How to discharge filter caps?

[Valvestate]

Hi,

I have had to open my amp up to perform some maintenance and I now want to discharge the filter caps before I remove the main board. I have done this in the past with a board that was already out of the housing using a 10W 10K resistor on the UNDERSIDE of the board. This time I want to discharge the caps while the board it in place and I don't have access to the underside...Is this possible, if so how?

How do the amp repair veterans in here go about doing this?

Thanks!

[Diablo]

If it's a tube amp, all you have to do is use a grounding jumper to any of the tube plate pins. 12AX7 tube is either pin 6 or pin 1. 6L6 tube is pin 3. You can put a resistor inline with your jumper if you like. Leave the jumper on a good long time to fully discharge all the caps. Measure the voltage to be sure it's safe.

[booj]

Make sure you ARE getting violtage to the preamp section first. If not you have an open somewhere between the HV and the pre supply, in which case the above method won't work. I had that happen the same day I gave the same advice to someone who worked at a music store!

[Valvestate]

I do have voltage. Surely if I didn't have voltage the amp would not be working well at all?

How long do those caps hold a charge for? I measured the voltage after having the amp off and unplugged for about 3 hours and I only was reading about 30V.

[Diablo]

The voltage you measure over time after shutting down the amp depends on the details of the circuit. If there are bleeder resistors on the main filter caps, then the voltage will drop to zero within a minute or two. Preamp filter caps make take a little longer to bleed down because of the dropping resistors on the B+, which slow the current flow through the bleed resistors.

You can put your own bleeder resistor in the amp too. You can wire in a 400K-500K - 2 watt resistor on the filter cap side of the standby switch. The other side of the resistor obviously goes to ground. After you hook up the resistor, check your work with a voltmeter to make sure it works properly. Even on amps that have bleeder resistors, I still go in and use a jumper cable to discharge the caps - just to be sure. The most foolproof method is to discharge each individual cap.

[acorkos]

If it's a tube amp, all you have to do is use a grounding jumper to any of the tube plate pins. 12AX7 tube is either pin 6 or pin 1.

flip the standby switch to 'operate,' too (with the amp powered down and unplugged, of course).

and i always check voltages (should be zero) before i start touching stuff inside the chassis

[vlahor]

You can put few 220k with crocodile clips and measure droping voltage on every cap