Is this red plating ?

[ruger9]

My amp has a little static sound recently. It's a Bad Cat Hot Cat, it's cathode bias, and I believe Bad Cat runs these EL34s pretty hot...

Wondering if this is red plating (only the one EL34 is doing it), and if that's likely the cause of the noise? I realize I could just start pulling tubes, but on this amp it's a PITA to start swapping preamp tubes...

I have another set of EL34s I could just pop in, I'm just wondering if this IS actually red plating, maybe I'll learn something today LOL.

 

[Wally]

Yes, the tube on the right is red plating.

 

[ruger9]

So... does this happen when the tube is nearing end-life? It's a cathode-biased amp, so there are no bias adjustments to be made, right? I just plug in a new set of EL34s and I'm good to go? And maybe (just a guess) always buy "matched pairs" of EL34s to avoid this in the future?

 

[Timbresmith1]

Plug a new one in there. If IT redplates, it’s time for service.
Pairs are a good way to go when replacing.

 

[Timbresmith1]

So... does this happen when the tube is nearing end-life? It's a cathode-biased amp, so there are no bias adjustments to be made, right? I just plug in a new set of EL34s and I'm good to go? And maybe (just a guess) always buy "matched pairs" of EL34s to avoid this in the future?

It CAN be a sign that the tube is nearing the end.

 

[Wally]

You could shut the amp down, let the tubes cool, and switch positions of the two power tubes to see if the condition follows the tube or stays in the socket. If it follows the tube, you need a new set of power tubes. If it stays in the socket, it is tech time.

 

[ruger9]

Great info! Thanks guys!

 

[AAT65]

You could shut the amp down, let the tubes cool, and switch positions of the two power tubes to see if the condition follows the tube or stays in the socket. If it follows the tube, you need a new set of power tubes. If it stays in the socket, it is tech time.

Excellent debugging advice!!

 

[Dacious]

If you should buy new tubes, ask the vendor for a matched pair with low(er) current draw.

Bad Cat does run tubes hard.

 

[gpw5150]

Had this ol’ chestnut in my Pro.

Yes, it is red plating.

Switch tubes when cold: if it follows the tube - get new tubes

If not and it stays on the socket:

Clean the valve socket
Re-flow the solder joints on the socket

Or

it can be either a coupling capacitor in the phase invertor or one of the resistors in that circuit, which you may need a tech to diagnose/fix

 

[ruger9]

It was the tube. I believe they were OEM (they were TAD EL34B), replaced them with a matched set of EHX EL34 (no B). No red plating, no noise.

But the "B" made me research EL34 vs EL34B lol. I had never heard of a "B" designation before.

Thanks again everyone!

 

[Henry Mars]

Look for burned resistors.

 

[Cjteleforum]

So... does this happen when the tube is nearing end-life? It's a cathode-biased amp, so there are no bias adjustments to be made, right? I just plug in a new set of EL34s and I'm good to go? And maybe (just a guess) always buy "matched pairs" of EL34s to avoid this in the future?

Glad to hear it was just a bad tube.

Any amp can have it's bias adjusted, whether it's fixed or cathode. "Adjustable" just means that there is a variable resistor(a pot) which makes it much easier to adjust the bias, since there's no soldering involved.

Cathode biased amps are adjusted by changing the value of the cathode bias resistor.