What resistor value to use to discharge filter caps?

Philtre

Some people say 10k at 5w, others 20k at 5w. Others say 10w. What's a good value to work with?

ICBM

I would go for a higher value - it takes longer to discharge, but you don't need such a high power rating and it's less hard on the caps. 100K 2W should be about right.

It's much easier just to turn off the amp in the right way though, it's rare that you actually need to drain a fully-charged filter cap. The problem is that the right way is not always the same for different amps.

Easiest if you do need to drain them to be sure is just to clip a lead from ground to one of the plate pins of the first preamp valve, that will drain all the caps via the B+ chain resistors regardless, as long as the standby switch is set to 'play'.

Philtre

Thanks @ICBM - I just want to be on the safe side when re-doing dry solder joints. I want to make a discharge connector using 12 AWG stranded wire, a suitable resistor and some alligator clips.

martinw

Just get a heavy duty test lead, cut it and solder in a resistor, and heat shrink it. I also use 100k/2W

RiftAmps

I just connect the plate of an output valve to chassis with a test lead. Only works if the valve is in the socket mind.

Philtre

I made up a lead using 47k 5w. Does the trick.

Question - once I've discharged the caps to less than 1v, I can see the voltage very slowly creeping back up again. What would cause this?

RiftAmps

Philtre said:

I made up a lead using 47k 5w. Does the trick.

Question - once I've discharged the caps to less than 1v, I can see the voltage very slowly creeping back up again. What would cause this?

Dialectric Absorption - http://www.robotroom.com/Capacitor-Self-Discharge-5.html

Philtre

@RiftAmps ; Thank-you sir!