Hello all,
I'm a little confused by all this tube hooplah and this mysticism surrounding power tube distortion without any hard details. I searched ALL over, and to no avail, so I turn to you

I own a Marshall JCM2000 DSL401 combo, and basically, on overdrive, there is a gain, channel volume (basically preamp volume I guess), and a master volume ( controls both channels). The manual states that the master volume is located at the end of the power amp, and this sorta makes sense cause the emulated DI stays constant level, even when turning that knob all the way down.... I think.

Anyway, if this all holds up, I know how to saturate the tubes with signal, by turning up channel volume, but my question is.. Does that yield power tube distortion? or does turning up the "master volume" do that? Sites that I have found master volume information on show only a resistance to the preamp signal as a means of volume control, but in this case, is mine differently wired?

I'm hoping this is what is really going on, as IMHO the amp sounds great at any volume with my settings - just wanna know what really is goin on for sure.. Any answers?

Lots of preamp gain/volume + master volume set low = no power amp distortion.
Less preamp gain/volume + master volume set high = power amp distortion and tonal bliss!

Pretty much what Platypus said. If you have tons of preamp distortion don't count on power amp distortion - the signal that goes into power amp is distorted and very compressed already, so you won't get much power amp distortion in this situation even if you crank your master volume knob to 11. When your preamp doesn't compress the sound that much (say you're using clean channel or "plexi" mode or something like that), by cranking the volume you can get some very nice breakup.

I wouldn't say it's a "tonal bliss" though. To each his own, and I prefer to rely on carefully tuned preamp distortion instead, even on clean channels.

Well, you'll still be getting power amp distortion- if you run tubes hot, you run tubes hot, it doesn't matter what the signal going in sounds like. It's just it won't be nearly as pronounced, because the signal is so saturated to begin with.

This is one of the things to keep in mind when using a tube amp, in fact- as the master volume goes up and the amp begins to compress, you need less and less preamp gain. Leaving your settings the same at 7 as at 1 will give you a VERY oversaturated tone...

-Drew

But the thing is... if you run a small signal to the power tubes even with master volume high, will you still drive the power tubes? Logically it doens't make much sense that a small signal would create power tube distortion no matter how high the poweramp volume is.

Also.. basically, I have gain at 3.. Very very driven. I try the same settings but reverse the volume settings (channel 3, master 7), but it is cleaner, but buzzier too..

If anyone has the amp, I mean, knobs starting from OD Gain - 3, 7, 3, 7, 4, 0, 5, 3

Running tubes hot has nothing to do with master volume or power amp distortion. To "run tubes hot" means to operate them at a high quiescent (bias) current, closer to (or in) class A. This doesn't necessarily mean that tubes are operated beyond their specified power dissipation either - they can be run hot, but at a lower voltage and they'll still be within limits.

What you said is correct, though. There still be distortion in the power amp. Power amp will even distort the signal when master volume is set to 1, especially the one without presence control (no negative feedback). BUT, this is not the power amp distortion guitarists are commonly referring to, it's way too small to considerably change the tone.

One other thing if someone looks for true power amp distortion is to avoid amps that have presence control. Presence is called "negative feedback" in engineering terms and here's now it works. It taps the signal at the output of the amp (AFTER the transformer in case of tube amps) and sends it to its inverting input (a point in the circuit). An "error signal" is formed then and the input signal is pre-corrected to pass through the power amp in unchanged form.

What does it mean for a layman?

This means that:
1. The amp will "resist" to distort except if presence control is set to zero (in Marshall-style power amps) or even if it's set to zero (in Mesa-style power amps)
2. The amp will be less dependent on the speaker because presence increases dampening factor. At the same time, the speaker will put out much more treble than it normally would, because of the same thing.
3. The power amp will have less voltage gain. But then you don't need much voltage gain here.
4. Fourth and the most important thing, at non-zero presence settings Marshall style power amp will distort abruptly when negative feedback loop (or more exactly the tubes amplifying the error signal) reaches the limits of what it can correct. This is not what "power amp distortion devotees" are looking for.

Ok.. I think I understand.. But, quoted from the Marshall DSL401 manual,
under Overdrive Volume
"When using the less gain/more level way of driving the power stage, keep the overdrive gain low and use this volume control to drive the power amp."

what does that mean?

also, under Master Volume in the manual,

"Apart from the obvious function of being the master level control for the whole amp (regulating teh output volume of both the clean and the overdrive channels), this control actually affects the apparent power level of the power amp section itself. This means that, at lower settings, the amp acts and feels like an even lower powered amp, along with the kind of smooth saturation that you get from pushing a power amp into distortion."

Now, all this hooplah from the manual seems to contradict any documentation or talk going around.. Maybe I'm misinterpreting, but can anyone shed any light into this? Are these Marshalls wired differntly or something? Cause apparently from other sites, I read that volume controls give resistance to the preamp signal, effectively controling the volume like that , but if Marshall's documentation holds,, woudln't that mean that the OD level controls preamp output and the master volume controls the final signal out of the power amp, post power amp distortion?

P.S. We are leaving all speaker distortion (major part of tone) aside...

edit: Some of you may wonder why I am so caught up in this... - It is because I feel that people say tonal bliss without being able to give me a specific answer to what they mean by power amp distortion, or rather, cranking the amp, what that actually is doing... So.. Can anyone help me?

Ahh, my bad. See, I'm one of those "laymen..." I meant more along the lines of "running the amp at high volumes so the power tubes are working harder," so i think maybe we're still talking about the same thing...

That's fascinating, about the negative feedback loop's interaction with poweramp distortion... gonna have to do some experimenting here and see what happens. Thanks,

-Drew

Waitaminute.. what about what I posted in response?

Logically, if your power tubes dont' have a big signal to process, how can they become distorted heavily just by cranking a volume control at the end of the power amp stage?

Thats becouse, for the most part, when you turn up the signal going into a tube it only increases the signal for certain amount of the turn. The rest of the turn is just increasing the gain. Thats in laymens terms. If you take a Marshall Plexi, for example, the amp will reach its loudest volume somewhere around 5 on the volume knob. The rest of the way you turn it just saturates the overdrive more.

Thats one reason but its a little more complicated then that, especially with new amps that use more then one preamp tube for gain. It doesnt sound right but basically what micromidity and platypus said is 100% correct. I dont really feel like going into why, or repeating what micromitidy posted again for everyone to understand, but it is true.

All the stuff your manual says about having poweramp gain at any volume is hooplah. You only get the desired power tube gain at very loud volumes.

Drew- you didnt say anything wrong. Amp people, not engineers, refer to a cranked amp that way. No offense micromidity. Even Aspen Pitman and Andy Marshall refer to it that way. Thier 2 of the top guys in tubes and amp circuitry. The english language isnt one that should always be taken literally with all the sarcasm, expressions and idioms.

Then if that is so.. what are each of the two volume knobs doing?

wouldn't having one down and the other up yeild a different result? They certainly sound different.. I'm just still a bit unclear

Ok. Perhaps I need to be a litlte more specific...

What eveyrone is saying, apparently, as far as I can guess, is that a volume knob saturates power tubes with signal, and once a certain limit of the power tubes is reached, gain starts to increase, and that's what I want. That would be before the power amp?

http://people.cornell.edu/pages/mt24/Amp/mm/mvols.html

So I assume people are talking about a volume control as specified there...

Well, I have two volume knobs.. Anyone able to look at the schematic and see where each is placed in the amp? and how they work? and therefore, how to use them best for power amp distortion?

A guess: if one of my volume controls (logically the master volume) was placed at the power amp stage, after going through the tubes... just guessing, wouldn't I be able to attunate the final signal post distortion / processing?

Sorry if what I am saying sounds silly to anyone who knows what they're talking about.. I'm no EI.

You know, I was actually thinking about this while showering before work. I know, I'm weird.

Here's what I came up with, near as I can tell. What we're looking at is a unusual cascading gain stage, with a gain control for both parts. The first one is your usual preamp cascaiding gain stage, where a signal is overdriven in one tube (or side of a tube, i think, they're split, right?) and then pumped back into another distored preamp tube to increase the level of overdrive even more. This is normal for a high gain amp, as my understanding goes. Here's the tricky bit, though. What happens now is that it goes into a third "gain stage," but this one is engineered to be as clean as possible. What it's responsible for is taking a quiet signal coming out of the preamp stage (i.e- one with a low preamp gain level) and boosting the overall volume of that signal up to a "hotter" signal, without drastically varying the gain structure, before it hits the power amp section, so the signal will be driven just as hard into the tubes as a heavily distorted (and in turn louder) preamp signal. However, because the signal is engineered to be as clean as possible, there's very little change in overall "gain." From there, it goes into a normal poweramp section.

Does this configuration simlate lower-powered amps when you have the master down and the channel level up? Of course not. Pure marketing hype. It just makes it easier to balance tones between the channels, and get earlier power-amp distortion out of low-gain settings (you don;t have to turn it up as far, but the overall loudness out to be about the same), and allows you to get more poweramp gain overall when you really push the thing.

Just a hunch, of course.

Micro- i tried setting my lead channel with the prescense all the way back, and compensated a bit with the treble, on my TSL last night when the parents were out... Man. It "feels" different- so much spongier, much more like a Mesa. And it seems somehow warmer, too, even at moderate levels (VPR (25 watt mode) on, master at 3-4). I like. It's not as forward and agressive sounding, so if i was playing speed metal or something maybe I wouldn't like it as much, but even for shred-style lead i was really digging the tones I got with the prescense all the way back. I don't know if it bypassed it or not, but....

-D

Heres how it goes SHsu516815

Input - Gain knob - Preamp tubes - Master Knob - Power tubes

Turning either knob up allows more of the signal to flow through the tubes that come after it.

Thats very simplified with many variables in between but thats the basic order.

If you still dont understand I cant help you, but the thing you need to realize is that strait signal volume isnt the only factor. The envelope of the signal going into the poweramp section is very important. It needs to be very dynamic for the power tubes to do thier magic. At least in the traditional sense. When you send an already distorted signal into the power section, it has no dynamics (in a guitar sense anyway). Its compressed. If you dont understand what that measn Ill explain it to you.

When you play a clean guitar, how loud it is depends on how hard you hit the strings. It has a natural decay that only lasts a few seconds. Its a very dynamic signal. You have an intimate control over volume and what Id call intensity or aggresivness. A very distorted guitar signal has nowhere near as much dynamics. No matter how hard you hit the strings your mostly getting the same sound. Its compressed. That means the lower sounds are made louder and the louder sounds are made lower. The decay is very long for that reason. Its envelope is closer to a violin while a clean sound is like a piano. Thats a crude analogy but its the best I can think of.

Now if you ever watch the power tubes when you have the amp cranked, youll see they get brighter the harder youy play. When your not playing they dim down and if you hit a big chord they start glowwing. Power tubes are sensative to the envelope of the signal sent into them. So if you send a signal thats compressed to hell like a distorted signal, the effects of the power tubes are greatly reduced. Thier percieved as making a small difference. Its a subtle difference. It sounds better cranked, but, your basic sound hasnt changed much. Its more of a feel thing. Its more noticable while your playing. If you recorded the amp twice at a lower and a louder volume, then went and listened to both, itd sound very similair.

If you come in with a cleaner signal or a slightly overdriven signal, the power tubes react very noticably to how hard you play. Since you have alot of control over the volume/signal by your playing technique, and the power tubes react as that signal gets larger, you can go from heavy to clean just by your attack. Thier effect is very present. It not only adds to your sound, it is half the sound. Thats pretty much all the hype about EVH's sound. All that gain and he used nothing more then a marshall plexi (usually thought of as a bluesy overdrive amp) but he made a few changes so he could safely crank the volume to 10 and have tons of power-tube distortion.

Many of the players today, I feel, dont like older style amps becouse they arent talented enouph to play them. Newer amps dont take much energy to get a certain sound from. You can play death metal with a super light attack on the strings. It still sounds heavey with tons of preamp gain. If your playing a cranked older amp, you really have to have the skills and technique to make the amp sound a certain way. Most people are surprised by how little gain there was in EVH's rigg. He didnt get gain by having tons of preamp distortion. He got it by playing aggresivly and digging in. His talent and abilities were able to coax a heavy sound from his rig, or a clean sound, etc.... Same with Hendrix, Page, Clapton and many great players today. I read another post that said almost the same thing on Jemsite a while back. Ill tell you one thing. My main amps are a Soldano SLO and a Bogner XTC, both high gain. When I jam on my cranked 67 plexi, I always need to get my chops together to play as well as I can on those other amps.

That was long. I hope its clearer to you now.

Thats what power-tube distortion is all about. Its not the end-all be all. Hell I use a few high gain amps as well as old amps. But it is probably the best thing to come from guitar amps.

If anyone agrees and wants to chime in to further explain or correct something feel free.

Dan- great analysis, man... I'm no expert on the subject, but it makes sense.

Of course, even a heavily distorted preamp section will still generate poweramp distortion... it's just that it won't vary as much. If a hot signal hits the tubes, they distort, no matter how "even" that signal is... The settings I always seem to like the most (when I have the ability to turn up that far, anoway, lol) have the preamp gain level on my TSL about halfway up, and the poweramp at halfway or over, to the poing where the tubes are being compressed and overdriven by the preamp signal (with the VPR switch in, this is a good "lead" level for the blues/rock bar band I play with, conveniantly. ) This still gives you a certian amount of dynamic control over the signal... but more importantly, it doesn't have nearly as much of that "fuzzy" edge to it as you get from a strictly preamp gain, but still provides enough compression for my hacked legato licks to sound ok.

-D