Debugging a failed channel in tube amp

[Alllensoncanon]

Looks like you have a leaking capacitor (0.33uf). When that happen, the positive grid voltage will cause three things to stress out, the tube, the output transformer and the cathode resistor (470 ohms). May be the blue flash you saw was the tube's Screen blowing out. Lets hope that saved the output transformer . Unless you have an expensive (zero inductance) cathode resistor. I would change that out while you are at it. Just in case it over stressed and will fail later. Don't think about changing the transformer however. The Cathode idle current is not that much off. It is likely still OK.

 

[pluplog]

Looks like you have a leaking capacitor (0.33uf). When that happen, the positive grid voltage will cause three things to stress out, the tube, the output transformer and the cathode resistor (470 ohms). May be the blue flash you saw was the tube's Screen blowing out. Lets hope that saved the output transformer . Unless you have an expensive (zero inductance) cathode resistor. I would change that out while you are at it. Just in case it over stressed and will fail later. Don't think about changing the transformer however. The Cathode idle current is not that much off. It is likely still OK.

Thanks for explaining, this has been a great learning example. I've been reading up on how tubes work but never actually been motivated to actually look at the data sheets, figure out the numbers until this thread. Are there any books that you would recommend to beginners on figuring more about how to debug tube amplifiers?

 

[Alllensoncanon]

Merlin Blencowe has a very good book for beginner. It is on Amazon. You can also read chapter one in his web site Valve Wizard. Do a search.

 

[famousmockingbird]

Cathode to ground measured 8.78VDC now.

Vg3=250 @ Vg1= -8.8 36mA cathode current seems normal

 

[pluplog]

Vg3=250 @ Vg1= -8.8 36mA cathode current seems normal

Thanks, I'm hoping the problem is just isolated to the faulty bypass cap. I'll know in a few days when Mouser finally sends them over 🙂

 

[pluplog]

Merlin Blencowe has a very good book for beginner. It is on Amazon. You can also read chapter one in his web site Valve Wizard. Do a search.

Got it. Interesting he has written 2 books, but I read from the reviews on Amazon they're similar, one tilted towards guitar preamps and the other hifi preamps. I got the latter, looking forward to reading it when it comes. Thanks for the recommendation!

 

[Trajan]

Radiotron handbook is worth studying and also much of the EE literature of the era. Lots of cool tricks.

 

[tequilamike]

Inspect the power tube solder joints or socket connections very carefully, for cracks or other.

 

[pluplog]

Radiotron handbook is worth studying and also much of the EE literature of the era. Lots of cool tricks.

Wow, that's 1482 pages of knowledge... I'll put that on the list. (and take hopefully not forever to digest it) 🙂

 

[pluplog]

So I replaced both 0.33uF caps this evening (with new ones rated at up to 400V as advised)

And the amp is working again...

Thanks everyone for the advice (and your time)!

 

[PRR]

Wow, that's 1482 pages of knowledge...

A fair amount is about Radio circuits and, in this crowd, may be skipped on first to third reading. However there are nuggets of wisdom even in the non-audio/power sections.