Tube Amp Mods - SAFETY Questions

[slejhamer]

I'm going to slightly modify my home-made 1/2-watt tube amp by changing some cap and resistor values. Already know exactly what needs to be done, but ...

I've heard that tube amps can be dangerous to repair, even when they are unplugged. Why? Voltage stored somewhere? Spiders? Eddies in the space-time continuum?

What safety precautions (besides unplugging it) should I take? I have a multimeter and know how to use it fairly well; just not sure what I'd be testing for.

[quarterfront]

The main thing is that you don't want to discharge the filter caps through your body 'cause that hurts and can kill you. Not good. The filter caps can hold a heap big charge for a long time after the amp is shut down and unplugged. Sometimes leaving the amp on standby after it's shut off, or turning the volume knobs all the way up or other methods are suggested, but the only way to be sure they're discharged is with a meter and the safe way to discharge them if they're not is with a snuffer stick.

Here's a link to a good page:

http://www.aikenamps.com/SafetyTips.html

There's a longish paragraph about half way down the page that deals with discharging the filter caps.

Here's a page with picture of the tool you'll want to build (mine's not nearly this pretty but it works fine)....

https://amptechtools.powweb.com/stick.htm

For reference, the B+ on my Weber 6A20 (BFDR type of amp) after it's shut down, unplugged, and has sat idle for 10 minutes, is about 350v. Don't want to touch that....

[slejhamer]

Excellent info! That's exactly what I needed to know, esp. the Aiken bit on the DIY snuffer.

[Ricochet]

Another great source of info: Go to http://geofex.com/ and click on "Tube Amp Tech Pages." In the "Tube Amp FAQ" and "Tube Amp Debugging Pages" you'll find safety info and an excellent troubleshooting scheme.

[slejhamer]

Very good, thanks, but it raised a question after I read this:

SUCK IT DRY
When you open up an amp, you need to find a way to drain off any residual high voltage. A handy way to do this is to connect a shorting jumper between the plate of a preamp tube and ground. This jumper will drain any high voltage to ground through the 50k to 100K plate resistor on the tube. To do this successfully, you will need to know which pins are the plate pins. Look it up for the amp you're going to be working on. You'll need to know this for the work anyway. Leave the jumper in place while you do your work ( high voltage electrolytics caps can "regrow" voltage like a battery sometimes. Really. ) Remember to remove it when you finish your work.

Shouldn't I be removing the tubes before doing the work? Or does that just mean to drain the caps with the tubes in, and then remove them?

[Ricochet]

Doesn't matter. Either way works fine. You don't need to drain caps to take tubes out (except where plate voltage is applied to a "cap" on top of the tube.) But to diagnose the amp you often have to be checking voltages on the working amp (with 400V+ on the plates and screens), and that obviously means with tubes in place. That calls for caution. Keep the hand you're not using in your pocket. Don't let yourself become part of a circuit. THINK before you touch anything, and be very careful that your probe touches only what it's intended to touch. Most every tech or hobbyist who's worked on tube gear still has stories about getting zapped or burning up something by making a mistake. Only takes one wrong move to be dead, if you're hit just "right." Be very careful! Know what you're doing before you do it.

[slejhamer]

Gotcha thanks; actually I'm not diagnosing problems, mostly doing some changes to the tone stack, but it's all scripted out and very straight-forward. Was just worried about touching my soldering iron to a component and getting zapped.

[Ricochet]

Problem is, lots of mods end up requiring some diagnosing when they don't work as expected.

[quarterfront]

Problem is, lots of mods end up requiring some diagnosing when they don't work as expected.

What he said. For grins, here's an article I wrote about troubleshooting a lead dress problem in my 6A20 build.

http://home.earthlink.net/~rowse1/6A20%20Build/pages/leaddress.html

The whole thing is that it involved poking the wires around and listening to what happened - which has to be done with the amp powered up. Which means, keep the free hand in a pocket, use only a DRY WOODEN chopstick at least a foot long to poke at stuff, keep the cat out of the room, don't work tired, don't drink, don't answer the phone, don't talk to anybody, all that stuff.

[slejhamer]

Problem is, lots of mods end up requiring some diagnosing when they don't work as expected.

Sadly true!

For grins, here's an article I wrote about troubleshooting a lead dress problem in my 6A20 build.

That's very interesting. So it was just the proximity of the wires to each other that was causing the problem? The other picture you link to of the amazingly neat dress job is, well, amazing -- looks like a 3D PCB! I'd be embarrassed to show the insides of mine; my soldering's decent but my wires are all over the place.

Mine's a Guytronix 1/2-watt Gilmore Jr. kit. http://www.guytronix.com/ I installed his "V-Mod" when I first put it together to get more of a Vox sound, but I'm sort of de-modding it now for the stock tweed sound.

[quarterfront]

So it was just the proximity of the wires to each other that was causing the problem?

Yeah, crazy stuff. I'd heard that lead dress is everything, but this really brought it home. Not just proximity but also orientation, I suppose. Really I didn't have to move them much, just a little this way and that, but it made all the difference. Learn something new every day...

The other picture you link to of the amazingly neat dress job is, well, amazing -- looks like a 3D PCB!

Yeah, that guy's got it going. Did you listen to his clips? Proof's in the pudding....

Mine's a Guytronix 1/2-watt Gilmore Jr. kit.

I've seen that website before, looks like a pretty cool amp. I just looked around it now, listened to some sound samples of the Jr. and they sound really sweet! I couldn't find details about the amp, though - what tubes does it use, and what are the controls on it?

EDIT: Okay, I see what tubes and controls. Right in front of me, but if it's not spelled out as a list I can't see it....

So... It's using two sides of a 6N1P as the output tube(s).... Looks cool. I have this 1w amp that uses one 12AX7 as the preamp and one as the output, and I've thought about experementing with it. Maybe this would be a way to go....

[TimeZone]

Eddies in the space-time continuum?

Ah. Is he? Who, exactly, is Eddie, then?

TZ

[Wes Inman]

Ricochet's advice about putting your hand in your pocket is good advice. My dad was an electronics tech, worked on military aircraft, but he used to build a lot of those old Heath Kits many years ago. I remember seeing a blue arc of electricity jump off and pop him good while he was working on one of these kits on the kitchen table.

But he always said the same thing, he recommended putting your left hand in your back pocket while you worked. Electricity always takes the shortest route, this way it goes down your leg, not through your other arm which takes the path through your heart.

I got shocked working on my Marshall amp about 6 months ago. Nothing major, but it got my attention.