Filter cap voltage

[grandmasterflash213]

When opening up an amp, can it just be left unplugged for a few days to let the filter capacitors drain this way, or would it take longer than that?

[MWJB]

It takes longer than that...much longer! An amp that hasn't been played for months can burn a hole in your finger if you inadvertantly (well you wouldn't do it on purpose) touch something that's still holding charge.

Play safe and discharge the caps with a 2 or 3W 10K resistor (crocodile clips at each end & shrinkwrapped for insulation) from + to ground, or ground a preamp plate. Doug Hoffman suggests switching the power off (standby still in play mode) and continuing to play until sound dies.

Amps that use 2x filter caps, bypassed with smoothing resistors, in a totem pole for the first B+ node (big SF/BF Fenders, NOT brown/blonde pre '64 amps) drain themselves if you power down, unplug and put the standby into play position.

[Amp Kat]

There are several nodes in an amp so just because the B+ main supply is drained doesn't mean a preamp or other nodes are drained depending on it's resistance to ground. The larger the resistance the longer it takes to discharge which is why when the standby switch is on that is basically an open circuit or infinite resistance therefore taking it along time to discharge. You can pop a tube out and check from pin 3 to ground and find out how much DC is present on the B+ supply.