Yall wanna nerd out for a second?
Caution - lots of info ahead. All of this is just my understanding and how I think of it. I might be wrong on some of this, but this is how I "get it".
Here's a pic of my scope screen showing oscillation.
This is the 2nd gain stage of the vibrato channel. V2b - second tube, second stage. FYI, 12AX7s are actually two tubes in one. Anyway, signal goes into the input jack straight to the first stage - V2a, it comes back to the volume pot, and the volume pot controls the level that gets sent on to this stage. This should be a nice even smooth sine wave. But it aint. Those little hiccups in there are oscillation. The tube is running a little bit haywire. Why? I'll show you.
This is the next tube socket - V3.
This tube takes a split of the signal from V2 and sends it through the reverb tank. The other half of the split V2 signal goes to the next gain stage - V4b. But we're only worried about this V3 half for now. Both halves of this tube are used to push the signal through the massive load that is the reverb transformer and tank. The reverb tank is essentially like a speaker load. With the reverb tank disconnected, or with this V3 tube removed, my sine wave at the previous stage was nice and even. So the problem lies with this tube/wiring - generally reverb related. See that little loop of blue wire connecting two tube pins? That wire bridges the plates of the tube and carries very high voltage. This wire was inducing spasms into the virtually empty wires around it. The green wire is a grid wire that carries actual guitar signal so there's very little voltage in there. And the yellow wire is a cathode wire - basically empty. It sets the operating bias of the tube. So those two weak ass wires are easily influenced by something as brutal as high voltage plate wires. When I'd move the blue looped wire around with a chopstick, the signal went batshit crazy and into full-on oscillation. The tube was losing it's mind. So I just desoldered that wire and flipped it around to loop to the other side of the tube socket. I should have done that to begin with, but whatever. My kinky signal at the V2b grid went nice and clean and perfect. Yay!
So here's where it is now.
These pics show the scope set for two channels. First off....
The yellow line is my signal input at the input jack. It looks big, but that channel of the scope is scaled down to show fine details. For the yellow signal, each Y-axis square on the screen is 50 mv. So as you can see, it's about 50 mv each way, or 100 mv peak to peak. Easy, right? That's like a very weakish guitar signal. But it's steady and set to 1khz.
The blue line is the output channel probing the speaker jack. It's going to get a much more substantial signal, so it's scaled to 10v per division. That's 200 times the scale but that's so you can see them on the same screen and on top of each other. If we set the blue channel to the yellow channels scale, the output signal would fly off the screen through the ceiling.
But that's just the set up. The yellow signal will be unchanged because it's steady at the input jack. Nothing we do on the amp affects that testing point. The blue line is what we're watching.
So here are the signals with the volume on 0. The yellow line is our input signal, the blue line is flatlined because the volume is on 0, duh. There's no output to measure.
And here we are with the volume on 5. The yellow signal is still the same because it's being read at the input jack and that doesn't change, but now look at the blue line. That's being read at the speaker jack. We have signal. And a good amount of it. But it's still pretty clean. The waves look about the same size, but remember the scaling. We're only at 50mv peak going in, but we're tipping over 10 whole volts coming out.
And now, for the grand finale....let's crank this bitch and see what the signal does. Volume on 10. Kapow.
As you can see, we're full on clipping. That's a lot of signal, and it's been hammered by sweet ass natural power stage clipping. We start with 50 mv and pump out over 20 volts. That's an amplifier doing work....amplifying! We've turned that puny sine wave into a pretty substantial symmetrical square wave, and that's our wonderful all natural amp distortion.
For clarification...that output voltage is actually lowered from what's inside the amp at the power tubes. That shit is hundreds of ass shocking volts on the inside, but it's low amperage. We can't drive speakers with all that voltage and no amperage. So enter the output transformer. It does exactly what it's name implies - it transforms the high voltage low amp inside signal to speaker driving low voltage high amperage signal.
And that's that for now.