Plexi Clone just one tube red plating

[rhaigh]

So I've been using this plexi clone I built for about 13 years and never had a problem. Today, I was playing at low volumes when the sound dropped out. I look into the amp and just one EL34 was red plating, so I shut it off. I checked all resistors and they are good. The Bias voltage reads between -35V and -40V. It measures equally to each power tube socket after the 220k resistors. Thinking it may have been a bad tube, I popped in another one and the same thing happened. I can turn it on with stand by on, but as soon as I turn stand by off, the tube red plates. Therefore I can't measure the bias current. I checked for shorts/ open circuits in the bias path but everything looks ok. No problem with the other EL34. Any recommendations? Thanks!!

[Enzo]

The other tubes remain OK? So pull THAT tube, and with the amp running, measure the voltages at that socket. Since it red plates I can assume you have B+ at pins 3 and 4. But check anyway. You say the bias measures 35-40 volts, OK. It can be anywhere it wants, but if it is not getting to pin 5 of the socket, it isn;t there for the tube. And you may have a lovely -40v in standby, but as soon as you flip it to run, a leaky coupling caps lets it drop to nothing. That is why the active readings matter. SO power on and ready to play, pin 5 says?

With the overheating tube's socket empty, does the amp make sound and otherwise behave?

[rhaigh]

I tried that and the bias voltage is on pin 5. Everything measures ok when the tube is pulled. Bad socket perhaps?

The other tubes remain OK? So pull THAT tube, and with the amp running, measure the voltages at that socket. Since it red plates I can assume you have B+ at pins 3 and 4. But check anyway. You say the bias measures 35-40 volts, OK. It can be anywhere it wants, but if it is not getting to pin 5 of the socket, it isn;t there for the tube. And you may have a lovely -40v in standby, but as soon as you flip it to run, a leaky coupling caps lets it drop to nothing. That is why the active readings matter. SO power on and ready to play, pin 5 says?

With the overheating tube's socket empty, does the amp make sound and otherwise behave?

[Enzo]

Sure, it is important for the voltage to get to pin 5, but it is equally important that the female pin 5 grips the tube male pin well enough to make electrical contact. Look down the hole, are the female pins spread? At least the pin 5 one?

[Enzo]

And one that I have seen only a few times in my career. Did you read that pin from up top or underneath? The female pin is a little piece of metal rolled into a cylinder with a tab sticking out to solder to. I have see a few times the cylinder part is broken off the tab part, but both pieces stay in the socket body. That means the voltage is on the solder tab underneath, but the part gripping the tube pin is not connected to it. SO, busted socket pin. Rare, but happens.

[isaac]

Maybe also your ground connection on pins 1 and 8 has broken. Also there could be a 1 ohm resistor between pins 1 and 8 that may have broken. Another thing to check.