Tube power amp simulation

Started by Sheldon, March 05, 2015, 02:42:15 PM

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sajy_ho

#20
It seems nobody from experts gonna explain it so a word from the idiot guy: :icon_mrgreen:

Actually both even order and odd order harmonics can be musical, kind of...

The warm feeling of the tone is about how much harmonics is added to the raw guitar signal. let me explain it this way, 2nd order harmonic is always the same as original note but one octave higher, so definitely it's musical. 3rd order harmonic is not the same note as original note but that doesn't mean it's not musical, because it will be always another musical note and will form a chord with original note, so on 4th and 5th orders...

But higher orders mostly aren't related to any musical note and tend to add some junky,fizzy,... taste to the tone.

So why tubes are more musical? That's because they tend to add a huge load of low harmonics (musical ones) to the signal and a little high order junk! And they do it in both clean amplification and clipping, but at higher signal levels and especially clipping condition the amount of generated harmonics will rise and the sound will get warmer and warmer.

Transistors work in another way, they do not add much harmonics to the signal at clean level, and at clipping they produce a huge load of ugly harmonics (high order ones).

Even order harmonics are produced by any type of assymetrical amplifier stage, like a triode gain stage, phase inverter, SE output stage. So if we want to generate even orders with transistors we need a "Clean Assymetrical Amplifier" stage.

PP output stages form a composite curve, and they tend to amplify the signal nearly symmetrical so they will cancel out the most of even orders, but odd orders still are there and they are part of PP tone!

I think it's best to focus on a particular amp's output stage and try to simulate all of its characteristics...
Life is too short for being regretful about it.

highwater

I had a similar idea, about a year ago. I read somewhere that the JFET transfer curve is more like a pentode than like a triode, and combined with the knowledge that tube amps are almost-always transformer-coupled, threw a JFET phase-inverter at a RadioShack transformer into a corner of my breadboard. It barely worked, and only sounded slightly better than crap, but was functional enough that I drew-up a generic class-AB push-pull power-amp topology in a sim. According to SIMetrix, it should work... I'm hopeful, seeing as I also simulated the proof-of-concept circuit, and the sim told me that *that* would never work (to be fair, I did have to crank my amp to even get bedroom-level output).

That sim file is on my other (currently broken) computer, though, and would almost certainly sound bad without tweaking anyway... meanwhile my breadboard has been perpetually occupied with pre-build tweaking of known-good circuits, so the project has stagnated. I expect to be finished messing-around with CMOS overdrives and/or buy another breadboard (or several) by the end of the month, though, and will at that point resume work on the concept.



CAVEAT EMPTOR:
This is *way* above my pay-grade, but on the other hand, it's also the exact reason why I hope my boss (me) will one-day give me a promotion. I assume there's a reason why my idea hasn't been done to death, but I'm the stubborn sort, and I've found that prop-wash is quite effective at keeping dust bunnies out of a car's engine bay, so I'm quite content to persist anyway... even if it's a fool's errand, I'm sure the topological similarity to a real tube amp will at-least give it some mojo ;).
"I had an unfortunate combination of a very high-end medium-size system, with a "low price" phono preamp (external; this was the decade when phono was obsolete)."
- PRR

Sheldon

#22
I have to admit I'm totally lost and I don't know where to start !

If I understand correctly, if I want to simulate the effect of the power amp stage in a modern push pull amp, I have to :
* split the signal
* invert phase of one side with a inverter op-amp
* use different diodes to do assymetrical clipping =>simulate Tube grid bias shift and Tube distortion (only first orders even and odd harmonics)
* combine the signals (substract???) so remains the signal with mainly third and some fifth order, and a little bit of second and fourth order, to simulate the fact that a push pull stage is never perfectly symetrical

And then trying to simulate sag compression and current feedback?


I'm trying to understand KGM Micro Amp Schematics that J0K3RX suggested but I admit I have difficulties to understand each part.
http://milas.spb.ru/~kmg/files/projects/fetpa/micro/MicroSchSS.pdf

Sheldon

@ Johan : as I'm exploring each possibility cause I'm trying to learn how things work, what is the role of the b (lower case) polarity and how is it created?

Johan

Its the bias/4.5volt for the opamp
DON'T PANIC

Sheldon

thank you. In fact it's the top right part of your schematics that I don't understand. The b polarity runs through the 100k pot and depending of its resistance it gives a PP or SE character to the tone?

Gus

IMO
It has a lot to do with the output tubes, output transformer and speaker interaction.  This mess is hard to model

There are amps that are IC until the output transistor/tube transformer out

BJTs might be better to use instead of jfets for preamp gain stages

sajy_ho

Quote from: Sheldon on March 08, 2015, 10:34:48 AM
I have to admit I'm totally lost and I don't know where to start !

If I understand correctly, if I want to simulate the effect of the power amp stage in a modern push pull amp, I have to :
* split the signal
* invert phase of one side with a inverter op-amp
* use different diodes to do assymetrical clipping =>simulate Tube grid bias shift and Tube distortion (only first orders even and odd harmonics)
* combine the signals (substract???) so remains the signal with mainly third and some fifth order, and a little bit of second and fourth order, to simulate the fact that a push pull stage is never perfectly symetrical

And then trying to simulate sag compression and current feedback?


I'm trying to understand KGM Micro Amp Schematics that J0K3RX suggested but I admit I have difficulties to understand each part.
http://milas.spb.ru/~kmg/files/projects/fetpa/micro/MicroSchSS.pdf

I don't think assymetrical diode clipping will do any favor in producing more even orders, because it's just clipping of "already symnetrically amplified signal". We need "assymmetrical amplification" to do this particular job. Also don't forget that tubes do have internal capacitances, so they will add low pass filters in the path.
Life is too short for being regretful about it.

Thecomedian

#28
See if you can find a PDF or hard copy version of The Stompbox Cookbook.

I believe that we've mostly figured out "tube sound" since people have tiny hand-sized amplifier emulator/simulators these days, but there is a section on the considerations of all the variables regarding what makes tubes sound "tubey". You might find it helpful if only as reinforcement. It may only be an analogue for a stomp box builder without software or hardware coding and microprocessors, but it can sound reasonably close.

I think rule #1 might be to prevent hard clipping as much as possible. In transistor terms, its the saturation or cutoff where no further changes take place. In preamp tubes and output tubes, as far as I know, no matter how hard they are driven, there is still at least a minimal change in the signal occurring at any given Time, because of the complex interactions of transformers, microphonics, operating point changes, etc. What is called soft clipping, the thing which tube amps sound is famous for, is the distortion of the signal so that it fails to follow the input accurately. Hard clipping, which happens because BJT transistors are so accurate at following input signal changes, is the point at which the signal completely stops following the input signal at all. Tubes still do follow, if at least highly inaccurately. I haven't really seen any information yet that shows true hard clipping in tube based amplifiers.

The section on tube emulation in TSC is a good read.

Quote from: sajy_ho on March 07, 2015, 06:09:27 PM
It seems nobody from experts gonna explain it so a word from the idiot guy: :icon_mrgreen:

Actually both even order and odd order harmonics can be musical, kind of...

The warm feeling of the tone is about how much harmonics is added to the raw guitar signal. let me explain it this way, 2nd order harmonic is always the same as original note but one octave higher, so definitely it's musical. 3rd order harmonic is not the same note as original note but that doesn't mean it's not musical, because it will be always another musical note and will form a chord with original note, so on 4th and 5th orders...

But higher orders mostly aren't related to any musical note and tend to add some junky,fizzy,... taste to the tone.


Definitely. It is my personal opinion that this is why metal is mostly power chords, possibly even where the term comes form, because with all that "power", you need to play fewer notes to have a sound that isn't garbled :P. After all, you're getting "extra notes" with the distortion, so you can play less on the guitar, and then use different combinations of the same notes of a chord for different musical sounds.

Quote from: Sheldon on March 07, 2015, 03:57:06 PM
I've always thought that depending how hard you drive it, the power amp section added a little bit of "warm distortion", less raw and more musical than the preamp one.
But I can't understand what is doing this.
As far as I can understand it, a push pull power tube section is attenuating most of the even harmonics, but I thought that it was those even harmonics that give the warm and musical tone?

Single output vs push-pull is also considered in TSC. Pre-amps are also given some attention. From what I remember off the top of my head, the claim was that more tubes in the pre-amp sound warmer because there's more cascading going on, softer clipping, than higher gain stages with fewer tubes. I believe the push-pull reasoning was along the lines of the shift in operating points, IMD, and such.
If I can solve the problem for someone else, I've learned valuable skill and information that pays me back for helping someone else.

Sheldon

thank you
One thing I find tricky is to simulate push pull amplification without actually amplifying the signal cause my aim is to have a line in and a line out

teemuk

#30
QuoteI think rule #1 might be to prevent hard clipping as much as possible.

A myth. People are brainwashed to this concept after they have been saturated with -drawn- images of what tube amps output in some weird imaginationland. Reality, that revelead by oscilloscope captures and such, is often totally different than drawn pictures.

If you actually scope output of a tube power amp you typically see plenty of hard clipping and other "nasties" like crossover distortion. Fortunately all this sounds much better than what it looks.

Given characteristic curves, pentode tubes are damn near FETs and beam tetrodes damn near BJTs so hard clipping is to be expected. Couple this to global negative feedback and hard clipping is guaranteed. So yes, tube power amps hard clip, like this Fender amp here:


...or like this Trainwreck clone here:



Actually when voltage can no longer swing upwards things just clip, whether they are BJTs or tubes. It's the limit. The tube isn't suddenly going to overcome it by some magic. If you look at it, tubes are equally limited by saturation and cutoff states. We can notice a slight "softness" in overall curvature of the characteristic curves but as I said, pentodes are damn near FETs and beam tetrodes damn near BJTs so don't expect miracles. The most prominent thing explaining why tube power amps behave like they do lies within the overall circuit architecture not in the active devices.

Besides, now that we talk about "change" in signal after sustained ovedrive we aren't neccessarily discussing about softness of clipping characteristics. Not at all. If you overdrive a typical push-pull tube power amp the signal will typically soft clip distort just a little then very quickly hard clip. THEN interesting stuff begins to happen: Voltages may sag due to increased current draw and this will furthermore reduce clipping threshold, making clipping even harder. Additionally voltage drop at screen circuit may introduce little bit of gain compression. Now the gradual voltage sags and increased distortion adds more and more harmonic distortion as overdrive sustains. It's not just static clipping to a static clipping threshold.

The signal is still hard clipping but due to such changes the clipping thresholds have changed, in fact they change in interaction with overtall signal envelope, and this affects harmonic patterns of the distortion. Continue overdriving and power tube grids begin to conduct and draw current, this both clips the signal at their grids and causes DC offsets (such as bias) change. This typically introduces crossover distortion to the signal so again more and more harmonics (now from crossover distortion) are getting layered to the distorted signal. Furthermore the harder we drive the power amp, we may have rising asymmetry in the PI output and other little details that have an effect on pattern of the harmonic distortion. Essentially we have several distortion/clipping mechanism that interact with overall envelope of the signal, not just with static thresholds of voltage. All this while our signal is still getting hard clipped, in fact all the time progressively more and more. Hard clipping does not exclude anything from this process. But we have a system where sustained overdrive not only increasas amplitude of harmonics but continuously changes the entire pattern of the harmonic distortion.

Preamps have similar mechanisms where clipping process is spread across multiple stages and asymmetry of clipping and capacitive signal coupling causes DC offset modulation. Such modulation naturally has further effect on overall asymmetry/symmetry of clipping and therefore also to harmonic patterns of the clipping distortion. The behaviour is totally different than static clipping to, say, just a single diode clipper where increased overdrive doesn't really change the harmonic pattern of the distortion per se, just increases amplitude of the harmonics.

Again, soft clipping is not a requirement. The phenomenon takes place as long as you have some asymmetric clipping and capacitive coupling between stages. In fact, if you break down a typical tube preamp it is quite common to find out that clipping in it happens in just one to three stages: Few of them clipping hardly at all and one practically clipping so much that it has nearly a square wave-ish output.

Thecomedian

#31
This is why I need to invest in oscilloscope. Thanks, Teemuk  :icon_biggrin: I believed in those squashed sine wave diagrams for "tube sound" mainly because FETs and tubes are not linear devices, and their gain tends to fall as their voltage swings get bigger.


How does the trainwreck clone compare to an equally overdriven BJT section? Does BJT maintain constant wiggle like on the plate of the clone?
If I can solve the problem for someone else, I've learned valuable skill and information that pays me back for helping someone else.

Sheldon

Today I began simulations with LT Spice, especially Johan's project, but with no success!

kingswayguitar

teemuk
those are great scope shots. can you get a shot of a screen grid above/below a plate trace? been googling for one lately with no luck :) thanks!

Johan

Quote from: Sheldon on March 09, 2015, 05:09:19 PM
Today I began simulations with LT Spice, especially Johan's project, but with no success!
Is there an accurate 12au7 model in LT Spice?   When i tried it a few years ago there were no tube models at all in there.
j
DON'T PANIC

Sheldon

#35
I simulate a 12au7 triode stage quite correctly, but when I try to simulate your schematics there are a lot of gain attenuation and the output signal is far thinner as the input one when I try with a guitar wave input

samhay

From memory, the valve models I have tried with LTSpice don't give sensible results at low voltages.

Sheldon. What do you hope to learn from a simulation regarding Johan's design? Can you replace the triodes with e.g. BJTs to get a feel for what's going on?
I'm a refugee of the great dropbox purge of '17.
Project details (schematics, layouts, etc) are slowly being added here: http://samdump.wordpress.com

Johan

A stupid thought, perhaps, but i have to point it out as i made that mistake myself..
..i drew the opamp +input at the top, and i think LT spice have them on the bottom of the opamp....?
J
DON'T PANIC

Sheldon

@Johan : I verified and the opamp I drew the opamp in lt spice as in your schematics

@samhay : In fact I'm quite a newbie in electronics (I had a few lessons when I was in engineering school but it's not my job) si I decided to simulate Teemuk, Johan... schematics with LT spice so I can try to understand part by part how they work!

samhay

#39
fair enough - it's a good general approach to understanding how these things work.

I found some old LTSpice simulations of circuits based on Johan's design. One is below - save the text as a .asc file and it should work once you add TL072 and 12au7 model files (.sub). The 12au7 model I used gives some gain with a 9V supply. I don't trust the amount of gain, but at least it shows it working.


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WIRE 3168 736 3168 704
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TEXT 1648 184 Left 2 !;.http://www.diystompboxes.com/smfforum/index.php?topic=107868.0\n;\n.tran 0 50m 0 1m\n;.ac oct 10 10 100000\n.lib usr/TL072.sub\n.lib usr/12AU7.sub\n;

I'm a refugee of the great dropbox purge of '17.
Project details (schematics, layouts, etc) are slowly being added here: http://samdump.wordpress.com