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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
 

From a diagnosis perspective, it seems odd that BOTH tube amps would suddenly develop the same problem at the same time. Thus, I like the hypothesis that there's a bad cap in the B section of the preamp.

And perhaps something was dislodged during the handling of the preamp taking it to and from the tube shop.

I don't know that these amps were so sensitive. But they have been in service for many years. 

What do you mean by "They will not fix it"? Can't these parts be replaced with something similar? I'm hoping my local guy can fix it, but if not there are some places I found online that works on VTL (High End Audio Repair in Brooklyn, don't really know anything about them.).

 

The original problem was definitely bad tubes in the preamp, but perhaps that resulted in some cascading problems down the line.

 

 

oddiofyl's avatar
oddiofyl
3,695 posts
 

Best to find someone local that can repair.  VTL, Luke and Bea are horrible people to deal with.  One of the worst in the biz....

czarivey
5,909 posts
 

@oddiofyl they don't mind providing schematics of the amps. That's how I fixed mine when I had it. After fixing and observing unacceptable built quality, I've decided to sell my MB100. 

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
5,909 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.  

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