Is "Power Tube Distortion = Holy Grail" a myth?

[mlp-mx6]

Can anyone cite a non-single-ended non-triode tone that is *absolutely* the power tubes distorting as a go-to tone? In other words, a "standard dual-pentode push-pull" output section tone.

Based on my understanding of feedback, the transition from clean to overdriven in the power section would be sudden and severe. Also, the push-pull xfmr would cancel any even-order harmonics.

What am I missing? It sounds like the "magic power section distortion" is another triumph of marketing over fact.

[Ken Moon]

TW Express - see some of the scope shots that show how the "clipping" progresses from the power tubes to the preamp as the input signal increases.

I'm not sure where I've seen them, but there are a bunch of TW experts right here, so they can point you to them.

[JamesHealey]

Ah this is the magic of the wreck! Very clever stuff! Power tubes are first to clip and it progresses back through the amp!

[jaysg]

My understanding is that it's really the PI's distortion interacting with the output tubes...which may not actually distort. Beyond that, you risk being thrown off the island for heresy.

[jaysg]

....

[Gibsonman63]

Merlin B discusses it a bit in his pre-amp design book. If I understand correctly, to create the magic harmonics you distort asymetricaly just a little at every stage to create the effect. He has a new book out on designing tube power amps and I assume that (musical) power tube distortion is covered. It will be interesting to get his take on the matter.

[mlp-mx6]

Yes. It is Merlin's book that has brought the question to mind - specifically, the "Feedback Theory" chapter. His description of how distortion behaves with feedback is exactly the reason I ask the question.

I'm completely willing to risk being thrown off the island.

Of course, it could be that "power tube distortion" belongs on the same island as "Vox AC-30 Class A"...

[snyder80]

I think it might be a myth, too.But after all it may depend on the ampdesign.

Those Highgain-Amps like Mesa, Engl, Hughes & Kettner, Diezel, Bogner and many Peavey and Soldano or Splawn are much more preamp-distorted.

It seems reasonable that PA-Distortion often means the PI is getting overdriven. This explains why those PPIMV-circuits work very well in these designs.
The PA is much more responsible for "pressure" und some dynamics I think.

[iknowjohnny]

I have long believed that it's not power tube distortion or tube distortion at all that is what makes tube amps so desirable. It's the tube tone itself, not the distortion they produce. Thats not to say they can't produce great distortion. But IMO it's not the distortion itself thats where the magic happens. It's just the dynamics of tube tone, period. Specifically the outputs. I first began to realize this when i was using SS distortion devices from pedals to full on preamps thru tube amps usually into the return so as to bypass the preamps. I got some seriously killer tones that way. Yet if i tried the same SS distortion thru any non tube PA it would sound like absolute garbage or at best just usable. Even preamp tube distortion had to go thru a tube output or it too sucked. I also looked at all the world class players i respected with killer tone and many of them got thier distortion with pedals into a clean tube amp. One of the best distorted lead tones i have ever gotten was a tube screamer used as a clean boost into a rat pedal set slightly dirty then into a very clean tube amp with the amp's EQ and the RAT's filter carefully tuned to interact just right. To this day i don't think i have ever achieved anything that good with tube distortion be it PA or preamp generated.

So in short, i believe that tube ARE IT, and totally necassary for great distortion. But they don't have to be where the distortion is generated. They just have to be there to sprinkle the magic dust on the distorted signal, wherever that may come from. And by the way, premap tubes can indeed make some great distortion. But just like SS generated distortion they must be fed thru a tube output section because thats where the real magic is.

By the way, ever notice there have never been highly respected amps with tube preamps and SS outputs, yet there have been some amps that have gotten reps as great amps with the opposite. Music man is one that comes to mind.

[Ken Moon]

Here are the Express 'scope shots:

http://home.polstra.com/amps/wreck1/scope/

[dave g]

My understanding is that it's really the PI's distortion interacting with the output tubes...which may not actually distort. Beyond that, you risk being thrown off the island for heresy.

This is correct. Power tube "clipping" is basically diode clamping; when the power tube grids are driven positive, the tube will continue to amplify the input signal as long as the driving stage can supply enough grid current. In this situation, the clipping is actually caused by the voltage drop across the driving stage's output impedance due the the grid current being drawn. The amount of grid current drawn just so happens to follow a 3/2 power law rather than an exponential law like a silicon diode, so the clipping is much softer and more pleasant. Each particular tube type has a different IV characteristic, so that's why they sound different. And of course, feedback makes everything more complicated!

Merlin's design philosophy regarding distorting preamps is effective for high gain preamps. What it really comes down to is avoiding a hard limit on any stage; you want to clip each stage enough to get appreciable distortion, but not so far as to reduce its incremental gain to 0. When you drive a stage into hard clipping, what results is the bee-in-a-paper-bag sound, as well as a loss of the "natural" sound of the guitar, because the high energy fundamental essentially slams into a brick wall, literally squishing the lower amplitude string harmonics riding on it. It's why high gain amps can sometimes make your guitar behave more like a MIDI trigger than an instrument.

It's not the only way to design a preamp, though. The Dumble preamp only clips a single stage (the 4th). However, the magic of that preamp is in the interaction between all four stages through the power supply. In response to an input signal from your guitar, the current draws of the 2nd and 4th stages are in phase, while the 1st and 3rd stages are in phase with each other but out of phase with the 2nd and 4th. Ever try to measure the amplitude of the signal on each grid under normal conditions? It's low (~100mV) on the grids of the 1st and 3rd stages, and much higher (on the order of a volt to several volts) on the 2nd and 4th stages. If you draw the load lines of each of the 4 stages and then inject a test voltage to the grid of the 1st stage, you'll see that the current through the 1st and 3rd stages drops about .1mA in each stage given a 100mV input signal, while the the current through the 2nd and 4th stages rises by >1mA. So you're talking about at the very least an additional 2mA being drawn by the preamp in response to this signal. Big deal, right? Except for the fact that there's a 15k-22k dropping resistor that will drop the preamp voltages 30-40 volts in response to that extra current draw!!! You never see a preamp power supply dropping resistor that big in a Fender This in turn drags the load lines closer to the origin on all 4 stages - basically preamp "sag"! This creates that extra compression the Dumble preamp is known for...that's why you get the dynamic, singing sustain when you dig in, but it's so clean and clear when you pick lightly or roll your volume knob down.

I think this is the source of the "bloom" - faced with a large input signal, the load line of the clipping stage will be pulled towards the origin, and the maximum voltage swing will actually decrease. As the input signal decays, the current draw of stage 2 shrinks, allowing the load line of the clipping stage to expand outwards again, which increases the signal swing seen on that stage's plate.

In this context, a lot of the "weird" things about the Dumble make a lot more sense. It's the reason why the input OD trimmer is always set so low, because allowing the 3rd stage to swing a large current will counteract the effect; hell, the only reason that the 3rd stage is even there is to basically act like an inverter, so that the large current swings of the 2nd and 4th stages can remain in phase! It's also the reason why the 1st and 3rd stages are biased colder than the 2nd and 4th, so that you've got minimal cancellation of the current swings in the power supply.

FWIW, I think this is also largely related to why driving the power amp makes the amp sound better. Even if you're not "clipping" the output section (in the sense that I described above), swinging a larger output signal creates sag in the power supply due the the increased current demands of the output tubes. This results in the power supply voltages being dropped across the board...it's why clean amps like a BF Twin have a "sweet spot" when you turn them up to about 5 or 6...

This is why I still maintain that chasing tone via different brands/compositions of caps, resistors, tubes, etc is a bit futile...topology is everything!

Sorry this may sound a little confusing, I'm writing this in a bit of a hurry. I'll see if I can post some plots when I get back from school and into my workshop...

[tictac]

I just wrote a thesis paper on tubes vs transistors and I ended up concluding that there are numerous factors that contribute to the overall sound besides the amplifying devices themselves.

Triodes do produce strong even order harmonics but feedback and yes a perfectly balanced push-pull power amp cancel even order harmonics which is why so musicians prefer an imbalanced power amp and/or PI. So the circuit is definitely part of the equation.

One of the biggest advantages of tubes that is often overlooked is their signal to noise ratio is way beyond what solid-state devices can deliver.

There is alot of good reading on this subject out there if you do a "tubes vs transistor" google search. Also search for Eric Barbour, John Atwood, and Russell Hamm.

Russell Hamms AES paper Tubes Vs Transistors Is There an Audible Difference? is excellent and CBS Fender did a research paper called Tubes Vs Transistors in Electric Guitar Amplifiers which is very good as well.

[David Root]

dave g, thank you for that remarkable post vis a vis the Dumble V1/V2 circuit! That answers some questions and raises not a a few more, I think.

Will have to think on that and look at some bias points too!

[Cliff Schecht]

Dave, very cool breakdown of the Dumble design philosophy. What you are saying not only makes perfect sense, I actually use a similar technique in my own designs.

Using really large dropping resistors can help to make a preamp stage more touch sensitive. As you play harder and more current is drawn, the plate voltages drop and you get more distortion. Play softer and the plate voltages shift up and you can get a (relatively) clean sound. The dropping resistors in the Express are relatively large too which I think helps to achieve the huge dynamic range the amp has.

As far as power tubes distorting, they most definitely do distort, and they distort big time. It's much more pronounced when no negative feedback is used, mind you, but a quick look on an oscilloscope from the speaker output to ground will reveal a lot of magic. If you can keep the entire preamp clean but push the power tubes into "clipping", or as others stated start drawing positive grid current, you will see just how hard the power stage can clip. This is much more pronounced in SE amps because of the lack of any cancellation and (usually) negative feedback.

The speaker/transformer/power stage interaction also has a LOT to do with the sound. The speaker is in no way an ideal load and it's impedance flies all over the place as it moves in and out. This presents a non-constant load and gives you some goofy artifacts that we take advantage of. Something I've noticed in SE amps is that when the power stage is clipped hard, you get a ton of flyback voltage on one side of the waveform. Since it's only on one side of the waveform (asymmetrical shaoe), the harmonic addition to your signal is completely even in order and makes the whole amp sound huge. The most pronounced I've ever heard this effect was in my KT88 SE design but I absolutely fell in love with that sound and have been meaning to build another one for myself..

[daydreamer]

Cliff, would you have a copy of that SE KT88 design you would like to share?

Ever since seeing that tube on a design I've been fascinated with it, and each time it's mentioned someone is praising it. It looks very versatile in it's power production as well, up to 42 watts in it's wiki?!

thanks
Andy