Tube Amp Rebuild (and Mod)

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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Comments

[FireandIcebike]

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?

[FireandIcebike]

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]

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.

[FireandIcebike]

I ended up doing just that. Thanks.

[gmoon]

Caps can hold a charge for a loooooong time (although the older ones usually don't, 'cause they're failing.) Were you pokin' around inside or just playing? Isolation can be a problem with these oldies, too.

I've been dumb enough to play my old Ampeg in the basement with barefeet....wasn't a painful shock, I could just feel the current flowing from my left hand to the floor. One lesson learned.

[titsanonymous]

i have an ampeg, its the reverb rocket. its busted tho. my dads friend said that a guy gave it 2 him and then it sat in his basement and he gave it to me. when i plug a guitar into it the light turns on but there is no sound. could it be the tubes?

[73lonestar]

a short in a output tube can cause the fuse to blow replace the fuse 250ma pull out the output tubes the larger ones and and look for voltage form chassis ground for g1 g2 and plate b+ should be about 350 to 450 volts g1 look for bias and make sure amp has speaker load bias differs on the type of tube el34 is about -42 6l6 -52 el84 -15 6550 -62

[titsanonymous]

do i need to discharge the amp to do that?

[73lonestar]

i find it so satisfying to build an amp from tubes the art work of human is awesome and money is the other drag parts are kinda pricey i can feel sick or stressed out but being in my work bench figuring out am i going to use 6aq5s el38s or how high of voltages will be best at this currant and making someones jaw drop after they see your work is cool after a week drilling the holes bolting up transformers and sockets at the end everyone is jamming out loud and sweet.

[73lonestar]

well i will say the voltage is high but you need to make these test with the amp on it is critical to measure the bias if not you can burn a set of good tubes the pin out on a 6l6gc or 5881 is 1 neutral 2 filament 3 plate or b+ 4 g2 screen b+ slightly lower than 3 5 control grid g1 bias is preasant 6 neutral 7 filament 8 ground notch clock wise upside down chassis between 1 and 8 i have 7591 tubes but they have the tall bases cant look to see how it works but you can get the pin out on google please be aware of the high voltage preasant and unplug the amp before you discharge the caps replace any resistors that are out of tolorance if it is self bias look at the 400 ohm resistor 7 watt or maybe 10 on a 6l6 it would be 220 ohms g1 grids are about 300k ohm to chassis 1/2watt resistors also look for proportion in the center tap primarys to the plates maybe around 30 to 60 ohms from center tap on output transformer and look at the preamp tubes and look at the voltages there to pins 1 and 6 are b+ on a 12ax t or u7

[gmoon]

It could be the tubes...but it could be other things too, like bad connections. Is there any sound at all? (humming etc.)

[titsanonymous]

no sound at all

[gmoon]

Yeah, that could be a lot of things (including bad tubes). Could also be a bad output transformer, disconnected speaker wires, etc.

[FireandIcebike]

I was just playing. Man, I didn't think an amp could be all lethal and junk! Thanks for the info.

[gmoon]

Yeah, actually quite a few people have died from this stuff. Very likely your guitar is wired to use you as a ground (through the strings.) On some old amps, the chassis ground might instead be a lethal voltage in respect to another 'ground' (you in your bare feet, or a mic stand, for instance.) There's a little info on Step 3 about that, if it's an amp without an isolation transformer. But even the amps with isolation can be dangerous--that's why I added the three-prong cord here....

[gmoon]

Here's an illustration of an older amp with an isolation transformer that still dangerous-- This is part of a schematic of my Ampeg Gemini II.

Because AC line plugs were all two prong, the amp has a two-position standby switch. This switch not only works as a standby, but it also connects the AC line to the chassis through a single capacitor.

Since the plugs weren't polarized, you could use this to connect the chassis ground indirectly to the line 'neutral' wire, and your amp would run quieter. But if the cap degrades and fails, and your guitar strings are grounded (as most are)--it's like poking your finger in the wall socket...

On cheaper amps without a standby switch, the cap was still there, but you'd change the plug direction to ditch the hum...

(picture: red square shows the switch and the 'death cap.')

[satanalingus]

 Shocking to say the least, especially when I'd be playing my guitar then would touch my friends at the same time...YIKES!!!!  that GeminiII....

[gmoon]

That Gemini cured me of ever playing guitar in bare feet...

[Philipeby]

I have a jcm900 How exactly do I drain the caps? I'm A little confused! thanks

[gmoon]

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...

[hotlickslynn]

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?

[gmoon]

Pin 7 --Here's a helpful link.

Leave it jumpered for a minute or two...it's probably draining through the output transformer.

[gmoon]

One more thing--if it's two EL84s in a push-pull circuit, shunting the plate to GND might not drain the caps...

If you have any questions, send me or post a schematic for your amp, and we'll figure it out.

[hotlickslynn]

No,it's a 1 tube single ended design,an Epi Valve Junior head.I want to put the Mercury Magnetics tranny mod kit into it.I've already put a Bitmo Tone/pull fat control in it.Thanks for your help.I really appreciate it.

[gmoon]

No problem. Hope the MM kit is worth the $$$$...you probably have more cash in the mod than you paid for the amp. You probably could have made an instructable on it, too. I imagine I'll pick up an Epi VJ at one point. Can't beat the price, easy to modify, too.

[hotlickslynn]

About the cap discharge:do you jumper from the tube socket or from the pin with the tube partialy pulled out?I'm now thinking of replacing just the OT with a Hammond.Any other sugggestions on it?Thanks.

[gmoon]

The socket would be the best bet.

Also, I don't know which version of the VJ you have, but the newer ones have a bleed resistor in the power supply to drain the caps. Since they need to be high resistance (so they don't mess with normal operation) they will take 30 minutes (or maybe more) to work.

You can always check the caps with VOM to be sure they are drained, too.

Here's a pretty good discussion of VJ mods, don't know if you've seen it.

[hotlickslynn]

Yes I've seen that discussion.I printed the whole thing for future reference and registered for the gear page.I have the revision 3 amp so I should be alright if I check with my multimeter.