Power tube OD in Push Pull

[lowell]

I've been pondering this quite a bit lately. I just built a single ended amp with power tube overdrive, and I biased the 6v6 colder to achieve distortion via cut-off. I am now thinking that this is impossible with a push-pull output right? In push pull the tubes technically go into cutoff anyway on the plate's positive going cycle. In other words, the plates of the tubes on both sides of the OT are always pulling down towards ground, therefore saturation is the only way to overdrive power tubes in push-pull arrangement. Of course I'm ignoring grid current limiting and blocking distortion. Am I correct or missing something. Finally this seems very hard to attain without grid clamp or blocking distortion happening first... thoughts?

[kg]

sounds like you're on the right track.

you don't get current cutoff in a push pull amp, unless it's a TRUE class B or C. neither is used in tube AF amplification.

[MWJB]

You do get crossover distortion if the tubes are biased cool enough, so that neither is really handling its half of the sine wave. This is a relatively common technique with fixed bias PP AB amps used for harp, not unheard of for metal guitar sounds either. I've known of guys who run idle currents in the 4-5mA region for 6L6 @ 470-520vdc. It's cold AB, rather than class B. When recorded, a cold biased PP AB amp can sound very similar to a heavily driven SE amp. Not saying that it's necessarily a nice sound, but cooling off current does still change the sound & introduce distortion of a different kind.

I'm not convinced that it's that helpful to really break down what is power or preamp distortion (unless you have unpleasant artefacts that you are trying to eliminate), much of the amp's character is determined in the preamp stages (usuall SE class A), if they are lacking in character, power tube distortion alone can sound harsh...it's really about striking a balance, stages complimenting each other. Instrument & subsequent signal voltage, sustain requirements are all factors too.

[TransLucid]

As far as lowell's observation regarding blocking distortion, there are techniques for preventing it (larger grid stop resistors on the output tubes and smaller coupling caps).

To elaborate on mwjb's comment, it's important to arrange the gain structure stage by stage to achieve roughly equal headroom throughout the circuit. Then you can jack up one part or another (for example, using the volume and master volume controls) to get one or another section to distort more or less. Having said that, I do believe that preamp and output stage distortions sound different, and both should be considered in crafting a dirty tone. The problem with output stage distortion, of course, is that you've got to play loud to get it...

[lowell]

I'm specifically just after push-pull saturation. I do agree though that a good sound is a combo of preamp and power amp distortion.

I don't have a plate curve for 6V6 in front of me right now nor can I draw on one and scan it for here. But... lets say theoretically we have a 2 6V6 push pull amp. Lets also say there's 300v on the plates and we're using a 10k plate to plate primary z. Now using the 1/4 of the primary z rule (2.5k) for push pull we'd have 120ma at the point of saturation. (300v/2.5k=120ma) Now if we were to bias this tube at Vgk of -20v, then when the grid signal reaches 40v p-p it's going to begin grid clamp and or blocking distortion right? And on the JJ 6v6 load lines this looks like it's near 60ma and Vgk=0. Va only goes down to about 150v at this point. So how is it that you can achieve saturation before this happens? Or is this an inherent unavoidable side effect of saturating power tubes in a push-pull amp - that there is always grid clamp happening simultaneously? Finally, if the grid "clamps" then the tube is technically saturated or not? Does grid clamp change the trans-conductance so drastically that it turns the tube into an "on" or "shorted" switch between plate and ground and is this what saturation is? So many questions I know sorry.

Thanks for your patience in answering all my q's.

[Chuck H]

I'm not nearly tech enough to answer this Q specifically. But I can say that the nature of tubes dicates that they are still amplifying to some degree even as they begin to clip. So, it seems logical that your implication of grid clamping and saturation can occure simultaneously. This is one reason why crossover distortion is very hard to avoid in OD'd tube power amps. I can say that providing a low time constant and current availability for the grid circuit will allow a tube to conduct more power (even to it's detriment). This can help avoid crossover distortion in the right application. That being the tube is not dissapating beyond it's max in this condition. But IMHE any time you saturate a power tube it produces a crosover distortion byproduct due to grid blocking. And the lower the Vp the less drive it takes to get there. So... If you run a tube conservatively and build a circuit that provides for some grid current you can achieve an almost crossover distortion free clip. But it will never be the same clip as cutoff distortion since the tube is NOT conducting in cutoff and is LIMITED in saturation. So the effect of cutoff clipping is very different from saturated clipping. I think this is part of the Q your addressing.

Please forgive as this is just intrinsic understanding and rote observation. But it does introduce the aspect of grid current.

[MWJB]

"I'm specifically just after push-pull saturation. I do agree though that a good sound is a combo of preamp and power amp distortion." The sound of every combo you have ever heard was preamp & power amp distortion, to distort the power tubes, they have to be hit with sufficient signal from the preamp. OK, so there is a big margin between preamp stages that distort from the off, up to power sections that distort early...but you need one to make the other.

Given the current required to saturate your 6V6s under load, there will be other aspects in the amp that contribute to and colour the sound...for example 2x6V6 amps tend to have PT B+ current ratings around 100mA, being cheaper amps older models nearly always had 5Y3 rectifiers (both this & the PT contributing to internal resistance in the power supply), cathode bias...all these factros will introduce artefacts into the final tone, that may be perceived as a saturated tone.

If we take an amp like a 5E3/5F3, these can be biased to just about class A territory (halfway between cut off & saturation at idle)...but they tend to make more heat than power, dissipating W in the late teens & kill OTs. When you cool off the current slightly, they become more dynamic, less garbled & more useful/reliable.

I guess that you could design an amp that ran the power tubes nearer to saturation, but you're probably going to have to drop B+ to achieve it...you might achieve what you are after but at the expense of output...you'll have a very hot & expensive, low powered 6V6 amp.

All this makes me wonder whether the tone you are looking for solely comes from bias conditions/saturation?

[kg]

I'm specifically just after push-pull saturation. I do agree though that a good sound is a combo of preamp and power amp distortion.

I don't have a plate curve for 6V6 in front of me right now nor can I draw on one and scan it for here. But... lets say theoretically we have a 2 6V6 push pull amp. Lets also say there's 300v on the plates and we're using a 10k plate to plate primary z. Now using the 1/4 of the primary z rule (2.5k) for push pull we'd have 120ma at the point of saturation. (300v/2.5k=120ma) Now if we were to bias this tube at Vgk of -20v, then when the grid signal reaches 40v p-p it's going to begin grid clamp and or blocking distortion right? And on the JJ 6v6 load lines this looks like it's near 60ma and Vgk=0. Va only goes down to about 150v at this point. So how is it that you can achieve saturation before this happens? Or is this an inherent unavoidable side effect of saturating power tubes in a push-pull amp - that there is always grid clamp happening simultaneously? Finally, if the grid "clamps" then the tube is technically saturated or not? Does grid clamp change the trans-conductance so drastically that it turns the tube into an "on" or "shorted" switch between plate and ground and is this what saturation is? So many questions I know sorry.

Thanks for your patience in answering all my q's.

well to answer your question i have to ask a question first:

what is your definition of "saturation"?

[MWJB]

KG put it much more succinctly than my rambling post, but yes, I think we need to make the distiction between whether you specifically want power tubes saturating under drive, or what folks simply perceive as "saturated" tone, which I have even heard used to describe fixed bias AB amps with moderate idle current.

[lowell]

I'm not after a tone or sound. I'm after a lucid understanding of the theory behind saturating power tubes in push pull power amps. I understand that this can and would lead to low power. I'm not concerned with that.

[lowell]

I'm not after a tone or sound. I'm after a lucid understanding of the theory behind saturating power tubes in push pull power amps. I understand that this can and would lead to low power. I'm not concerned with that. Thanks in advance for any more info on this... specifically the relationship between grid clamp and saturation.

[MWJB]

So you're looking at approaching 110mA per tube? 120vdc on the plates? Personally I don't think you need to get anywher near those plate currents before you start hearing problems, or replacing tubes at an unusual rate...so I have to ask "why"?

[lowell]

Okay fine. Then how would you go about setting up a push pull power amp to achieve saturation without eating tubes and without concern for power? Again I'm just curious about the theory.

[MWJB]

I wouldn't because you seem to be proposing something akin to a square wheel. You could take a low voltage 6V6 amp & push up the current until something gave (at max volume that you would use it at...the thing with finding a components limits is that you have to exceed them & destroy it), then backtrack...but you would probably still find yourself in Class A territory.

What would be the purpose/desirability of a very low powered amp, with expensive & heavy transformers...it would keep you warm in the winter, but a heater would do a better job.

I'm not trying to put you off experimenting, lord knows I have some funny ideas & curiosity gets the better of me & I execute them, but you sometimes need to temper that with a goal/unique advantage over the status quo, ask yourself what the design has to offer?

[lowell]

Can we ignore practicality for a moment? Or look at the 5E3. When a 5E3's power amp is overdriven is it being partially saturated or is it in grid current limiting, or maybe both?