin with. But there can be stability problems at either end of the audio band, where the fed-back signal can eventually go in-phase because of phase shifts caused by mainly capacitors (also the output transformer in tube circuits). This will result in oscillation, or at least in spurious instability under certain signal conditions.
It is the job of the designer to see that response at band-ends is such that stability is always assured, and there are simple rules to achieve this. The basics are: The circuit should, before feedback, have negligible phase shift over the audio band. Then feedback is applied and one can perform the necessary tricks outside the audio band to keep things in order. This is relatively simple, and proven principles have been there since the beginning of electronic amplification. The distortion generated in any proper design should be low enough before feedback, that no more than 26 - 30 dB is needed to render an entirely acceptable product (that is a voltage factor of 20 - 30). I repeat that this is entirely feasible; it has been done in a multitude of successful designs, both tube and semiconductor, over many decades.
You mention putting the signal through the same imperfect components several times: Don't be worried about that. It is a very pious idea (one of many floating around) that since no component is perfect, every component must deteriorate the signal. Nonsense. Our ears are extremely sensitive to certain artifacts, but are mercifully still limited. The "imperfections" caused by good quality modern resistors and capacitors are orders of magnitude below this. E.g. the "extra" deterioration caused by the few extra components used to implement feedback may be say 0.1%, while the gain in quality is the feedback factor, say 20x. So there.
There are a few successful tube designs (I must watch the length of this) which I can pass on (perhaps off-site); practical construction will depend on available components, especially the output transformer. Semiconductor designs are freely available, e.g. the Douglas Self designs, but also others.
As said this topic cannot be covered here up to design level, but I trust these few basic remarks helped. I am certain there are other honourable members on-site that can comment.
It seems i've been talking with the 'feedback is bad brigade' !!!!
So, every tube and transistor will have non-linearity...gain of quality is much bigger than the loss by injecting it through the components again (but it will encounter the non-linearity of the tube again.......)
is feedback always in the region of 20-30db??? because that would mean you'll get 6-9 times less output (-3db is halving the power)
So it seems the trick would be to build an amp that is very stable and lineair of itsself and just implement as little feedback as possible.....
(i don't have a clue how to use nfb, so it will be without for now)
to get the most linearity use a current-source on the output and input tubes (as this has the effect of horizontalizing the load line)??? and then one could implement if necessary a little feedback....
@johan potgieter,@miles prower: i was not thinking of a bread-bord....the amps shall be built on wood and with point to point soldering (like always with a test-amp).
it doen't have to be a complete amp, it can be just one stage...
I am still very interested in some 'test'- questions (off-site)
I really want to grasp this...and am very worried that i will teach myself 'wrong' things
btw.: I Respect voltages!! and always work with one hand!!
@johan potgieter: I've seen that outpout xfrmr'can cause oscillation; the original xfrmr in the schematic i built caused this...it has been changed from 10k to 6k and....no more problems!
So, every tube and transistor will have non-linearity...gain of quality is much bigger than the loss by injecting it through the components again (but it will encounter the non-linearity of the tube again.......)
is feedback always in the region of 20-30db??? because that would mean you'll get 6-9 times less output (-3db is halving the power)
So it seems the trick would be to build an amp that is very stable and lineair of itsself and just implement as little feedback as possible.....
(i don't have a clue how to use nfb, so it will be without for now)
to get the most linearity use a current-source on the output and input tubes (as this has the effect of horizontalizing the load line)??? and then one could implement if necessary a little feedback....
@johan potgieter,@miles prower: i was not thinking of a bread-bord....the amps shall be built on wood and with point to point soldering (like always with a test-amp).
it doen't have to be a complete amp, it can be just one stage...
I am still very interested in some 'test'- questions (off-site)
I really want to grasp this...and am very worried that i will teach myself 'wrong' things
btw.: I Respect voltages!! and always work with one hand!!
@johan potgieter: I've seen that outpout xfrmr'can cause oscillation; the original xfrmr in the schematic i built caused this...it has been changed from 10k to 6k and....no more problems!
A few more thoughts . . .
A little more technical: Worst-case distortion occurs when the device internal resistance (Rp in tubes) equals the load resistor. Thus, typically with pentodes or transistors where the Rp is high one uses a load resistor significantly lower in value. With low mu triodes (there is no transistor equivalent) one uses a relatively high load resistor (but there are also other factors governing these values). Linearity will be improved with current sources for loads, but again one cannot simply do that and then create a low output impedance with feedback where such is required. Low impedances created with lots of feedback have their own pitfalls - nothing in this game is ever simple! Doing the above in an output stage to get a high damping factor can lead to all sorts of nasties when the load is a loudspeaker. Prof Matti Otala wrote a landmark article on this (the amplifier-loudspeaker interface) some 25 years ago, but again this will become long and boring for some to fully explain here.
Feedback: Don't see this as passing the signal around and around. In its simplest form, where the gain is high the fed back signal is high, where the gain is low the fed back signal is low. Thus the result with feedback is more linear. The fed back signal can never completely cancel the input, then there will be no output! Thus the fed back signal cancels at most part of the problem, although it can be a large part.
Feedback is not always 20 - 26 dB. Some semiconductor amps use 60 - 70 dB! But to me that is an indication that something is wrong; why is so much needed? The above figure mainly comes from the fact that a well designed amplifier will have distortion of the order of 1 - 2% before global feedback. If we consider that distortion of 0.05% is inaudible, that gives a factor of 20 (26 dB). But I must immediately add that this is an over-simplified statement. The nature of the distortion (high-order harmonics or not plus many other factors) plays a large role.
I must also not generate a false impression re output transformers. When a specific transformer causes oscillations in a certain circuit, it does not necessarily mean that the transformer is inferior. It simply means that the loop phase shifts must be analysed and stability restored by the proper choice of phase correcting components. E.g. when a wrong value capacitor causes oscillations, it does not mean that there is a fault with the capacitor, only with the value.
I am by no means the only one (or the best) here with knowledge, but should you desire you may contact me direct (off-site) with questions, where matters can be explained without the limitation of space.
A little more technical: Worst-case distortion occurs when the device internal resistance (Rp in tubes) equals the load resistor. Thus, typically with pentodes or transistors where the Rp is high one uses a load resistor significantly lower in value. With low mu triodes (there is no transistor equivalent) one uses a relatively high load resistor (but there are also other factors governing these values). Linearity will be improved with current sources for loads, but again one cannot simply do that and then create a low output impedance with feedback where such is required. Low impedances created with lots of feedback have their own pitfalls - nothing in this game is ever simple! Doing the above in an output stage to get a high damping factor can lead to all sorts of nasties when the load is a loudspeaker. Prof Matti Otala wrote a landmark article on this (the amplifier-loudspeaker interface) some 25 years ago, but again this will become long and boring for some to fully explain here.
Feedback: Don't see this as passing the signal around and around. In its simplest form, where the gain is high the fed back signal is high, where the gain is low the fed back signal is low. Thus the result with feedback is more linear. The fed back signal can never completely cancel the input, then there will be no output! Thus the fed back signal cancels at most part of the problem, although it can be a large part.
Feedback is not always 20 - 26 dB. Some semiconductor amps use 60 - 70 dB! But to me that is an indication that something is wrong; why is so much needed? The above figure mainly comes from the fact that a well designed amplifier will have distortion of the order of 1 - 2% before global feedback. If we consider that distortion of 0.05% is inaudible, that gives a factor of 20 (26 dB). But I must immediately add that this is an over-simplified statement. The nature of the distortion (high-order harmonics or not plus many other factors) plays a large role.
I must also not generate a false impression re output transformers. When a specific transformer causes oscillations in a certain circuit, it does not necessarily mean that the transformer is inferior. It simply means that the loop phase shifts must be analysed and stability restored by the proper choice of phase correcting components. E.g. when a wrong value capacitor causes oscillations, it does not mean that there is a fault with the capacitor, only with the value.
I am by no means the only one (or the best) here with knowledge, but should you desire you may contact me direct (off-site) with questions, where matters can be explained without the limitation of space.
> A common BJT, even run at emitter currents as low as 1.0mA, will have u= 3076. Run that current up to 50mA, and u= 153846.
I think you will find that Early effect limits BJT Mu to 300-3,000, at any current. Certainly not 150K.
Of course for another $0.02, you can cascode to Mu of a million.
> It is the job of the designer to see ...that stability is always assured, ... proven principles have been there since the beginning of electronic amplification.
Just to be pedantic: we had electronic amplification and even some use of feedback before Black and others laid out the theory of why it works and what criteria control stability. There's maybe a 10 year gap between wide use of electronic amplifiers and wide use of feedback theory.
OTOH, you could argue that feedback principles were discovered long before electrons. The speed governor on a steam engine is a feedback system, and sure can become unstable. But simple low-gain systems can usually be made stable with simple hacks; tubes and near-perfect electric reactances made fancy instability possible and easy, and easier to analyze than practical mechanical systems.
> So it seems the trick would be to..... ...... ...... ......
There is no trick! Just different ways to skin the cat. Fashion plays a large part. Whatever you do today will be laughed-at in 20 years, and may be revered in 40 years. Who remembers 1970, the race to point zero-zero-zero THD? Who knew that 20 years later naked SETs with 10%THD would be cool again? Losing as many coupling transformers as possible was one of the early "optimizations", yet iron is in again.
Do what you feel comfortable with.
> blast from the past
I respect Crowhurst and feel everybody should read about everything he wrote, he too was blown around by the winds of fashion. Less than others, because he pretty much knew what he wrote about, but take it as the view of one man at one time.
> just implement as little feedback as possible.....
My current fashion sense says that "small feedback" is bad. Either use NONE, or use a LOT.
Injecting a little output back to the input does raise the issue of re-re-distorting the signal. IM distortion on a simple one-stage amplifier actually rises for 6dB NFB. It may not get back down to no-NFB levels until you get to 30dB-40dB NFB. I think that is an over-simplification, and I know I have enjoyed many low-NFB amps. It is pretty inevitable using pentodes on modern speakers: you need NFB for damping, but you can't get huge NFB around an output transformer.
> Worst-case distortion occurs when the device internal resistance (Rp in tubes) equals the load resistor.
I don't see it that way.
For triodes, THD is caused by current swing, and reduced by plate voltage swing. In small-signal work, you can just make signal current swing small compared to idle current. In large signal work, economics force you to a large signal current swing. Then THD is reduced by large plate voltage swing, i.e. hi-Z loading, and is worst for low-Z loads.
For the other devices, "Rp" is meaningless in power stages. (It may set the no-load gain and this one corner of the stability envelope for a practical amplifier, but becomes irrelevant under load.)
For pentodes, for medium power, percent current swing determines THD, and plate voltage swing has no THD reduction effect. For maximum power, you cut down into the knee of the pentode curve, which has a reverse curvature to the main current-swing nonlinearity. The reverse curve part-cancels the 2nd but raises the 3rd. For best power at lowest THD number, you pick a point in the knee that gives best THD cancellation. This is a paper exercise: THD will vary with minor change of load shifting you to either side of the knee. Also lowest THD is not best-sound: the 2nd is inoffensive, the 3rd is edgy.
For BJTs and FETs, the knee is so low-volt and so sharp that no THD cancellation is possible. Particularly for BJTs, the distortion due to current swing is so bad that we never run them without feedback, at least local feedback, which becomes a whole different problem.
I think you will find that Early effect limits BJT Mu to 300-3,000, at any current. Certainly not 150K.
Of course for another $0.02, you can cascode to Mu of a million.
> It is the job of the designer to see ...that stability is always assured, ... proven principles have been there since the beginning of electronic amplification.
Just to be pedantic: we had electronic amplification and even some use of feedback before Black and others laid out the theory of why it works and what criteria control stability. There's maybe a 10 year gap between wide use of electronic amplifiers and wide use of feedback theory.
OTOH, you could argue that feedback principles were discovered long before electrons. The speed governor on a steam engine is a feedback system, and sure can become unstable. But simple low-gain systems can usually be made stable with simple hacks; tubes and near-perfect electric reactances made fancy instability possible and easy, and easier to analyze than practical mechanical systems.
> So it seems the trick would be to..... ...... ...... ......
There is no trick! Just different ways to skin the cat. Fashion plays a large part. Whatever you do today will be laughed-at in 20 years, and may be revered in 40 years. Who remembers 1970, the race to point zero-zero-zero THD? Who knew that 20 years later naked SETs with 10%THD would be cool again? Losing as many coupling transformers as possible was one of the early "optimizations", yet iron is in again.
Do what you feel comfortable with.
> blast from the past
I respect Crowhurst and feel everybody should read about everything he wrote, he too was blown around by the winds of fashion. Less than others, because he pretty much knew what he wrote about, but take it as the view of one man at one time.
> just implement as little feedback as possible.....
My current fashion sense says that "small feedback" is bad. Either use NONE, or use a LOT.
Injecting a little output back to the input does raise the issue of re-re-distorting the signal. IM distortion on a simple one-stage amplifier actually rises for 6dB NFB. It may not get back down to no-NFB levels until you get to 30dB-40dB NFB. I think that is an over-simplification, and I know I have enjoyed many low-NFB amps. It is pretty inevitable using pentodes on modern speakers: you need NFB for damping, but you can't get huge NFB around an output transformer.
> Worst-case distortion occurs when the device internal resistance (Rp in tubes) equals the load resistor.
I don't see it that way.
For triodes, THD is caused by current swing, and reduced by plate voltage swing. In small-signal work, you can just make signal current swing small compared to idle current. In large signal work, economics force you to a large signal current swing. Then THD is reduced by large plate voltage swing, i.e. hi-Z loading, and is worst for low-Z loads.
For the other devices, "Rp" is meaningless in power stages. (It may set the no-load gain and this one corner of the stability envelope for a practical amplifier, but becomes irrelevant under load.)
For pentodes, for medium power, percent current swing determines THD, and plate voltage swing has no THD reduction effect. For maximum power, you cut down into the knee of the pentode curve, which has a reverse curvature to the main current-swing nonlinearity. The reverse curve part-cancels the 2nd but raises the 3rd. For best power at lowest THD number, you pick a point in the knee that gives best THD cancellation. This is a paper exercise: THD will vary with minor change of load shifting you to either side of the knee. Also lowest THD is not best-sound: the 2nd is inoffensive, the 3rd is edgy.
For BJTs and FETs, the knee is so low-volt and so sharp that no THD cancellation is possible. Particularly for BJTs, the distortion due to current swing is so bad that we never run them without feedback, at least local feedback, which becomes a whole different problem.
Who remembers 1970, the race to point zero-zero-zero THD? Who knew that 20 years later naked SETs with 10%THD would be cool again? Losing as many coupling transformers as possible was one of the early "optimizations", yet iron is in again.
The one thing is probably related to the other. That monomaniacal obsession with ultra-low THD numbers (blame the marketing weenies for that) led to a generation of bad sound. Folks decided to try something different. As for "iron is in again", today's iron is better than ever. Lundahl/MagnaQuest/OneElectron/Sowter/etc. all have access to exotic materials that didn't exist back when they were trying to eliminate as much "iron" as possible.
I respect Crowhurst and feel everybody should read about everything he wrote, he too was blown around by the winds of fashion. Less than others, because he pretty much knew what he wrote about, but take it as the view of one man at one time.
Suspect the demands of the "bottom line" here, more so than "fashion". Good practices remain good practices regardless of what absurd ideas the public-at-large falls for. At one time, everyone wanted SW receivers that came loaded with enormous amounts of gain ahead of the first detector. That remains a bad practice, and you don't need any front end gain (and even then, just a few db's) above 40m (and then only if you are blessed with an extraordinarily quiet QTH). You don't have to fall for anything: if it doesn't improve performance, then don't do it. If it helps, then go with that.
That's the advantage of DiY.
The one thing is probably related to the other. That monomaniacal obsession with ultra-low THD numbers (blame the marketing weenies for that) led to a generation of bad sound. Folks decided to try something different. As for "iron is in again", today's iron is better than ever. Lundahl/MagnaQuest/OneElectron/Sowter/etc. all have access to exotic materials that didn't exist back when they were trying to eliminate as much "iron" as possible.
I respect Crowhurst and feel everybody should read about everything he wrote, he too was blown around by the winds of fashion. Less than others, because he pretty much knew what he wrote about, but take it as the view of one man at one time.
Suspect the demands of the "bottom line" here, more so than "fashion". Good practices remain good practices regardless of what absurd ideas the public-at-large falls for. At one time, everyone wanted SW receivers that came loaded with enormous amounts of gain ahead of the first detector. That remains a bad practice, and you don't need any front end gain (and even then, just a few db's) above 40m (and then only if you are blessed with an extraordinarily quiet QTH). You don't have to fall for anything: if it doesn't improve performance, then don't do it. If it helps, then go with that.
That's the advantage of DiY.
The dangers of efforts to be brief and not waste readers' time with with extensive cutting/pasting of previous narrative. . . . . .
PRR: I don't feel that we DISagree, rather that you went W - A - Y further than I did. I tried to reply very basically to requests from Kathodyne (posts #27/28) as a starting point - only! I believe one should first get the basics right before advancing to specific applications, and I got the impression that that was what he wanted at the time.
You said that you do not see the matter of R.int/R.load the same as I do, but in your (valid) expansion, for a triode high voltage/small current for least distortion conformed to what I said, and for pentodes the opposite - basically! I commented along tube lines since that was the subject initially. Anyway, let us not enter into a sparring match over this - not my desire. There are obviously specific ways of dealing with every case, after one has the basics right. We also agree, I think that THD is useless especially with semiconductor circuits; the various harmonic products present are what counts.
To move on, the "development" over the years astounds. Single stage output triodes were superceded by push-pull because of obvious sonic advantages, but now we are back with SET - exactly the same thing as 70 years ago! (We are now rather in the thread "The many faces of distortion", but these things are never watertight). I think it is recognised that the reason is that a small quantity of 2nd and 3rd "additions" actually add to the so-called sweetness/musicality sound of this branch - not all distortion is objectionable! . The signal is no longer an exact replica of the original - but it is the customer's money, which he can spend any way he pleases.
But a few years ago HiFi and RV published a list of the 5 best equipment reviewed by some 5 of their top critics, plus their personal choice. One may presume that those gentlemen knew something about realism and faithful reproduction. Yet the top preferences of some did not even appear on the list of others! As a designer, for whose taste must I design? One gets nowhere with this; one can merely design a "clean" amplifier and hope the customer is satisfied. Perhaps I was singularly lucky in that so far all my customers were highly appreciative. Individuals can of course tinker and adapt to their taste.
About what happens at band ends (specifically the high end); this is again a quite complex story, but many of the compromising results can be attributed to what happens to super-audio signals. They operate in a part of the frequency region above say 20 KHz, where they can cause severe (audible) difference products, as has already been alluded to here. I personally believe that this is the reason for much of listener fatigue, with CDs capable of containing a whole band of signals up there. For this reason I am a staunch supporter of proper filtering (cut-off) to cut anything above 20 KHz. Blind tests did reveal that greater satisfaction was obtained with such filters in place than without them. It will absolutely depend on amplifier design, but I am rather safe than sorry.
PRR: I don't feel that we DISagree, rather that you went W - A - Y further than I did. I tried to reply very basically to requests from Kathodyne (posts #27/28) as a starting point - only! I believe one should first get the basics right before advancing to specific applications, and I got the impression that that was what he wanted at the time.
You said that you do not see the matter of R.int/R.load the same as I do, but in your (valid) expansion, for a triode high voltage/small current for least distortion conformed to what I said, and for pentodes the opposite - basically! I commented along tube lines since that was the subject initially. Anyway, let us not enter into a sparring match over this - not my desire. There are obviously specific ways of dealing with every case, after one has the basics right. We also agree, I think that THD is useless especially with semiconductor circuits; the various harmonic products present are what counts.
To move on, the "development" over the years astounds. Single stage output triodes were superceded by push-pull because of obvious sonic advantages, but now we are back with SET - exactly the same thing as 70 years ago! (We are now rather in the thread "The many faces of distortion", but these things are never watertight). I think it is recognised that the reason is that a small quantity of 2nd and 3rd "additions" actually add to the so-called sweetness/musicality sound of this branch - not all distortion is objectionable! . The signal is no longer an exact replica of the original - but it is the customer's money, which he can spend any way he pleases.
But a few years ago HiFi and RV published a list of the 5 best equipment reviewed by some 5 of their top critics, plus their personal choice. One may presume that those gentlemen knew something about realism and faithful reproduction. Yet the top preferences of some did not even appear on the list of others! As a designer, for whose taste must I design? One gets nowhere with this; one can merely design a "clean" amplifier and hope the customer is satisfied. Perhaps I was singularly lucky in that so far all my customers were highly appreciative. Individuals can of course tinker and adapt to their taste.
About what happens at band ends (specifically the high end); this is again a quite complex story, but many of the compromising results can be attributed to what happens to super-audio signals. They operate in a part of the frequency region above say 20 KHz, where they can cause severe (audible) difference products, as has already been alluded to here. I personally believe that this is the reason for much of listener fatigue, with CDs capable of containing a whole band of signals up there. For this reason I am a staunch supporter of proper filtering (cut-off) to cut anything above 20 KHz. Blind tests did reveal that greater satisfaction was obtained with such filters in place than without them. It will absolutely depend on amplifier design, but I am rather safe than sorry.
> let us not enter into a sparring match over this
Oh, foo, I didn't mean to be so in-yer-face.
But if I make an objection, I "need" to be sure of it, and sometimes I think out loud at excessive length.
But I still must object to "Worst-case distortion occurs when the device internal resistance (Rp in tubes) equals the load resistor." It implies that low distortion operation is possible in low-Z, lower than when Rl=Rp.
In-context (apparently push-pull), I don't object much. I must have had SET on the mind. In push-pull, Rl matters much less except as it kills power.
When you get to Pentodes(*), Rp in a loaded power-amp is just meaningless. No useful Rl is anywhere near Rp, nor is there any good ratio of Rl/Rp. (At least for basic design; the way Rp falls around the knee in some EL-series tubes is interesting to guitarists who live around the knee.) (* "Pentode" here includes BJTs and FETs, though UltraLinear is in-between.)
> not all distortion is objectionable!
Not even audible. Yet other distortion grates even at very low level.
> Single stage output triodes were superceded by push-pull because of obvious sonic advantages
I'm not sure of that. No hard evidence. I wasn't actually there. But I think P-P soon dominated "high power" work because it was cheaper, and because it measured "lower THD". In short: To Save Money. But is 2% 3rd harmonic less offensive than 5% 2nd harmonic? Depends a LOT on the music, and a lot on your experiences (I've had phases where I loved the 3rd on the right music). But my guess is that 5% 2nd "should be" less-nasty than 2% 3rd, as a general rule.
> As a designer, for whose taste must I design?
Not being personal, but in general: Assuming you design for profit, you have two options. Maximum buzz-words (high selling price), or maximum value for money (high volume). While "clean" is a generally safe goal, I'm not even sure that is what all buyers want. Or rather: a bit of an "edge" will sometimes stand-out as "better", at least to somebody. While I like to theorize the SET fad is about low-low high harmonics and low musical-IMD, I do suspect the sweet 2nd going into 3rd at 2 Watts is part of the attraction for people who grew up on "clean" amplifiers (unlike the 50L6 of my youth).
Oh, foo, I didn't mean to be so in-yer-face.
But if I make an objection, I "need" to be sure of it, and sometimes I think out loud at excessive length.
But I still must object to "Worst-case distortion occurs when the device internal resistance (Rp in tubes) equals the load resistor." It implies that low distortion operation is possible in low-Z, lower than when Rl=Rp.
In-context (apparently push-pull), I don't object much. I must have had SET on the mind. In push-pull, Rl matters much less except as it kills power.
When you get to Pentodes(*), Rp in a loaded power-amp is just meaningless. No useful Rl is anywhere near Rp, nor is there any good ratio of Rl/Rp. (At least for basic design; the way Rp falls around the knee in some EL-series tubes is interesting to guitarists who live around the knee.) (* "Pentode" here includes BJTs and FETs, though UltraLinear is in-between.)
> not all distortion is objectionable!
Not even audible. Yet other distortion grates even at very low level.
> Single stage output triodes were superceded by push-pull because of obvious sonic advantages
I'm not sure of that. No hard evidence. I wasn't actually there. But I think P-P soon dominated "high power" work because it was cheaper, and because it measured "lower THD". In short: To Save Money. But is 2% 3rd harmonic less offensive than 5% 2nd harmonic? Depends a LOT on the music, and a lot on your experiences (I've had phases where I loved the 3rd on the right music). But my guess is that 5% 2nd "should be" less-nasty than 2% 3rd, as a general rule.
> As a designer, for whose taste must I design?
Not being personal, but in general: Assuming you design for profit, you have two options. Maximum buzz-words (high selling price), or maximum value for money (high volume). While "clean" is a generally safe goal, I'm not even sure that is what all buyers want. Or rather: a bit of an "edge" will sometimes stand-out as "better", at least to somebody. While I like to theorize the SET fad is about low-low high harmonics and low musical-IMD, I do suspect the sweet 2nd going into 3rd at 2 Watts is part of the attraction for people who grew up on "clean" amplifiers (unlike the 50L6 of my youth).
Incredible thread!
I'm learning a great deal
One of my early teachers ("triode dick") is a non-feedback guy.
i built his kt88 design
Maybe of some value to this thread; i switched from UL to triode to. actually a strapped pentode (?)
I know little about the details, just made the bold move to swap the kt88 for GU50's.
I've been listening to the amp for a few weeks now. I think it sounds good. But then again, it's my first amp and don't know houw good it "should" sound.
Th bass is very neat. No hummy humms and lots of "pop".
But i think it might be a little weak, as my highs sometimes get a little metallic.
I can think of a few scenario's:
- I completely messed up with the GU50's
- The GU50's need bigger coupling caps
- My speakers suck
-I'll have to get used to "real" sound and not the exaggerated bass of commercial amps
I'll post the schematic. Maybe somebody could criticise in a way that makes the output stage better understandible for me and everybody else that's relatively new to this?
Bas
I'm learning a great deal
One of my early teachers ("triode dick") is a non-feedback guy.
i built his kt88 design
Maybe of some value to this thread; i switched from UL to triode to. actually a strapped pentode (?)
I know little about the details, just made the bold move to swap the kt88 for GU50's.
I've been listening to the amp for a few weeks now. I think it sounds good. But then again, it's my first amp and don't know houw good it "should" sound.
Th bass is very neat. No hummy humms and lots of "pop".
But i think it might be a little weak, as my highs sometimes get a little metallic.
I can think of a few scenario's:
- I completely messed up with the GU50's
- The GU50's need bigger coupling caps
- My speakers suck
-I'll have to get used to "real" sound and not the exaggerated bass of commercial amps
I'll post the schematic. Maybe somebody could criticise in a way that makes the output stage better understandible for me and everybody else that's relatively new to this?
Bas
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