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VAC 30/30 & Sophia 6SN7's

I recently purchased a VAC Ren 30/30 amp that is an incredable little amp and the best so far in my system. I decided to try some tube rolling in the front 4, 6SN7's and purchased a matched pair of the highly reguarded, Sophia 6SN7's to try out. When I install them in the inside (splitter positions) and then power up, a very loud hum is generated to the extent that I am afraid I will damage something so I shut it down. If I install them in the outside (driver) positions everything is fine and the sound is great. My question is: Has anyone experienced this? Are my tubes bad?
I do have an email in to VAC but thought I'd seek out any experiences that other, more knowledgeable audiophiles might have.
Thanks
Markus
markus1299
markus1299 OP
252 posts
 
Update- Kevin Hayes of VAC emailed me about my experience with the Sophia's and would like me to send him the tubes so that he can test them and sort out the problem I am seeing. How many manufacturer's are willing to do that for you? Anyway, I'll probably take them over to him (I don't live to far away) and I'll let everyone know what his results are. Should be interesting.

Thanks

Markus
almarg's avatar
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almarg
9,670 posts
 
Wow! As always, Kevin is terrific.

FWIW, my speculation is that a little bit of 60 Hz and/or 120 Hz ripple is present on the DC that is supplied to the filaments of the tubes. Perhaps its magnitude is a bit greater than when the amps were new, due to aging of the filter capacitors. The excessive heater-to-cathode leakage of the tubes, in turn, couples some of that ripple to the cathodes of the tubes, thereby causing it to enter the signal path.

Thanks for the update!

Best regards,
-- Al
markus1299 OP
252 posts
 
Thanks Almarg for the info. I will pass your suspicions on to Kevin for consideration. I believe, for what we paid for these tubes, we should not be seeing this trouble But, it's all a learning process in this hobby isn't it?

Markus
raquel
841 posts
 
At least on the VAC Renaissance 70/70, the outer sockets are for the driver tubes, which must be the higher voltage GTA/GTB spec 6SN7's (450 volts vs. 300 volts for standard 6SN7's), while the two inner sockets are for the splitter tubes, which need only be standard-spec (low voltage) 6SN7's.

So if the 6SN7 array on the 30/30 is the same, one would think that hum problems would occur if you were running standard-spec 6SN7's in the high voltage driver sockets, not the other way around.

It could be that there is another issue - your tubes may be defective - but I suggest that you ask Kevin about this.
markus1299 OP
252 posts
 
Raquel,

The Sophias are not standard spec 6SN7's. Their data sheet reflects that they are higher voltage equal to GTA or GTB prefixes.
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