Bad Socket?

[MoparWade]

Been trying to track down a buzz in an AB763 Twin Reverb build. Determined it was coming from reverb circuit. In probing and for not good reason I stuck my wood probe into the center pin of tube 4 and gave it a wiggle. When I do this I get a couple things, a loud buzz like you get from heater wires shorting to signal wires and increased buzzing. All I can think is some solder made its way down there and is shorting between pins. Thoughts?

[Paul1453]

This center pin, is it the one on certain 9 pin sockets (many don't have a pin here) which is often tied to ground?

If so it could be that instead of being tied to ground it is possibly somehow connected to heater voltage instead.

Some resistance measurements from that pin to other circuit points might give you a clue to what the problem is.

If not replacing that socket with one that doesn't have a center pin might do it too.   

[MoparWade]

This center pin, is it the one on certain 9 pin sockets (many don't have a pin here) which is often tied to ground?

If so it could be that instead of being tied to ground it is possibly somehow connected to heater voltage instead.

Some resistance measurements from that pin to other circuit points might give you a clue to what the problem is.

If not replacing that socket with one that doesn't have a center pin might do it too.   

Yes, it's a 9 pin socket. Nothing is tied to it now, at least nothing on the outside. I'll replace the socket and see what happens.

[eleventeen]

This is the kind of problem you usually just have to bend your brain over. I tend to doubt it's the socket. The metal cylinder in the center of a 9 pin socket is only tied to ground if the user makes that connection; at least as far as I've ever seen. Most modern circuits (if you can even say that wrt a tube circuit) ignore that pin.


Do you have a real or synthetic center tap on your heaters?


You say the buzz comes from the reverb circuit, which of course could be the driver or recovery tube. For the driver, are you pretty sure you have cleanish DC power there? Remember the rev driver takes the second highest B+ in the amp, if your B+ isn't fairly clean by then (perhaps a bad filter cap in that #2 position) then big ripple would flow through the little reverb tranny and transmit through to the recovery tube. The rest of the amp, the preamp, would have its/their own filter caps and maybe they would work OK without the filtering provided by the reverb B+ takeoff. Maybe measure for AC (yes, AC) on the rev driver cathode; actually all its pins. Look for AC on the rev recovery tube and check things out by pulling it out. A scope would be super helpful here. Watch yourself on the rev driver plate, big volts there. Can you kill the buzz by pulling out the rev driver tube?


Those are my first thoughts. 

[MoparWade]

Do you have a real or synthetic center tap on your heaters?

Synthetic tap.

For the driver, are you pretty sure you have cleanish DC power there?

Maybe measure for AC (yes, AC) on the rev driver cathode; actually all its pins. Look for AC on the rev recovery tube and check things out by pulling it out. A scope would be super helpful here. Can you kill the buzz by pulling out the rev driver tube?

I will check for AC, have a scope. Will pull the tube and see what happens. Only reason I was thinking anything had gone wrong with socket is because wiggling that center pin gives me a loud pop and changes the amplitude of the buzz. Will hook the center pin to a DMM and monitor it for any voltage when I wiggle it.

[PRR]

You can isolate reverb driver and recovery troubles by unplugging the tank. Maybe shorting the input to the recovery stage.

[MoparWade]

Tank has not been plugged in. I will do some more diagnostics and see what comes up.

[MoparWade]

Just wanted to pop in and let you know I haven't forgotten, can't stand it when a thread is left incomplete. My 1000x probe apparently quit working so I have been waiting for a new one before I go digging around in the B+ at the B node. It has arrived so I should have some diagnostics done soon.

[MoparWade]

So just for my own satisfaction I hooked my scope up to the center pin and with a wood stick wiggled it. Looks like I have some sort of problem with shorting inside the socket. In standby that is a nice sine wave, becomes what you see below with the amp out of standby. Looks to be shorting between more than a couple pins otherwise I would only see signal from the heater circuit.