Super Champ:
How the Effect Switching Works
By Andrew Waugh of the Princeton Reverb II site

Reverb
Without Footswitch
V2 drives the reverb transformer, which moves the reverb springs. The motion of the springs is picked up and amplified by V3c, which passes this pure reverb signal to the reverb control. That sends the chosen amount of pure reverb via 470Kohm resistor into the main signal path at the input to V3b.

With Footswitch
Same as above, except the pure reverb signal is also sent all the way out to the footswitch on the plain jack ring terminal. "F/s reverb off" switches the pure reverb signal direct to ground in the f/s - a zero resistance compared with  the high (470k) resistance it would have to overcome in order to get into the main signal path; so you don't hear any reverb. "F/s reverb on" opens the f/s reverb switch, so the ground path isn't there and the pure reverb signal behaves as though the f/s wasn't there at all, ie, it heads for the main signal path through the 470K resistor. In this setting the f/s also allows the -6.2V supply, through the red tip terminal, an LED and a 390ohm resistor, to go down to ground (all inside the f/s), so you get an LED indication that reverb is on.

Lead
The overdriven, distorted sound made by taking a huge, boosted signal from V2 via the input (primary) side of the reverb transformer and sending it to the main signal path at V3b, thus distorting V3b and everything after it. But how does that big signal get there?

For this you have to understand how the optocoupler works. It is shown on the schematic like this. I'll label the 4 terminals a,b,+ and – .

Inside the optocoupler, a is not connected to + or - at any time. Also b is not connected to + or - at any time.

When + is connected to a positive DC voltage and - is connected to something at a lower voltage, a connects to b by a low resistance. It's not quite a perfect zero resistance (like a good mechanical switch) but it's close. Let's call this "on".

When + and/or - is disconnected, the resistance from a to b goes very high. It's not infinite (as in a good mechanical switch) but it's high enough to be called "off".

In the Super Champ the opto has - permanently wired to -6.2V. That's 6.2V BELOW ground level. So connecting  + to ground will turn the opto "on" because + will be 6.2V ABOVE  - . (The -6.2V is also used to power the LEDs in the footswitch; it comes from a 6.2V zener which is connected to the bias supply circuit for the power valves/tubes.)

Without Footswitch
The 'pull for lead' knob closes 2 mechanical switches on the back side of the volume knob. One switch is drawn near the opto and when pulled it allows the boosted signal to go straight to the input of V3b via a 220K resistor. The other switch is drawn near the red f/s socket. When pulled it connects the non-signal end of the lead level pot to ground, thus allowing the pot to act like a volume control for the whole amp in conjunction with the master volume.

When 'pull for lead' isn't pulled (ie you want the clean sound), both switches are open. The boosted signal never gets into the signal path. The bottom end of the lead level pot is disconnected - it just 'floats'. Therefore it can't act as a volume control and its setting doesn't matter.

Without the footswitch the opto is always off and so never affects the signal routing.

With Footswitch
For operation according to the manual, "Pull for lead" should always be pulled when the f/s is connected, so both pull-switches are on. When the f/s red plug is connected, this operates an extra switch in the red socket so that the ring terminal isn't always grounded. This means the lead level pot can only be grounded via the f/s 'lead' switch.

With the f/s set for 'clean', the lead level control is out of circuit as described above. Meanwhile the opto is turned on by having its + grounded by the lead switch in the f/s, via the plain jack tip terminal. Therefore the boosted signal goes down to ground via the opto's a and b. So you don't hear the boosted signal and the lead level control has no action.

With the f/s set for 'lead', the lead level control is in operation because its bottom end is grounded in the footswitch via the red ring terminal. The opto’s + is disconnected from ground in the f/s, so the opto is off and the boost signal heads up to V3b instead of down to ground. Also the lead switch in the f/s grounds another f/s LED, which is fed -6.2V via another 390ohm resistor, so you get an LED showing lead is on.