BZZZZZZZT!!!! Suprise!

[Axtman]

I got the snot shocked out of me yesterday. I was working on a Univox tube amp trying to reattach the choke transformer. My hand must have slipped and touched the back of the standby switch. I had a firm hold of the chassis with both hands. Boy did I let go quickly. Electricity must have raced through my arms and across my chest. Luckily I am fairly young and healthy. My fingers hurt the most. The amp was unplugged and off. The funny thing is that it felt like AC current.

If the filter caps don't have bleeder resistors, what is the safest way to drain them. I don't want a repeat experience.

If you don't hear from me again, send flowers.

[tubeswell]

Edit - oOps I didn't read the bit about the amp already being unplugged. You should always drain the caps and check the voltage before working on the amp. You need to drain them thru a resistor. You can use one of the plate resistors in the pre-amp. Hook one side of your meter to the chassis ground, and the other end to the plate side of teh plate resistor and watch the voltage go down.

You should also go and get a check over ASAP.

[CharlieP]

You can use a resistor of high ohm , say 33k or more. Bend the leads to short across the cap leads. Hold on each cap approx. 20 seconds on each. Take a voltage reading and if voltage is still there repeat until voltage is low. Remainder can be shorted to chassis without causing a welding arc. Use an insulated pair of pliers and grab the resistor by the body only. Use an insulated probe to short to chassis.

[Capacitordeath]

oootch. My name pretty much sums up my worry.
I've seen a few posts on here, and a few posts around the 'net, and the biggest suggestion is to get one of those big, ugly concrete resistors and short each capacitor with it. I picked a couple of 10kohm/10watt resistors at radio shack for like 3 bucks.
then...bend those leads out wide, grab your resistor with a pair of rubberized handle pliers, and short the leads to the cap. there's an equation to figure out how long to do it, but odds are you'll have it drained down to weakness in 30 seconds.
The smaller a resistor you have, the faster it goes. you go too small, and you'll pop the resistor and shoot burnt epoxy into your fingers and eyes. too big, and your arm will get tired. (I guess that's the bad side). One source I read said to stay away from gator clips, as the insulation may not be good enough to stop the current.
I guess the theory is to run the current through the resistor and convert it to heat, dissipating it. Smaller resistors will heat up faster, and not be able to take it. You'll never run the current out completely, but if you get it down low enough, the current can't get through your resistance.

I STILL have yet to do this to my project, out of sheer lack of time. Tell me how it goes...if you're still alive. If not, please send someone an email via seance.

[Don Moose]

Rather than handle a leaded resistor and try to touch it to capacitor terminals, solder an insulated gator clip to one (sleeved) lead, and a foot or so of insulated wire to the other (sleeved) lead with another insulated gator on the end. This is a gadget you're (hopefully) going to use on every amp you open, so you might as well do it right.

Clip the short gator to chassis, hit the other caps with the long lead, leaving it clipped to the main filter when you're working.

This is a gadget you're going to use on every amp you open, so you might as well do it right.

Value? Not terribly important - at least 33k, not more than 1M. Fixing it up so the gators carry your meter leads as well is good - forces you to watch the voltage sink.

Hope this helps!

[tubeswell]

Or put permanent 200k 2W bleeder resistor(s) in parallel with the reservoir (input) filter cap. Then your amp will drain automatically everytime you switch it off.

[tim]

I cut a hefty alligator clip lead in half and soldered a 47K ohm 5 watt resistor in the middle of it and then covered the cement block resistor and solder joints with a piece of heatshrink.

Alot of times, I'll clip one lead on the chassis and the other end on a long thin screwdriver and then touch the tip of the screwdriver to the various high voltage points in the circuit for a few seconds each....the added safety to this is as you are holding the screwdriver, your hand is on an insulated handle and thus less likely to have your hands close to the high voltage.

Don't forget, the higher the resistance of the resistor, the longer you have to hold it to the high voltage (RC time constant).

Also, occasionally check your resistor with an ohmeter...I've had mine become open before...not very good protection in that condition.