My Dynakit ST120 is playing along 17 hours a day with one 3300 uf cap to power both channels. No hum much, I get a lot more from the mixer when the power is on. My Wurli 4500 has such wimpy looking heatsink and one really old output transistor per rail, I think it might be only a 15 W amp. The power transformer is really tiny also, 2.5'x2.5'x3.5', whereas the St120 transformer is about 5'x5'x3.5'. Paul has had some Wurli 4x00's blow the power rectifier before, so if you do decide to upgrade the main capacitors, put in a NTCR like a CL90 in the power transformer feed. The actual one would depend on your hot and cold current. Or, really cheaply, put in higher current rated rectifiers. About 1965-70 some people were still using germanium rectifiers because of cost that looks pretty silly today. Ge's are ~300 mv forwards, SI are 450-700 mv. Schottky recifiers now drop no voltage forwards, but they are only good up to 45 PIV (peak inverse volts). If the hum level varies with the expression pedal, it is probably from before the signal reaches the amplifier chassis. You need to identify whether it is power related e.g. from faulty regulation, or pickup on the signal path. If it is mostly 120Hz then probably power supplies are at fault.
