HR Deville Red Plating when signal is applied

[twilightofthedogs]

Howdy folks. I am troubleshooting an HR Deville that is red-plating when a signal is applied. When Idling the bias is stable at 60mv. However, when I apply a 4mv 1khz signal to the amp with all pots mid way it spikes very quickly into the 100-200 and up and doesn't come back down? Any ideas?

I think I have ruled out all the easy stuff. Tubes, dirty pots, dirty jacks (jumpered the pre out and power in). DC voltages seem to be correct. I even checked the coupling caps that go from the PI to the Power tubes. I should also say when I first got the amp V1 was bad so it wasn't making any noise. Replaced that, now the preamp appears to be working. When I plug directly in to the power out, I don't get much of anything. Thanks in advance!

[52 Bill]

Howdy folks. I am troubleshooting an HR Deville that is red-plating when a signal is applied. When Idling the bias is stable at 60mv. However, when I apply a 4mv 1khz signal to the amp with all pots mid way it spikes very quickly into the 100-200 and up and doesn't come back down?

What happens if you turn down all of the controls when this starts, does the current come back down or does it stay up?

[Enzo]

Get a nice bright light and look CLOSELY at the solder on the power tube socket pins. Any cracks? Whenever i am in any of this series of amps, I routinely resolder the power tubes sockets whether I SEE cracks or not.

Your amp wakes up and idles nice, but as soon as you run some current through it, it takes off, right? I can imagine a poor connection to a socket pin heating up and parting.

[twilightofthedogs]

Thanks a lot guys. I have been pokey about getting back to this amp, but will definitely try these two things first tonight!

[twilightofthedogs]

@52 Bill. Basically, the bias shoots ups when the volume is turned up and a signal is passing. The bias returns to normal as either the master is turned down on the dirty channel or the volume is turned down on the clean channel. @enzo, no luck on the tube sockets. I touched up all the jointed related to the power tubes and PI. Guess I am gonna have to pull the dreaded circuit board and see what is up on the other side. Also, chopstick test had no effect.

[52 Bill]

@52 Bill. Basically, the bias shoots ups when the volume is turned up and a signal is passing. The bias returns to normal as either the master is turned down on the dirty channel or the volume is turned down on the clean channel.

It might be oscillating. Do you have a scope? You could try paralleling a known good filter cap across filter caps for the preamp stages.

[Enzo]

Well hold on, the test point measures tube current. The idle current is just that, idle. Yes the curent goes up when you are playing, then it is no longer idle, then it is just tube current during operation. The bias reading is idle current. If you are running a signal through the amp, it is not at idle, and the reading has nothing to do with bias setting.

You say tube current rises, but what does the bias VOLTAGE at pin 5 of each socket do?

[twilightofthedogs]

Thanks again guys.

@ 52 Bill. Tried the parallel filter cap thing to no avail. I do have a scope though. Can't really claim to be an expert at using it though!

@ Enzo. The voltage on pin 5 of the power tubes is about -47 at idle. When I strum a guitar with all controls at about half way it dips down to about -65 for as long as I am playing. when I stop, it goes back to -47ish.

I should also say that V1 was blown when I first looked at this amp. Not sure if that's a clue.

[twilightofthedogs]

All right guys. It's amateur hour over here. Had the speakers plugged into what is apparently the "external" speaker jack, not the "internal." Everything hunky dorey now. I am almost too emabrassed to type this, but hey, other might learn from my foolishness.

[Enzo]

Actually, I ought to apologize to you for not thinking of that. That particular mistake is made a LOT, and we usually ask that first.