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Baffling Tube Preamp Problem--Rhythmic Thumping after Tube Replacement

VTL Deluxe Preamp and VTL Compact 80W monoblock amps. I've had these for 30 years.

The two amps had tubes that were biased correctly, but there was a lot of static that I narrowed down to the preamp. The preamp has 5 tubes, four 12AX7 and one 12AT7. I tried replacing some of these with my spares but the noise persisted and the tubes were making noise when tapped.

I took the preamp into a local shop (the guy has tube amps and also has a vast tube collection), he tested/matched new tubes and installed them. He tested the preamp with a solid state amp and it worked fine. He used the "A" out puts which are designed for solid state. I took it home and plugged it back into my tube amps with the "B" outputs which are designed for tube amps.

 

The speakers on both channels started making a low rhythmic "thumping" noise, like a heartbeat, and then one of the amps blew a fuse.

 

I tested the tubes on both amps. The amp that blew was all messed up, but the other amp tested fine (at least the 4 power tubes 5881), I didn't test the two AT7's on there.

 

So I connected the preamp only to the amp that had OK tubes and the thumping problem persisted.

 

We are both baffled. It must have something to do with the B output on the preamp but the schematic is not particularly helpful and the company hasn't been communicating.

 

The thumping problem didn't occur until I brought it back from the shop, but maybe something came loose in transit.

Another possible issue is I may have put the wrong tube in a spot (i.e. AT where it needed AX) as my eyes are not great. But again, after he replaced everything, it worked fine with the solid state amp in the A output.

 

Any ideas on what to check next?

drbeechwood
drbeechwood OP
14 posts
 

I have a schematic. I like the idea that the B section Capacitor is the problem. It wasn't the original problem, the tubes were the original problem. but maybe in transport it was compromised. I think that it's also possible that the tubes and/or capacitors in the amp became damaged when putting the bad signal through them after I got the preamp back. It is frustrating but I think based on what you guys said and what I have read, this is likely the problem given the age of the amps. Thanks again for your feedback.

czarivey
5,674 posts
 

If B supply resistor blows, that huge voltage will be applied at bias cap and destroy it. It acts like circuit protection in a such weird way that cap will go bad as well. Upon visual you will see that cap bubbled, but B supply resistor is the first item to inspect.  

drbeechwood OP
14 posts
 

GREETINGS and here's the update. I went to my electronics guy and he said "Take my Dynaco Tube amp and test your preamp".

Basically it worked fine from both the A and B outputs.


So czarivey said "So far the issue seems like tubes and caps going away in the power amp, not preamp."

 

And that does seem to be the issue given that my preamp worked fine with other solid state and tube amps.

 

So we are going to test/replace the capacitors and anything else on those amps. I have no idea WHY the power amps went bad suddenly, given that the preamp definitely had bad tubes, which is why I brought it in. Maybe some bad signal triggered the capacitor failure in the power amps.

 

Making me think about selling everything after I get it all functional again. Or, if it's functional again, maybe just keep it running for another 30 years? 

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