Help Draining Caps on amp

[Jtwusmc]

Hey forum. Probably a simple question, bit i want to be safe than sorry...

I'm working on a bugera 6260 amp today. I need to drain the caps, but they are different than what I have worked with. Usually I drain the cap from the ends/ wires coming off each individual cap. This amp has the caps plugged directly into the board where I can not get to the ends (like both wires coming out of one end that are face down into the board).

I have heard about touching 1 side of a resistor to a pin in a tube (pin 1 of preamp tube 1 in a 12Ax7 setup) and one to a ground. I wanted to see if this is correct. Also, does it matter if the tubes are still in the amp when I do this?

Thanks.

[Jtwusmc]

Heres a pic of the 2 biggest caps. Does the resistor nextto them indicate that tey could be self draining?

[Jazz P Bass]

I cannot tell from the photo what the designations are on the resistors.
I have attached that part of the schematic.
The top of R51 is the B+ voltage.
The bottom of R52 is ground.
With power removed you can attach a 1K 2 watt resistor between these two points.
Hook up your meter to those points first.
Set it to read volts dc.
Then attach the resistor across the meter leads.

[Wittgenstein]

I made a cap discharge tool using a 5W 15K wirewound resistor, plastic pen tube, wire, alligator clip, and a nail. Just solder it all up with the resistor housed in the pen. It looks something like this:
http://i.stack.imgur.com/ZqRX1.jpg

Then just clip the alligator clip to the chassis or ground, and touch the nail to the cap and hold it there for a few seconds. You can put the multimeter in line to make sure that it is dissipated. NOTE: You should always do this with one hand behind your back or in your pocket. NEVER have your other hand holding on to metal grounding out.

If you use the chassis or the bottom of R52 as P Bass suggested than having the tubes in will not matter. If you are trying to ground out to pin 1 of the preamp tube, than yes you would need the tube to act as the resistor.

[Steve Conner]

I think it's a fair bet that the two 220k resistors directly under those caps are for voltage balancing and bleeding. In other words "self draining" within a few minutes. However I accept no responsibility for your death by electrocution.

[52 Bill]

If Jazz's schematic is correct, both plate and screen supplies have the 220K bleeders installed, so it should be safe after a few seconds, even with the standby switch off.

The safe way would be to just meter the voltage to see that the voltage has drained down.

[Gingertube]

More generally the scheme of shorting from pin 1 or pin 6 (anodes) on the first preamp tube to 0V mentioned in the original post is a good scheme - you will always have anode load resistor and B+ dropping resistors in series. You can safely use this scheme with no knowledge of the actual circuit.

Cheers,
Ian

[Steve Conner]

You can safely use this scheme with no knowledge of the actual circuit.

Not quite, you still need to know which of the tubes peeking out at you is the "first preamp tube". It's not always conveniently labelled V1, or placed next to the input jack.

To be fair though, it would probably work with any of the preamp tubes, though you might get a surprise from a DC coupled cathode follower.