Only seen this with mismatched power tubes where no matter how you bias, if one tube doesnt match the gain characteristics of the rest it will cause hum.
So, I highly suspected that your 6SN7s are matched at all.
Never had this problem on my Phi70 monos no matter I use NOS, Sovtek, EH, or the orig red plastic base VAC Chinese tubes. |
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 |
You need to change to a 6SN7 tube with a GTA or GTB suffix. Almarg's suggestions of going with vintage 1950's Sylvania 6SN7GTB's is a good one. |
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 |
Prior to insalling the Sophia's, I was running Sylvania 6SN7GTB's in outer sockets and RCA VT231's in the inner sockets. I did notice on the Sophia site that there 6SN7 spec's were the same as the GTB's so I thought I'd give them a try. They are amazing in the outer sockets and really take the sound up a notch or 2 but will not work in the inner sockets. Surprised! Guess I need to get the tube tester out per Al's suggestion about leakage. I did get the 1 year warranty so we will just have to see how this goes down. Thanks guys for your input.
Markus |
an incredible little amp You must be a power lifter, Markus. I thought that my Doshi/Lectron JH-50 was HEAVY until I got my Ren 30/30!! Highly recommended, as long as your health insurance covers disk surgery ;-) I'll have to check out my Sylvanias but I will comment that the Shuguang Black Treasure 6SN7 equivalent (CV-181?) sounds pretty darn good. I have them in all 4 slots on my Ren 30/30. Not to hi-jack this thread, but has anyone tried the Shuguang Black Treasure 300bs in their VAC Ren? |
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 |
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 |
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 |
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 |
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. |
Raquel,
The Sophias are not standard spec 6SN7's. Their data sheet reflects that they are higher voltage equal to GTA or GTB prefixes. |
I had read your post and forgotten what you and Almarg had written about the voltage of the Sophia 6SN7's. I was commenting mostly to point out that driver tubes used in the Renaissance amps must be the higher voltage GTB or GTA spec 6SN7's - this is not mentioned in the 70/70 or 140/140 owners' manuals.
While I'm at it, I will also mention that any 300B's put into the Renaissance amps must be standard WeCo spec 300B's - "super 300B" variants like the KR BLX 300B, as well as meshplates, will damage or destroy these amps.
Finally, any 300B used in the Renaissance amps must be carefully vetted to ensure that it can handle the stress put on 300B's by the circuit, specifically, as for transconductance, output tubes for the Renaissance amps should test in the 3,000-5,000 range. In addition, the plate-to-cathode voltage is approximately 430 volts dc, with idle current approximately 85 to 90 milliamperes in a self-bias (cathode bias) circuit. This is approximately 5% below the maximum rating for the WeCo spec 300B, meaning that the 300B's in these amps are run close to flat out. The milliamp and transconductance testing for purposes of matching and vetting 300B's must be done at those voltage levels, and anyone you buy 300B's from has to be someone you can trust to cull tubes to meet these demanding standards.
This information is likewise not in the owner's manuals, but will be confirmed by Kevin Hayes if you ask.
Returning to the issue you face with you Sophia 6SN7's, it sounds like Almarg has identified the problem. For whatever it's worth, I've been running Sylvania chrome tops since 2003 with my Renaissance amps and they've always been trouble free (although my VAC is no longer my primary amp and it's seen only very sporadic use since 2008). While some production out of Shuguang that is quite good, there is no reason, in my opinion, to purchase any current production Chinese 6SN7's, as there remains a large supply of high quality new-old stock 6SN7's available that are not really expensive (for example, $50 per tube). |
