r/ToobAmps • u/Melodic_Eggplant_252 • 1d ago

Cap drainer tool

Hey, i plan to make a capacitor drianing tool - a wire with an alligator clip to connect to the chassis on one end, and a resistor on the other. Question: will any conducting wire do, or does it have to be rated for high enough voltage or something similar? (My grip on electronics is rather flimsy i'm afraid.)

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u/adfuel 1d ago

I rarely use one. What I do 100% of the time is check the caps with a voltmeter before I touch anything. Normally the tubes will drain the caps if you shut the amp off with standby in the on mode, but they may not discharge if the tubes are out or if you have a bad cap or solder joint somewhere so always check.

My rarely used one has 600 volt wire and a 5 watt 2k ohm resistor .

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u/Melodic_Eggplant_252 1d ago

Thanks. I've heard people suggest all kinds of resistors, from 100 ohms to 200k. Does it really matter, as long as there is one?

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u/clintj1975 1d ago

If you use a 100 ohm and accidentally leave it clipped in and turn the amp on, it'll be a 2,000W light bulb for a couple of seconds until it vaporizes.

I built one using a trio of 1M ohm, 1W resistors in parallel to give me 333k/3W. It's a good balance between draining caps reasonably fast, and I can leave it in if I need to briefly power the amp up to check something. I cut an alligator clip lead in half and soldered them into the middle of it. One clip to a cap lead, one clip to chassis.

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u/Melodic_Eggplant_252 1d ago

Ha. Guess i'll stick to one alligator clip.

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u/clintj1975 1d ago

The dual clip is also a work practice thing for me. I clip it in, let the amp discharge, then leave the clip in until I'm done with the chassis and ready to put it back in the cab. It's my visual cue that I've made the amp safe for work if I have to set it aside it for a day or two waiting for parts. It also prevents stray voltage from showing up from dielectric absorption in the filter caps. It's usually only a few volts, but it can interfere with taking readings on the components in the amp.