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Tube Amp Rebuild (and Mod)

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Step 4: DISCHARGE THOSE CAPS!!!!!

Seriously. Do this EVERY TIME you work on the amp. If you don't, DO NOT complain if you loose the use of your hand. DO NOT come back and haunt me if you die....

The 'filter' caps can store fatal amounts of electrical current. The caps are connected near the rectifier and are part of the power supply, and aid in converting AC to DC. In fact, they are a standard component in any power supply.

If you're completely lost, and don't understand this, DON'T MODIFY YOUR AMP. You haven't enough knowledge to work on high voltage/current circuits safely...

Several ways to discharge caps:

FIRST, UNPLUG THE AMP! (But that doesn't make it safe....)

THEN,
--Take a screwdriver or a jumper and short the capacitors leads.
--OR jumper the power amp tube plate pin to GND for a minute or so (Class A, single power tube only.)
--OR jumper the positive (+) lead of each large cap to GND for several seconds. A jumper with a built-in resistor (10K or so) will help prevent sparks here...

Some, or all of these methods may result in a spark... Obviously, your flesh can act as a jumper also.

It's NEVER a good idea to touch amp circuitry when it's switched on. If you're stupid enough to do it, NEVER USE BOTH HANDS--that way, there's at least a slim chance the current won't flow through your heart.

And don't work on amps in bare feet, on a damp basement floor. :-(
Don't even play your amp in this fashion...


 
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Man, it's crazy that you posted this because I was just wondering why my old Gibson Falcon amp shocked me when I was barefoot! How long do the filter caps hold a charge?
I'm pretty sure if you're not a licensed electrician, you shouldn't be farting around in the back of an amplifier. At least that's the impression I'm getting. I finally found a guy that specializes in this stuff, so hopefully he can fix me up and I'll be jamming out instead of worrying about dying working on my amp.
gmoon (author)  FireandIcebike5 years ago
The last person you'd want inside your amp would be a "licensed electrician." They deal with a very specific set of predefined circumstances--NOT what you find in tube amps.

If you're not willing to learn (or unwilling to take the risk), seek out an experienced amplifier technician.
I ended up doing just that. Thanks.
Philipeby5 years ago
I have a jcm900 How exactly do I drain the caps? I'm A little confused! thanks
gmoon (author)  Philipeby5 years ago
Locate the high-voltage filter caps--they should be the largest caps. On many amps (including Marshall), they are large "can" caps that are mounted on top of the chassis. Modify a alligator-clip jumper by adding a resistor (say 20K to 35K) into the jumper. UNPLUG THE AMP. Attach one end of the jumper to the chassis, the other to the live cap. Leave for 20 or 30 seconds. Move to the next cap tab; or, just leave the jumper in place for a minute or two--all the caps should discharge through their voltage-dropping resistors. Test with a multimeter before touching, though...
I'm trying to learn something about this.Which is the plate pin on an EL84 so I can safely discharge the caps on my amp?