This amp has a lot of features and I'm not following everything going on in this circuit, but here's what I see:
Hope this is helpful!
- The power amp section uses discrete transistors. The op-amps are all used in the preamp.
- U1A is the first gain stage in the "norm gain" section. How much of its output ends up in mix is controlled by the norm "pre" knob.
- U1B is the first gain stage in the "lead gain" section. The "saturation" knob adjusts the amount of hard clipping and tone shaping (simultaneously, it's a dual-gang pot) in this stage. How much of this stage's output ends up in the mix is controlled by the lead "pre" and "post" knobs (which I think adjust the volume before or after the tone stack).
- I'm not following all the details of how it works, but it looks like U2B is involved in the working of the foot switch, which connects to the lead gain, reverb, and tone sections. Does it turn them all on and off at once?
- U2A is a gain stage driving the tone stack. High/mid/low/thick controls happen after this stage.
- U3A recovers gain after (most of) the tone stack. Presence adjusts the high frequencies in the feedback loop of this stage.
- You're correct about U4. U4B drives the reverb tank and U4A amplifies the reverb output to recover volume.
- U3B mixes the dry preamp and reverb outputs at its input, and drives the preamp out / effects send.
Hope this is helpful!
Wow, thanks so much for that - that's super helpful.
I was mainly interested in figuring out which part of the circuit is responsible for the "saturation" clipping in the preamp. I guess that's happening in U1B. I had thought maybe that's what U2 was for. That's really interesting that U1 includes the 'tone shaping' (probably a low cut) - I wondered what that dual gang pot was about.
I was also interested in why the U3 gain stage was needed. I guess maybe this is like the phase inverter stage (without the phase inversion for push-pull?) in a tube amp (?)
I should have mentioned that the footswitch (reverb/channel select) is necessary in order to use it. Without it plugged in, you can't use the 'normal' channel at all - in my case I don't have it so I'm just on the lead channel.
I was mainly interested in figuring out which part of the circuit is responsible for the "saturation" clipping in the preamp. I guess that's happening in U1B. I had thought maybe that's what U2 was for. That's really interesting that U1 includes the 'tone shaping' (probably a low cut) - I wondered what that dual gang pot was about.
I was also interested in why the U3 gain stage was needed. I guess maybe this is like the phase inverter stage (without the phase inversion for push-pull?) in a tube amp (?)
I should have mentioned that the footswitch (reverb/channel select) is necessary in order to use it. Without it plugged in, you can't use the 'normal' channel at all - in my case I don't have it so I'm just on the lead channel.
I believe U3 is a dual op-amp chip with two op-amps in it, U3A and U3B. Both of those appear in the circuit around where a push-pull tube amp would have a phase-inverter, but they don't serve the same purpose at all.
U3A I think is mostly just amplifying things back up after the tone stack loses some volume. Lots of amps with TMB-style tone controls have a stage like this. The presence control also happens in U3A's feedback loop. In tube amps presence is usually part of the feedback loop from the output transformer back to before the phase inverter, so it's happening at a somewhat similar part of the circuit, but not exactly the same.
U3B is needed as an effects loop buffer. At this point in the circuit, the dry signal from the preamp and the wet signal from the reverb tank are mixed, using some mixing resistors. But the next thing that happens is the effects loop, where a long cable can be inserted. Resistance right before a long cable rolls off high frequencies. (It creates an RC low-pass filter, because cables have some capacitance between their conductors.) We need a low-impedance output to push as much current down the cable as is necessary to maintain the desired signal voltage all the way to the other side, despite cable capacitance. That's what U3B does.
If you want the normal channel, I think you could get it just by plugging in something like a pedal patch cable with nothing connected on the other side. The T/S (no ring) guitar plug will connect the "ring" and "ground" conductors in the jack, putting the effects select in the other position.
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