TPR

I have an ancient no name amp with 6973 power tubes (3 of them ) and 2 12ax7s. It was probably not originally a guitar amp. It seems like a good one to learn more about basic repairs. It's functional, but absolutely needs new tubes and a cap job.

The wiring is a complete mess. Any idea where I might find info on dealing with something like this? I'm sure a basic amp building book would help, but instead of starting out with a clean slate, there's quite a bit of cleanup involved. Thanks for any tips.

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The awesome tone coming out of my amp almost makes up for my inability to play.

Frankie

Well, safety first. Make sure you discharge that amp properly before you even consider touching it. Plug it into the wall so it has a proper ground, the connect 2 alligator leads like:

chassis<>---<>100kohm resistor<>--<> THEN CONNECT THIS LAST to one of the postive leads on the cap can.

Wait 2-3 minutes, test the positive lead on the cap can for voltage to verify it's not storing 500vdc anymore, then have at. I'd leave the alligator clip plugged in and the plug in the wall to keep a ground on my amps, but who knows if that has a death cap in it. For you, I'd alligator lead the ground prong or chassis right into the wall ground.

Seriously, I'll say it again, 500vdc from a cap across the chest can and will stop your heart. Be careful! I've been hit twice, not across the chest, and it tensed me up so bad I couldn't turn my head for a week.

The mess of wiring might be on purpose. Twisted pairs of wires are normally AC runs, and untwisted, they'll cause hum in an amp. Does the amp even work?

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TPR

Thanks, Frankie. I learned about discharging the caps a while ago because I do minor work inside amps, so I'm used to being careful about that.

I also take your point about the electrical wiring, but I think there's more to the mess than that. And yes, the amp does work. I got through a band rehearsal as a test, but it started to fizz out after a while.

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The awesome tone coming out of my amp almost makes up for my inability to play.

TPR

Oh, and I should have mentioned before that the amp has been sitting unused and unplugged for many months. I just went out and checked the voltage on one of the filter caps, and also checked plate voltage. As I expected, there ain't a bit o juice in that thing right now.

Unfortunately, I discovered on closer inspection that the 2 can caps appear to be riveted to the chassis. I guess I'll have to see about tapping or drilling out the rivets to get them out, then install new braces. Argh.

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The awesome tone coming out of my amp almost makes up for my inability to play.

thrawn86

.

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American Viking

Wow! You leave them plugged in when you work on them! You can still get shocked if you touch near where the cord comes into the amp. This is how I've been zapped before.

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Procter2812

haha i think he means turn the socket off so that the chassis is grounded to the wall through the power cable.

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Riffraff

Here's a place that has some good debugging tips for you.

Tube Amplifier Debugging Page

Wilder Amplification

He posted that before I called him on it in another thread a few weeks ago.

There seems to be some confusion between mains ground and circuit ground. The only reason why the chassis is referenced to mains ground when it's plugged in is so that if you have a hot lead short to the chassis on the mains side only (like a power transformer winding shorting to the core/chassis or something of that nature), it creates a short to neutral via the bonding strap that connects ground to neutral in the service panel and trips the circuit breaker, thereby killing the circuit before you have a chance to come into physical contact with the live chassis and get shocked by the mains voltage. If that mains ground wasn't there (like it never was on some amps), the chassis becomes live with mains voltage in the event of a mains hot/chassis short and would have no way to trip the breaker.

The only thing that can trip a circuit breaker is to have more current flowing between hot and neutral than the breaker is rated for. This means that both hot and neutral have to be connected to the chassis for a mains-chassis short to trip the breaker. This is the reason for mains ground...it connects the chassis to neutral via the bonding strap in the service panel to make the mains safety ground work.

Amp circuit ground is ONLY part of the mains ground when the amp is plugged into the wall. Unplugged and it's its own seperate entity. The negative side of the circuit is referenced to the chassis by default, including the negative side of the filter caps. By shorting the positive side to the chassis, you end up shorting the positive to the negative and your charge voltage on the caps drops to nothing because a difference in charges between positive and negative no longer exists due to them being connected together via the chassis and shorting strap. Mains ground has absolutely nothing to do with this.

So for safety's sake, unplug it from the wall when working on the amp with it powered off. It DOES NOT need to be grounded to mains ground when discharging filter caps.

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Wilder Amplification

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