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trombone 4th December 2011 03:08 AM

Power amp sputters on turnoff
 
My tube power amp has just started sputtering quietly on turnoff. It's not bothersome, but it is new, which means it is of concern. All the power caps are new, though I realize one could be leaking. It would not be fun trying to find out which one. There are also small bypass caps.

There are bleeder resistors on there that aren't so new. Could they be the problem? I'd try replacing them first, but if anyone can suggest anything else to look at while I'm in there, I'd appreciate it.

The am sounds just fine when playing and is very quiet.

dgta 4th December 2011 03:14 AM

Please describe "sputtering".

GloBug 4th December 2011 03:33 AM

I would try the bleeder resistors.

Does the amp usually fade out, or go silent instantly?

ChrisA 4th December 2011 04:28 AM

Quote:

Originally Posted by trombone (Post 2805654)
My tube power amp has just started sputtering quietly on turnoff. It's not bothersome, but it is new, which means it is of concern. All the power caps are new, though I realize one could be leaking. It would not be fun trying to find out which one. There are also small bypass caps.

There are bleeder resistors on there that aren't so new. Could they be the problem? I'd try replacing them first, but if anyone can suggest anything else to look at while I'm in there, I'd appreciate it.

The am sounds just fine when playing and is very quiet.


I suspect the B+ voltage is not ramping down smoothly. Could you put a volt meter on the power supply node that powers the power tubes? It should just ramp down to zero and not oscillate.

Maybe the bleeder resistor is open or over sized.

What happens is the amp will make sound if a "close" filter cap has enough volts still in it, the the conducting tubes quickly drain down that cap and then stop conducting, some how the cap then gets re-charged, the tube conducts and so on. Sounds like maybe you power supply is a complex network. It is storing energy some place. A bleeder resistor will fix that

trombone 4th December 2011 09:42 PM

Power amp sputters on turnoff
 
Quote:

Originally Posted by dgta (Post 2805656)
Please describe "sputtering".

A kind of quiet, fading popping. Not motorboating.

trombone 4th December 2011 09:42 PM

Power amp sputters on turnoff
 
Quote:

Originally Posted by GloBug (Post 2805666)
I would try the bleeder resistors.

Does the amp usually fade out, or go silent instantly?

Instantly.

trombone 4th December 2011 09:44 PM

Power amp sputters on turnoff
 
Quote:

Originally Posted by ChrisA (Post 2805700)
I suspect the B+ voltage is not ramping down smoothly. Could you put a volt meter on the power supply node that powers the power tubes? It should just ramp down to zero and not oscillate.

Maybe the bleeder resistor is open or over sized.

What happens is the amp will make sound if a "close" filter cap has enough volts still in it, the the conducting tubes quickly drain down that cap and then stop conducting, some how the cap then gets re-charged, the tube conducts and so on. Sounds like maybe you power supply is a complex network. It is storing energy some place. A bleeder resistor will fix that


The supply is not complicated, no regulator for instance, but it has a lot of caps. A couple of you have mentioned the resistors, and I'm thinking that, too. I'm going to change them.

Thanks, guys.


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