Strange glowing plate problem - help.

[justonwo]

I seem to be a magnet for these things. I just bought a used Suhr/CAA OD100 SE+, though it was brand new as of a few months ago (i.e., the previous owner didn't have it very long). I noticed tonight as a studio mate played through it that the plates on the left two power tubes (V9 and V10 of a 10 tube amp) were glowing quite orange. V7 and V8, the other two power tubes also had some fainter glow.

The volume and pre-amp adjustments were at about noon while this was happening. Then my buddy stopped playing altogether, keeping all the amp settings the same, and the tubes REALLY lit up. The plates were glowing bright, bright orange. I've never seen plates get brighter when you stop playing. Usually, it's the other way around - they get brighter when you're playing.

Any ideas? Is it something as simple as the bias being adjusted too high? Normally, I would say yes but the strange behavior has me wondering.

[chaz]

Yes, have the bias checked. If it's a class A amp it would get brighter at idle. You may need to retube as the plates get damaged by this.

[John Phillips]

Any ideas? Is it something as simple as the bias being adjusted too high?No.

Best guess:

It sounds like you have a leaking PI coupling cap, which is pulling the bias voltage way down on that side, and leaking a little across the bias supply to the other side as well, making the tubes run too hot.

When you play, the average voltage at the PI plates falls a little, so less then leaks into the bias supply and the tubes will run cooler.

Drop John Suhr a line, he should be able to confirm that or give you a better guess...

[justonwo]

I just sent you an email, John. Wow, that's incredible customer service.

[jezzzz2003]

I just sent you an email, John. Wow, that's incredible customer service.

Yer damn straight!
tell us something we dont know.:D

[justonwo]

Here's a recap. I find out there's a problem on Sunday night. John describes the source of the problem and sends out the replacement parts the next morning. I'll probably have the thing fixed by tomorrow or Wednesday. Incredible. What's more, John posted a video describing how to replace the two faulty caps to make it trivial.

Incidentally, I popped open the chassis tonight and found out that, sure enough, I have the wrong values John described. This may be the easiest amp fix I've ever had to deal with.

[justonwo]

John sent replacement parts and I installed them last night. Problem solved. And what a great, great amp.

Hats off to John for being so responsive and helpful and also to John (Phillips) for his accurate cyber-diagnosis.