Gain on a Guitar Amp

[CeaSaR]

How does the gain on a guitar amp work?

On all the amps I have seen, the gain doesn't really up the volume, but works to distort the signal.
If all it does is distort the signal, why call it gain?

I know that distortion circuits operate very near saturation so that small adjustments overdrive the
amplifier into distortion while not really adding much amplification.

Would something like a low gain stage set very near saturation at the beginning of the signal chain
be considered "gain" in a guitar amp? For example, my Mini Amp schematic at the bottom of the linked
page could have a very low gain stage right after the signal generator, before R9 (100k level control)
that would serve as the "gain" section. Am I picturing this right?

The music business definitely has it's own way of naming things. Boy am I confused, I think...

Thanks,

CeaSaR

[MrAl]

Hi Ceasar,


You linked to the first page of a several page thread. Could you link to the schematic
so we can take a look?

They might call it 'gain' because it's the gain of the stage that drives the next stage
into clipping. The higher the gain, the more the clipping that takes place, and the
clipping is what makes the sound of the distortion.
There is a secondary effect too however. That is that the extra gain causes the
signal from the guitar to "last" longer, because even though the signal from the
pickup starts to diminish over time the gain keeps it up higher than normal, so
the second stage gets a very high level. Since the second stage clips, it's acting
as a sort of gain limiter, which means it ends up having a fairly constant output
signal level, with the time duration of this action also depending on the 'gain'
setting. This is commonly referred to as "sustain" in the guitar realm.

So in the end the correct terminology would not really be 'distortion' either, because
it also controls the sustain time. We'd have to call it "distortion and sustain time" i guess,
so that's probably why they went with the simpler "gain".

[CeaSaR]

Sorry about that MrAl. I went to page 6 and copied the address in the bar. When I click on the link, it goes to page 6. Hmmm.
Anyway, here's the schematic:


Yep, know about sustain, just not how it works.

Distortion + Sustain, I guess that DistStain sounds too much like disdain, so Gain would sound like a better name.
(Say that 3 times fast! LOL)

CeaSaR

[CeaSaR]

I saw a schematic for a 10 watt guitar amp that shows the "overdrive" as being a capacitor that is switched
in or out of stage 1 of the preamp. IIRC what MrAl taught me in my preamp thread, this allows for more gain -
hence overdrive - of that particular amp stage. You can see the schematic here at electronics-lab.com.
Do I understand that correctly? I like to think that at least some of what all of you have shared with me has
sunk in.

On another point, redcircuits.com has quite a few schematics that might be of interest to some of you, at
least those with a bent towards audio.

CeaSaR

[MrAl]

<snip>
Would something like a low gain stage set very near saturation at the beginning of the signal chain
be considered "gain" in a guitar amp?
For example, my schematic <snip> could have a very low gain stage right after the signal generator,
before R9 (100k level control) that would serve as the "gain" section. Am I picturing this right?

Hi again Ceasar,

I think you have the main idea right. Now all you would have to do is to test that injecting
more gain at that point would actually work with this particular amplifier.
Along those same lines, i got lucky a long long time ago with a guitar amp that had two
preamps and only one power stage, by taking the output of one preamp and connecting it
into the input of the second preamp. Wow, did i get a lot of distortion that way, and that's
just about what you intend to try here except you have to supply a second preamp.
Something overheated one time during a performance and the amp quit, until it cooled off,
so you also have to make sure nothing overheats. Lucky it wasnt during a solo

Another idea that is sometimes used is to take two diodes and connect them anti parallel,
then drive them with a high gain amp with some resistance in series with the output, and
take the output from across the two diodes and amplify that a bit too or maybe feed that
into the main power amp. The two diodes clip the incoming signal and provides lots of
distortion and a bit of a limiting action to get the sustain. The input gain has to be
varied a bit to get the right level for the desired effect.
Diodes like 1N4148 or Schottky should work fine, but probably not regular rectifier diodes.

[sofaspud]

CeaSaR, you do have it right, and if I might add ('cause it keeps my gray matter from atrophy)...
switching the capacitor in puts the emitter at AC ground, bypassing the resistor. Mr. Ohm says
less resistance equals more current, equals more gain from the same voltage. The choice of
capacitors would then each have their own tone due to Xc. While not a guitarist, I am aware of
their quest for tone. Hope this helps you visualize the circuit operation.
P.S. And I guess in this case, it's the source not the emitter.

[CeaSaR]

Sofaspud,

Yeah, MrAl taught me that also. I used it in that preamp (see here, about 3/4 the way down). See C5-C7?
They are the bass "contour" caps. The 100 uF sounds much more "full" / has more bass, but the 10 uF gives
what some "artists" like as that thinner sound that lets you present the guitar "more".

Thanks again,

CeaSaR