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In Reply to: RE: Art Audio Phono Stage Only Putting Out One Channel posted by AnalogJ on August 04, 2024 at 18:19:31
Dollars to donuts one of your 12ax7's used for gain has died. Just because the filament lights up doesn't mean that it's not dead.
A review of that box I found says the 6FQ7 provides power regulation to the 2 gain tubes (12ax7's), and then a third 12ax7 is employed as a cathode follower. The other possibility besides a dead gain tube is one of the triodes in the cathode follower is muerte. Hopefully you have some spares. If not, I can wholeheartedly and highly recommend the JJ E83CC frame grid tube. If you decide to try this tube, be careful to order the correct version as JJ makes multiple versions of the 12AX7, and other versions do not sound the same as the frame grid.
BTW, flashing on startup means next to nothing. Some tubes do it and some do not.
Follow Ups:
It's just that there's been some crackling at start up prior to the channel going out.
If the 6FQ7 is used in the way it's used here, could one channel go out as a result of it failing?
Yes, I suppose it is possible, but much more likely IMO that one of your 12ax7's is the culprit.
The flash on startup is a fairly common trait in Euro made tubes.
Then I suppose if I switch the tubes and the channel that's out reverses, then I'd know it's the tube?
Don't forget to try swapping the cathode follower if swapping gain tubes left to right doesn't cause the problem to move with it.
If swapping all 3 12ax7's around doesn't affect the failure condition, then it would be logical to assume it's the 6CG7.
If it should come to it and replacement of the 6CG7 is warranted, keep in mind that the 6FQ7 is for intent and purpose the exact same tube as the 6CG7. The difference between the FQ and the CG is immaterial in this application, and the 6FQ7 is far more abundant in the market. The only difference is the presence of a physical shield or barrier between the two triodes inside the bottle to cut down the possibility of crosstalk. Not really a consideration as a regulator.
I swapped all three around with each other, initially switching the left and right channel tubes. The right was still essentially out, but, oddly, I could hear a bit out of the right channel. The left channel was quite dominant, though.
I tried doing some more swapping, but with every combination, the left channel was all I heard. I could not even hear the right channel at all. I could not recreate a combination where I could hear anything out of the right channel.
Only thing left is the 6CG7. Try a new one there if you have one.
If that doesn't fix it you'll probably need to see about sending it in for diagnostics and repair.
Again, how important is sound quality when it comes to the tube in that part of the phono stage? I have bought from Vintage Tube Services in the past, and they have not only different brands but graded with regard to microphonics.
The 6CG7/6fq7 are electrical equivalents of the older and venerable 6SN7. Cg/Fq are merely miniaturized 6SN7's with a nine-pin base. By far and away THE most common use of these tubes is for signal amplification. Most suppliers assume you're going to use it as a an amplifier. Since in your phono pre it's used for voltage regulation and not amplification, it need not undergo the same rigorous testing you might apply if it were an amplifier.
Not very. It's not amplifying anything. Voltage regulation. Microphonics is irrelevant in this application.
The 6CG7/6FQ7 are common and available practically everywhere. Any number of sources. My one and only experience with Andy left much to be desired, but that was over twenty years ago.
I've used Andy a lot and had very good service from him. But I'm looking at any options.
The Tube Depot out of Memphis has become my preferred provider. There are certainly others.
Even eBay.
One last question. With selecting a tube, in this scenario, is it worth the $4 upcharge for low noise/microphonics?
No, not really. Better to have balanced sections.
I'm not sure I understand your answer. I need one 6FQ7 tube. The Tube Depot offers a NOS GE tube, with a $4 upcharge for a low noise/low microphonics one.
Sorry. I think they refer to it as balanced triodes.
There's two 12AX7s, one for each channel. And then there's the single 6FQ7.
Is this what you have?
Oh yes, looking at it again, you're right. And that's the one!
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