The relay drive circuits use sections of the LM324s as comparators. A given input goes "low" to turn on the relay, i.e., to enable the circuit section in question. Inputs can be driven "low" either with the six front panel switches, or using the optional footswitch unit. The LM324s drive 2N2222 transistors for greater current capacity; the transistors should handle up to 600mA each, more than enough to drive two parallel relay coils (as in the reverb section).
The 1M resistors produce a hysteresis band through positive feedback: i.e., if the input voltage drops to a low enough value to just barely flip the output to "high", it must then rise more than the width of the hysteresis band (perhaps about 0.5v) before the output will flip back to "low". This prevents oscillation or noise when the input voltage is close to the threshold voltage; the threshold shifts slightly, making the output "reluctant" to change.
This control circuit is completely isolated from the audio portion of the amp (note the use of a different ground symbol). Power comes from a small plug-in DC power supply (aka "wall wart"), which plugs into an AC outlet provided for this purpose, inside the amp. This outlet is switched by the "preamp" power switch.
Originally, I thought I could use the stereo effects return jacks as power amp inputs; doing this would have required some tricky wiring in the bypass relay circuits, so that the jacks would function properly even when the preamp power is off. However, I have since relocated the effects returns such that they can't double as power amp inputs, anyway. So, no tricky wiring, all six bypass circuits operate the same way. And there will need to be four additional jacks provided on the back panel: stereo pairs of "pre out" and "pwr in".
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