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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
atmasphere's avatar
atmasphere
11,564 posts
 
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.
lewm
12,429 posts
 
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.
atmasphere's avatar
atmasphere
11,564 posts
 
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.





lewm
12,429 posts
 
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.
peguinpower OP
59 posts
 
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.

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