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Changed the cap and resistor, the signal is increasing in volume and it is brighter but the other way around. Turning bright off is loud, turning it on is not as loud

Hello Tuck
yes - exactly like the Ampeg
You could always remove the input jfet buffer and connect the input socket directly to the normal/bright switch - in this case you would have to scale up the resistor values x10 (100k, 47k, and 5M6) and decrease the capacitor value x10 (220p) - then it will be exactly the same input impedance as the Ampeg SVT
cheers
bajaman

I was just experimenting with running this pedal at 12v instead of 9v. With the higher voltage, I'm getting more headroom, but also a low-level hum that's absent otherwise. Any ideas why that might be?

What sort of 12v supply are you using ?
It could be your 12v supply that is causing the noise, especially if it is plugged into a power point.

Yeah could be. I haven't tried any others yet. It's just a generic wall wart supply, and it's labeled as 9v even though I happened to measure it one day and found out it's putting out 12. I'll try another.

Whoops...I plugged in a 12v supply but forgot that its polarity was the opposite of what the pedal expects, and now it's distorting in a way it wasn't before. Did I fry an opamp maybe?

Looking at the schematic there's very little else to go wrong. Check your voltages at the IC pins.

I'm way out of my depth here, but I'm reading some mismatches of 50-150 mV on most of the + and - input voltage pin pairs, which, if I understand correctly, is often a sign that something is wrong. I'll just replace them, use sockets this time, and hope for the best.