Gain

[trem]

I realize this is kind of a rudimentary question, but I wonder if, maybe, a lot of people are just as confused as I am.
Seems like when I read about amps, the words Gain and Distortion kind of get used synonymously. But are they.?
I thought gain was the amplification factor of a tube.....the net "gain" in the difference between the amplitude of the input signal and the output signal.
Assuming we are dealing with amplification on the linear part of the tube.....you can have more gain that Is Not distorted.....it just has more gain/amplitude. Am I right about that.?
So what are the benefits of a "High Gain" amp. ? Why do players (that want high gain) want "a lot" of gain.?
What can you do with an SLO that you cannot do with a Super Reverb.?
I just repair, and work on amps (to my ability) I do not play guitar. I am a drummer.
Thank You

[wizard333]

Colloquially, "gain" is synonymous with "distortion". Technically, gain is as you describe above. Sort of like how a "Tremolo" bar is really a vibrato bar, and the "Vibrato" on a Fender amp is really tremolo.

[eschertron]

A lot of writers go to great lengths to describe the confusion in terms and the technical difference in terms, just as you did.

What makes a 'high-gain' amp, is indeed gain. If a 25mv RMS sin wave will clip the output tubes of an amp like this, an input signal many hundreds of times larger will still clip with the same absolute output peak-peak voltage. More distortion in the second example, surely. The gain of each stage made that distortion, as each stage over-drove the following stage.

I'd hazard to say that the number of gain stages (preamp tubes) is a good yardstick for whether an amp is a 'high-gain' model or not. I'd also hazard to say that nobody has ever bought a high-gain amp with the intention of playing exclusively in the linear portion of the amp's dynamic... not unless they were amplifying brainwaves, of course!

[Chuck H]

not unless they were amplifying brainwaves, of course!

We ARE talking about the crunch monkey metal heads, right? I'm not sure technology has invented a device sensitive enough.?.

[trem]

Thanks for the reply's.
But I am still confused (sorry).
I remember seeing an SLO in Eric Clapton's back-line for several years. What was he able to do with that that he could not do with a Tweed Bassman, Tweed Twin, Blues Breaker, etc etc etc.?
I have always associated "high gain" with that kind of squealy, high pitched, just about ready to feed back, Eddy Van Halen kind of a sound.
Thank You

[trem]

Thanks for the reply's.
But I am still confused (sorry).
I remember seeing an SLO in Eric Clapton's back-line for several years. What was he able to do with that that he could not do with a Tweed Bassman, Tweed Twin, Blues Breaker, etc etc etc.?
I have always associated "high gain" with that kind of squealy, high pitched, just about ready to feed back, Eddy Van Halen kind of a sound.
Thank You

[defaced]

Not all high gain amps are created equally. The SLO tends to be a little more on the raw, open, and "middier" side of things. It doesn't really clean up well with a volume knob, but sounds other than "metal" can be coaxed out of it. Unfortunately, there are fine points to a high gain amp that generally just get lumped into "dumb metal sounds", which is often the case, but isn't really true. I chalk that up to poorly informed geriatric players, but that's another discussion. A good amp is as much an instrument as the guitar.

escheron is correct, the number of gain stages is a very good indicator of how much distortion one can get from the amp (and amp alone). Beat the strings as hard as you like, you won't make a Twin/Deluxe/whatever sound like a SLO. Now some will say "use a pedal", "the rat/MT-2/whatever is as much gain as anyone would need". Sure you can take a Twin and toss some gain stages in front of it in the form of a pedal, and the circuit gain might be equivalent, but they will not sound the same. Yea, it'll be distorted, but a good high gain sound is more than just distortion - sounding good is a requirement. This is because high gain amp design requires some fineness to get to sound good.

Uncontrollable high pitched and squealing generally means someone didn't design the amp correctly, it's not set correctly, or the pickups are microphonic.

If you think of high gain amps like the drag racing cars of the amp world, then things start to make some more sense. Sure, you can race your Honda, but you're going to get stomped by that GTO - but they both have an engine, round wheels, a windshield, etc, but they're far from the same animal.

[trem]

^^^^^ Not Bad..... Thank You

[eschertron]

I chalk that up to poorly informed geriatric players

:shakey: Hold on there, sonny, (pant, pant) I resemble that remark!

[defaced]

I'm glad you saw the humor in that. I generally don't poke back, but I get a little tired of unsolicited condescending opinions, something this board has started to get alot of and is part of the reason I tend not to contribute (over nit picking/arguing is the other).

I put this together for another forum a while ago. It's a very broad stroke "literature review" of amp topologies and gain stages. Obviously there are more examples, but I tried to stick to what I knew so I wasn't completely talking out of my ass, but even then, I don't claim to be really all that familiar with the 5150, Mesa Mark series amps, or ENGLs.

Here's a link to the thread so you can read it in context. Gain stages: How many in series?

I think answering your question is more a matter of intent than theory. How much distortion do you want? That will drive the number of stages. Sure one can take it to the extreme, but practically there are only a handful of useful answers. Looking at some classic amp designs can provide some insight into what sorts of sounds you can get from a particular number of gain stages and a given topology. There tend to be two camps for topology:
Tone stack early in the circuit and plate fed (Fender inspired)
Tone stack late in the circuit and driven by a cathode follower (CF) (Marshall inspired)
And of these two basic topologies, there are mixes of each that provide their own "flavor" to the mix.

Twin - two gain stages. The first one brings the signal up, it feeds a tone stack, and the signal is amplified again. Because the tone stack is in the middle of the two stages, it's a clean setup.
Plexi - two gain stages. The first one brings the signal up, the second clips depending, a CF drives the tone stack.
JCM800 - three grains stages. The first brings the signal up, the second and third clip, the CF drives the tone stack. One of the stages is biased colder to force asymmetric clipping
SLO100/Recto/Uberschall/countless other "high gain" amps - four stages. Similar to the JCM800, but instead of two clipping stages, there are three. CF drives the tone stack.
ENGL - four stages. Just like the SLO100, but instead of a CF driven tone stack, it's plate fed. TS is at the end of the chain.
Mesa Mark V - five gain stages. The first one brings the signal up and feeds a tone stack off the plate, four more provide clipping, and the last of the four drive a plate fed GEQ. This is like a combination between the Fender topology and the Marshall topology, EQ early, EQ late, and plate fed for all (though that doesn't make much difference for the GEQ because it's actually being driven by NPN BJT transistor (MPSA20)).
5150/6505 - six gain stages. The first one brings the signal up, the next five provide clipping, the last of the six stages drives a plate fed tone stack.

So there's alot of permutations of the different number of stages and ways of arranging the parts. By listening to these amps, you can get an idea of what "universe" each configuration can provide. But generally speaking with standard preamp design practices (increase high frequency content while also reducing low frequency content early in the circuit, divide down signal between stages to control clipping, and only use the active elements for clipping), the sound becomes more compressed, saturated, and noisy with an increasing number of stages. Not a very surprising summary, but worth keeping in mind.

[loudthud]

Since tubes respond to an input Voltage on the Grid, the term Gain usually means Voltage Gain. In it's simplest form Voltage Gain, A(v), would be measured as Output Voltage divided by Input Voltage. In the typical preamp stage used in guitar amps, the Voltage Gain is about 50. That assumes that the signal is small enough that it is not clipping on the output. If the signal is clipping, it doesn't really change the Gain, but the Voltage on the output will be less than the product of the input voltage times the Gain.

The amount of signal that a tube can have on it's output depends to a large degree on the B+ Voltage. The tube type and the circuit also have a strong influence. But most tube preamp stages fall into a narrow range. The two stages of a blackface Fender preamp can't get much clipping on the output. If you install a Master Volume after the second stage, you don't get much overdrive and it doesn't sound good until you crank the Master enough to overdrive the power amp. The third reverb recovery stage doesn't add much gain overall because the signal is attenuated between the 2nd and 3rd stage. If you crank up the Master Volume, you can get quite a bit of clipping at the speaker, but the amp is very loud. Guitar players would like to get a strong overdrive and still be able to control the overall volume with the Master Volume control.

Because of the inability to get a really strong overdrive before the Master Volume control, more preamp stages were added. It was discovered that simply adding more preamp stages didn't produce a good sounding overdrive, the circuits needed subtle tweeks between stages. You'll find this in amps like the SLO and 5150. You could obtain the strong overdrive (gain) with pedals, but the feel of tube amp compression would be lost or diminished.

[wizard333]

the number of gain stages is a very good indicator of how much distortion one can get from the amp (and amp alone). Beat the strings as hard as you like, you won't make a Twin/Deluxe/whatever sound like a SLO.

Disagree. 3 gain stages is enough to do insane amounts of distortion, if arranged properly. That's the same 3 stages you have in a Fender verb channel, just they aren't arranged for high gain there. I've done a number of preamps that do SLO kind of distortion with 3 stages. I'm not sold on 4 and 5 stage gain channels. More stages = more noise, and harder to get low gain tones as well as high.

[Chuck H]

I agree. With the caveat that to get modern "uber gain" distortion tones you need to include the PI and power amp. Making five stages in the chain. You can certainly get the same level of saturation with ONLY three stages, BUT, the over drive character of a couple of cascaded stages over driven a lot is not as good (in general) as three stages over driven a little less each. If that makes sense. And, as stated above, over driving the power stage doesn't always suit guitar players that need that level of clipping at less than ear bleeding volumes.

Just to add my .02... The big difference between a Twin Reverb and an SLO, or any other "high gain" amp is a mater of sensitivity and harmonics. It's true enough that viewed with a scope the square wave at the end of the chain looks sort of similar. But the devil is in the small differences. Modern "high gain" amps will clip a signal and then clip that clipped signal. And possibly do it again! What you end up with is a lot less of the original note and a lot more of the overtones and harmonics. If kept stable, this makes for a very dynamic effect that vintage style amps just can't do. At the risk of repeating what has already been said... Gain is "voltage gain" If a stage is overdriven the gain suffers. But the waveform is clipped analogous to that stages inability to amplify the input signal. Somehow this has become a modern description of gain. So we are dealing with different phenomenon applied to the same word. And as far as I know there is no common nomenclature to avoid the dual terminology.

[defaced]

Disagree. 3 gain stages is enough to do insane amounts of distortion, if arranged properly. That's the same 3 stages you have in a Fender verb channel, just they aren't arranged for high gain there. I've done a number of preamps that do SLO kind of distortion with 3 stages. I'm not sold on 4 and 5 stage gain channels. More stages = more noise, and harder to get low gain tones as well as high.

Re-read what I wrote. I never said you couldn't get an insane distortion of gain from 3 stages. I said you couldn't make a Twin/whatever sound like a SLO by any other means than copying the SLO circuit. They're not the same statement; please don't misconstrue them.

With regard to noise, that's a design issue. Plug into an amp made by Fryette and you will understand how quiet a 4 gain stage amp can be. Same for clean/distortion dynamics.

I agree. With the caveat that to get modern "uber gain" distortion tones you need to include the PI and power amp.

Where did you get that notion from? Lots of preamp distortion + PI and power amp distortion = a muddy, uncontrollable mess. There's a reason why the master volume is positioned between the preamp/tone stack and the PI/power amp, and these amps are usually 50/100 watts output. It's because more/less, *all* of the distortion is intended to come from the preamp.

[guitician]

What came first the harmonic or the distortion?

My favorite gain is a wide open single 12ax7 into the power amp(Phase Inv/PP)