VAC 30/30 & Sophia 6SN7's

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

Yes, I had similar results when I installed a matched quad of Sophia 6SN7's in my VAC Ren 70/70 MkIII, except that the hum level was not "very loud." It was acceptable for the first few months, just about inaudible at listening distance (after I put the worst tubes in the outside positions). However, eventually it worsened somewhat in one channel, and that prompted me to recently replace all four tubes with vintage 1950's Sylvania 6SN7GTB's. Quoting from my recent post in my system description thread:

Replaced the Sophia Electric 6SN7s I had been using in my VAC amplifier with 1950's Sylvania 6SN7GTBs, a matched pair of "chrome domes," and a matched pair of "chrome tops." A slight hum that had resulted from excessive heater-to-cathode leakage in two of the Sophias recently worsened, prompting the change. The Sylvanias have no measurable leakage, and do not produce any hum. Preliminary listening impressions are that the change resulted in significantly improved dimensionality and instrumental "body," while retaining the timbral accuracy of the Sophias, which I had been very pleased with.

The heater-to-cathode leakage in my Sophia's, as measured on my vintage Hickok tube tester, was as low as 3 or 4 megohms on some sections of some of the tubes, and was in the 5 to 10 megohm area on most of the others. That is very disappointing, especially for such expensive tubes. At some point in the future I'll contact Sophia to ask what they would be willing to do about that, but not immediately as I'm just at the mid-point of the one-year warranty I purchased.

FWIW, as I indicated in my system description thread I was very pleased with the sound of the Sophia's apart from the hum problem, which would not have been evident on speakers having significantly lower sensitivity (mine are 97db).

Regards,
-- Al

Thanks, Paul (Pdreher). I don't think that in this case, though, the lack of a suffix is the problem. The datasheet that is linked to at the Sophia site for their 6SN7 is the datasheet for the General Electric GTA and GTB tubes.

Best regards,
-- Al

Highly recommended, as long as your health insurance covers disk surgery ;-)

Michael, you should try lifting (actually, you SHOULDN'T try lifting) the 70/70!! :-)

Best regards,
-- Al