Need some tube amp work safety tips

[red]

Hello, my Peavey Classic 30 amplifier has a minor issue with the mids pot probably touching with the mid boost button, because when I work the pot sometimes the mid boost activates itself while the mid boost button itself is in the off position. This just fixes itself when I stop rotating the mids pot, but it makes adjustment annoyngly difficult.

So I was thinking about popping the hood up and looking inside. I am competent with a soldering iron, and have very basic experience with electronics (I can tell a diode from a resistor from a capacitor). I've so far succesfully modified a stock Ibanez TS9 pedal to Keeley specs, and built a BYOC Overdrive 2 and BYOC Large Beaver pedal.

The thing with tube amps, though, is that everywhere I've looked they say that you should be very careful because they have caps that store potentially lethal charges even when the amp is not plugged in. So professionals don't take chances, and discharge those before starting work inside.

If we have any experienced fellow forum members that would care to give me some tips on how to approach this specifically for a Peavey Classic 30, but generally for any tube amp, I would really appreciate it. I will keep looking up information on the Internet as well in the meantime. I won't go in until I can make sure it's all understood and with 0% safety risks.

Thank you!

[sumitomo]

I play with amphs,So probe with a sharpie and the other hand behind your back and learn how to discharge capacitors for starters.Sumi:D

[duhvoodooman]

You already know this, Red, but it's important enough to bear repeating, if only for others who read this post: Be very, very careful working inside an amp, even an unplugged one--you can get killed by the lethal voltages there, if you don't observe proper safety practices. Here are some good links:

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

http://www.geofex.com/tubeampfaq/tube_amp.htm#Section0

http://www.mackamps.com/tube-guitar-amp-safety/

[red]

I play with amphs,So probe with a sharpie and the other hand behind your back and learn how to discharge capacitors for starters.Sumi:D
Thank you for your answer!

Can you recommend any specific website or tutorial that you've found accurate based on your experience? I've read some stuff, but I can never be sure I've covered everything or that the person who put up the website isn't some deranged guy delighting in frying unsuspecting idiots :).

[red]

You've no doubt already read this, but it's important enough to bear repeating--be very, very careful working inside an amp, even an unplugged one--you can get killed by the lethal voltages there, if you don't observe proper safety practices.
Thanks duhvoodooman! I was kind of hoping you'd answer, you being the resident man with the soldering iron :).
I've only read the geofex one before your post, will read them all carefully.
Much appreciated!

[duhvoodooman]

Thanks duhvoodooman! I was kind of hoping you'd answer, you being the resident man with the soldering iron :).
Thanks, but don't give me too much credit! I still consider myself a relative novice when it comes to working on amps. There are a couple of guys around here that are MUCH more experienced with such work than me. Now, where has that Tunghaichuan gotten himself to??....

[red]

Thanks, but don't give me too much credit! I still consider myself a relative novice when it comes to working on amps. There are a couple of guys around here that are MUCH more experienced with such work than me. Now, where has that Tunghaichuan gotten himself to??....
Heh, I'm sure you're just being modest. I meant no disrespect to all the other knowledgeable users, I just don't know everybody well enough I suppose.

I am thankful for all the information from everyone willing to share it!

[sumitomo]

1lozkT1zrIUHere ya go!!!Sumi:D

[red]

Hahahahaha excellent stuff!!! And just as he was saying that you should always be careful...

[tunghaichuan]

All good tips so far.

One thing I would recommend is getting the schematic for your amp, it should be readily available if you Google it.

Keeping one hand behind your back when probing in an amp is essential, it keeps the current from flowing up your arm and accross your heart, thereby stopping it. The sharpie pen is a great idea as well. I use a plastic chopstick.

The Kendrick video shows that very clearly. If you get bit, it will hurt but more likely than not it won't kill you.

Another useful thing to make is a discharging tool. I have insulated alligator clips soldered onto both leads of a high value/high wattage resistor. Anything from 10k at about 5W will work well. This will not only discharge the caps, but if you leave it in place, it will keep the caps from charging up while you're working on the amp. Just remember to remove it before putting the amp back together.

[red]

One thing I would recommend is getting the schematic for your amp, it should be readily available if you Google it.

Yes, the schematic is not a problem. There's this site with a bunch of mods and schematics for the Classic 30 and generally good stuff: http://www.blueguitar.org/. I also think I have the schematic in the manual for the amp, which I've kept but don't know where it is at the moment :).


Another useful thing to make is a discharging tool. I have insulated alligator clips soldered onto both leads of a high value/high wattage resistor. Anything from 10k at about 5W will work well. This will not only discharge the caps, but if you leave it in place, it will keep the caps from charging up while you're working on the amp. Just remember to remove it before putting the amp back together.
So I should have one of those ready for each cap, right?

Thanks for your answer!

[tunghaichuan]

So I should have one of those ready for each cap, right?

Thanks for your answer!

No, just one. Put it on the first filter cap in the node.

Also, it is a good idea to actually check the voltage with a meter to make sure the discharger has drained the caps.

[markb]

See those gaps with the jumper wires bridging them in the schematic? Inside the Classic 30 the board is folded around into a sort of box with the guts on the inside and the jumpers are short lengths of insulated wire.
This is one amp I would give to a pro not least because he'd be liable to fix any damage he did "unfolding" the board assembly.

[red]

This is one amp I would give to a pro not least because he'd be liable to fix any damage he did "unfolding" the board assembly.
I agree, and I'll do that if it comes to having to do any non-trivial work in there - however I'm hoping that making sure the mids pot doesn't somehow touch the mid boost button in the circuit can be done either with minimal tampering with the board, or without pulling the board out at all.

Another question I forgot to ask is this: the amp has been unplugged and not in any use for almost a month now, in a different city from where I'm now typing this. I'll get there in a few days. Shouldn't the caps be all discharged by now anyway? Wouldn't a first (and only) look inside when I get there and before plugging it in be safe "by default"?

[Goose]

A couple of quick discharge questions:
1. I have heard and read that if you turn the amp off and unplug it while the tubes are still warm, then play the guitar this will discharge the capacitors through the tubes. Correct or not?
2. For just changing out the tubes, is discharging necessary/recommended.

[red]

A couple of quick discharge questions:
1. I have heard and read that if you turn the amp off and unplug it while the tubes are still warm, then play the guitar this will discharge the capacitors through the tubes. Correct or not?
2. For just changing out the tubes, is discharging necessary/recommended.
1. Might work with some amps, won't work with others. The safest way is to properly discharge all capacitors and check your work with a multimeter.

2. No, it's neither necessary nor recommended. Just make sure the tubes are not hot (leave it for a while to cool off) before swapping them, and be careful not to touch the metal legs while the tube is still partially connected to the socket.