Draining Filter Cap Question

[bhuether]

I understand (from a previous post of mine) a common method involves connecting jumper wire from the chassis to pin 3 of a power tube. Just wondering what type of cable is acceptable and what type of insulation on the cable as well as the alligator clips. I can picture draining these caps generating a lot of heat and want to be absolutely sure that the insulation holds up! Also, would it be worthwhile to connect a power resistor in the middle of the cable? If so, what specs?

I am on the mouser site now ordering parts for a mod that Chuck H recommended me and figure I may as well order parts for the filter cap draining.

thanks,

brian

[defaced]

Heat is a function of power dissipation, and the filter caps in guitar amps don't store enough current to really produce excessive amounts of heat when draining.

A 220k, 2w resistor will be plenty to drain the caps. I wire them directly across the cap so that they bleed the voltage off when the amps is turned off. These are called bleeder resistors. If you want to do it as a jumper, then normal 300v PVC coated stranded wire will work. I'd use anything between 20 and 18 gauge.

[Chuck H]

As Defaced said...

FWIW your amp DOES have bleeder resistors in the circuit, and so drains itself whenever it is turned off. Also, your amp does not need to have the standby in the "play" position for this to happen, where some amps do.

The method I described for you is a universal, fool proof method that will keep you from getting shocked no matter what amp you work on. Some seasoned amp techs use this method no matter what they are working on just to be safe because it is repetition of good general safety practices.

If you want to idealize the system a 100k 3 watt resistor in between the alligator clips is a good idea. It slows down the discharge which I suppose is better for the capacitors. And as Defaced said, 20 to 18ga. wire is fine, Just to be on the safe side use 600v insulation of any kind (pvc, silicone, etc.) Use stranded wire. I have a half dozen alligator to alligator leads that I made from broken test leads. Nice and flexible.

Chuck

[Robmosis]

What is the best way to insulate the alligator clips themselves. All I can find locally are all metal clips. Would wrapping them with electrical tape be good enough for draining purposes.
Thanks Rob

[MWJB]

"I understand (from a previous post of mine) a common method involves connecting jumper wire from the chassis to pin 3 of a power tube." I'm not aware that this is a common method at all - the more common method would be to utilise the dropping resistors in the power supply and clip the jumper to pin 1 or 6 of a 12AX7 style PREAMP tube & wait 30 seconds. Don't need big wire, those bags of insulated jumper leads work fine.

[billyboy]

Sometimes when i'm in a hurry i put screwdriver accross stdby or pwr switch or even caps to ground. Am I damaging caps or something doing this?

[boyt_bpc]

I have a question on this same topic. I was draining the caps on my Blues Junior. After letting them drain for a few minutes, I checked them with my VOM and there was still around 10~20 mV. Is that normal? Would they ever drain to 0?

Thanks,

Boyt

[defaced]

With infinite time, yes. This is due to the draining and charging properties of an RC pair. Also, that little voltage with your DMM set to the high voltage scale could be error.

Am I damaging caps or something doing this?

This is functionally how a transformer/rectifier arc welder works. Unless you are exceeding some rating of the cap while doing this, you're not going to harm it.

[boyt_bpc]

I was getting this with my VOM set on the minimum DC voltage. After letting them drain for a while I deduced that this voltage would remain and that I'd avoid the caps and be fine (I was only working on the tone pot).

[Enzo]

A volt or two sitting in a cap offers no danger.

If you short out a cap to drain it, it will tend to recharge itself a little due to "dielectric absorbtion." Furhtermore, that cap is connected to a whole circuit, and there are other capacitances scattered around in it removed by resistances. SO shorting the main cap doesn;t short those other capacitances, it tries to discharge them through those resistances. Remove the shoort from that main cap and the remaining charges scattered about will try to even out by moving some charge back into the main cap.

SO to actually get absolutely every volt out of a power supply can take some time. But it also isn't necessary.

[Chuck H]

Some alligator clips are sold with insulated booties. Sometimes the booties are sold as accesories. I think even Radio Shack sells insulated alligator clips but if you have trouble finding them you can order them from Mouser. In the interem you can just use an uninsulated clip to ground, then hold the other end of the lead with a pair of insulates pliers and touch it to the + terminal of a preamp filter for about a minute. Always test voltage AFTER draining the caps. Excess flux or some other grime can sometimes prevent good contact and the caps will not drain.

Chuck

[boyt_bpc]

Some alligator clips are sold with insulated booties. Sometimes the booties are sold as accesories. I think even Radio Shack sells insulated alligator clips but if you have trouble finding them you can order them from Mouser. In the interem you can just use an uninsulated clip to ground, then hold the other end of the lead with a pair of insulates pliers and touch it to the + terminal of a preamp filter for about a minute. Always test voltage AFTER draining the caps. Excess flux or some other grime can sometimes prevent good contact and the caps will not drain.

Chuck

I've made two cap draining tools. The first has insulated aligator clips on both ends and a 2watt 5k ohm resistor. The second is a test lead form an old VOM that I clipped the banana plug off of and installed an insulated aligator clip. That one is real convenient for getting into the tight places and draining those caps.

[jrfrond]

I don't know why anyone gets all fancy on this! Plug a cable into the amp, turn it on, hold the plug tip on the other end to make the amp hum loudly and the turn it or at the off switch or unplug it. When the sound dies, the caps are discharged. I always check with my DVM to make sure, but this is what I usually do.

[boyt_bpc]

I don't know why anyone gets all fancy on this! Plug a cable into the amp, turn it on, hold the plug tip on the other end to make the amp hum loudly and the turn it or at the off switch or unplug it. When the sound dies, the caps are discharged. I always check with my DVM to make sure, but this is what I usually do.

That strikes me as a seriously bad idea.

[jrfrond]

That strikes me as a seriously bad idea.

OK, I've been doing this for over 30 years. What strikes you as a bad idea? Essentially, you drain the caps through the speaker load. You need to provide a signal in order to help the caps drain faster, so I use the hum from an open cable being held in my hand. It could be from a guitar or signal generator as well. Best of all, it's totally non-invasive and does not require thinking about where to place a bleeder resistor. If the amp, by some chance, will not power up, the caps are probably discharged anyway.

And, like I said, I ALWAYS double-check when I open the amp with my DVM, and you (meaning everybody) should too!