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VAC Phi 300.1a Single Channel Ground Loop Hum

I borrowed a Vac Phi 300.1a amp to test in my system.  What a beast to move around but that's another story...

With the amp in my system instead of PS Audio BHK 250 I get ground loop hum in a single channel (right).  I've swapped input tubes, no change.  Speaker cables not crossing power cords.  Same hum whether the preamp (NAT Audio Magnetic) is connected to the amp or not so I know it's not the preamp.  VAC passes audio in right channel but the hum is pretty loud.  The amp was a trade-in to the dealer who didn't bench test it before letting me borrow it.  I guess could be a bad outuput tube but it will be kind of a PITA to swap them around because it's on a shelf in my rack.

I've never experienced single channel ground loop noise before but my dealer says he has with a couple of tube amps.  I've got no financial exposure since I'm borrowing it but I really did want to hear it especially since it was such a bear to install it (~130lbs of amplifier).  Any ideas?

Thanks much!

Ian

P.S. Not that it means anything but Kevin Hayes from VAC was a classmate of mine in grad school.  I was his TA in a quant course.  I had no idea then that he was into audio.  Wish I had.  When I met him at a show about 10 years after we graduated he was very cool.
ihmeyers
ihmeyers OP
61 posts
 
I hadn't either.  Dealer said he once had a CJ amp that had a ground loop in a single channel.  Given that a 'cheater' cord didn't help it sure points to an output tube.

My son will be thrilled (lol) to have to move that beast from the rack.
stereo5's avatar
stereo5
2,766 posts
 

"My son will be thrilled (lol) to have to move that beast from the rack."


That is what our kids are for.

rhljazz
1,117 posts
 
I would concur on a bad output tube.  I had a set of cryo treated tubes and they went bad in that manner.
jbrrp1
215 posts
 
I ran the VAC Phi 200 before I switched to PS Audio BHK 300 mono's (better fit for my speakers, for sure).  The VAC amp was very sensitive to bias balance between the output tubes, and when it was off (can be from a bad output tube) it created a God awful buzz in the one channel that sounded like some harmonic of 60 hz., but with a more "electronic" distortion complexity to it.  Does this sound like what you are hearing, or is it a fairly pure, low tone?  If this sounds familiar, you could try re-setting the biases on the output tubes.  If that doesn't work, then it is likely that an output tube is shot, and they push-pull cannot balance.

I hope you get a chance to compare this to your BHK, as I would be interested to hear your findings.  And what speakers will it be driving, if I may ask?
brf
1,699 posts
 
You mentioned that you tried swapping the L+R input tubes but did you try swapping the L+R output tubes?  This will isolate whether it is a bad output tube or not. 
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