Multiple amplifiers shutting down when playing vinyl through a tube phono stage


This has got to be the weirdest audio connundrum, and Ive seen many, I have ever experienced, or even seen.

I have a Balanced Audio Tech VK-P5 phono preamp. It was retubed recently after having the original tubes in there for some 15 years by the previous owner.

I have been using it for 2 weeks since being retubed. My system is composed of the VK-P5 phono preamp, a VK-3iX preamplifier, a Pass Labs amplifier and a digital source.

In that time, my Pass Labs amp blew a fuse. I replaced it and it ran again. 4 days later the fuse blew again. I put the Pass amp aside. I changed the fuse and it started up again. So it still works, but I'll try something else.

Thinking something is wrong with the Pass amp, I replaced it with a very new Audio Research tube amplifier. Twice, the AR amps lights flickered, the sound distorted and bogged down. Being a tube amp, I guess it doesnt blow fuses as fast as the solid state Pass unit.

The amp issues only happens when I am playing vinyl and using the VK-P5. It does not happen with a digital source. As a matter of fact, when the AR tube amps starts to distort and bogged down, the problem stops when I switch inputs on the preamp away from the VK-P5.

Further test were done, I swapped the VK-P5 with a solid state phono preamp, no issues.

I opened the VK-P5 and found 3 of the eight new 6922s to be weak. This was not the case when I tested them for installation.

My suspicion is that something is wrong with the VK-P5, that it is sending a signal/voltage/DC, going through the preamp (how is this even possible?), and then stressing my amplifiers. Oddly the VK-P5 will work and only start having issues 15 to 30 minutes in.

Again, with the VK-P5 out of the system, I don’t have any issues playing vinyl. The system is stable and reliable.

Any ideas to troubleshoot better highly appreciated. Can a ground fault do this? When the amps shutdown, the rest of the stack doesnt even flicker. Ive got streamers, DACs, turntable, both preamp and phono pre dont miss a beat.

I've written Balanced Audio and your input could be helpful.
peguinpower
I think you isolated your problem. Have BAT check it out. 
Yes absolutely sounds like DC the timing of retubing is likely just coincidental.
Do you have the old tubes?  Can you re-install them and see whether the phenomenon recurs?  That would establish that you have one or more defective new tubes.  What do you mean when you say some of the new tubes tested "weak"? Does your tester measure transconductance?  It is not necessarily the case that a tube amp and solid state amp are per se different in their tendency to blow fuses.  It depends entirely upon the two different circuits, how much current they draw, and how much safety margin the designer decided to go with, when he or she specified the current rating of the fuse.  One possibility: one or more of the tubes in the VK5 is losing bias voltage on its grid.  As bias goes toward zero, the tube draws more and more current, which is wearing out the tube at best and could cause the tube to self destruct if allowed to continue for a long time.  But I don't know how to tie that scenario in with blowing fuses in the Pass amp or drawing a lot of current from the ARC amp.  Another thought, if the VK5 is capacitor coupled at its output to the amplifier, and if that capacitor has failed, THAT could send DC into the downstream amplifier, which could upset the bias of the input stage in the amplifier.  I like that idea better.
Yes. I have the old tubes. 
Im putting them back in and sending the unit back to the seller. The tubes arent the issue. The unit is sending some sort if signal that is stressing the ampliers. I suspect its DC due to a capacitor failure. :( 

Its too bad. The unit sounds magnificent.
I suspect the output coupling capacitors. DC at the input of any amplifier with a direct coupling input will give it conniptions. With solid state blowing a fuse would be one scenario. With a tube amp it would cause the input circuit to not be at its proper operating points and so would likely make more distortion. 


Some of the older VK-P5 used coupling caps that look like oil-filled parts. If that is what is in this unit, that would be my first guess. But since you're sending this back its probably moot.
Oh yeah.  Oil caps are like little time bombs built into a circuit, especially when used in a coupling application, and especially if "Jensen".  I forgot that BAT was enamored of oil-based capacitors.  FWIW to the OP, this is an easy fix.
We stay away from oil and paper caps in our stuff because regardless of the manufacturer, its reasonable to expect that sooner or later an oil and paper capacitor will develop a bit of electrical leakage- and so pass a small amount of DC voltage while otherwise not outright shorting.





I always had a feeling that in the good old days, with the good old oil and paper capacitors that contained PCBs, now outlawed, leakage was less of a problem. I have had a personal bad experience with one or two of the modern era oil capacitors, and I would no longer ever use one in a couple in capacity especially.
Viktor Khomenko at BAT personally told me that it is likely DC.

Sucks. I dont want to tell the seller to pay for repair, that would sound uncool. I do like the sound it makes, but I will have to send it back.