Ok, I did some A/B/C testing with the Mesa, Jan, and the Tung-Sol.
Quick note: The Jan is a 5751, but supposedly it's a particularly high gain one, so fairly comparable to a 12ax7.
Here are the comments on each one from the Tube Store where I got them...
Legend:
Manufacturer/Model,
Microphonics, Noise(Hiss, Pop, Hum), Musical Detail & Dynamics
Tung-Sol/12AX7,
5, 4, 5
JAN-Philips/5751,
5, 5, 4
Tung-Sol 12AX7 - There are only so many ways to describe tube tone and most have become cliches. The Tung-Sol 12AX7 has the gain and drive of a Chinese 12AX7 and the pure tone of a Mullard or Brimar from the U.K. I've had two people come to me recently with amps they thought were in need of complete overhauls. In both cases, careful examination revealed no serious problems and all the tubes "tested" as good. At the end of the day, I replaced the NOS Mullard and RCA pre-amps (one in each amp) with a reissue Tung-Sol 12AX7. In both cases the owners were very impressed and thought that their amps had been restored to full health. Believe it or Not.
JAN-Philips 5751 - Now here’s a surprise, and a good one at that. The 5751 is typically referred to as “the lower gain” 12AX7. The fact is they are just dual triodes with the same pin-out as others in the 12AX7 family. This particular batch of NOS 5751 fails to perform as most 5751 tubes. It’s not lower gain, it’s not darker. These JAN-Philips 5751 tubes are one of the best dual triode tubes I’ve ever heard. They have nearly 12AX7 gain, extremely low noise, and frequency response that covers any audio application. They sound good in everything I’ve tried them in but particularly shine in single ended, simple amp circuits. Good examples would be a Fender Champ, tweed Princeton, Gibson GA-5 or any amp that has a preamp, power tube and rectifier. One of the best deals available in an NOS 12AX7, is actually a 5751!
Results:
Tung-Sol: I definitely heard a little less noise and microphonics with Tung-Sol, a modest tonal improvement, and if anything, it might have been even hotter than the stock Mesa Chinese (Sovtek, I believe, tho Mesa branded on the tube) 12ax7-a. Not night/day at all, but a noticeable improvement overall.
Jan: This one was dead silent from a microphonics standpoint, but it also didn't have the musical dynamics of the Tung-Sol or even the Mesa. A noticeably more fundamental tone. It also does have a noticeable gain reduction, but with an amp as hot as the M6, that really isn't any big deal. If it had better tone, I probably woulda' left it in there just for the extra quiet operation. Make no mistake, it did sound very good, in fact, for someone looking for a slightly less dynamic tone, it'd be a great option.
I decided to leave the Tung-Sol in there. Very low microphonics (though not completely silent), great tone, and great gain.
No, for all you hiss-talkers reading this, no significant hiss reduction. If I put my head right in front of the cab, I can still hear hiss coming from the tweeter, but at least from my perspective, it was never a problem before, and while the TS and Jan may have slightly reduced this, it wasn't enough to talk about. What that tells me is, whatever hiss is being produced, is being produced before the tube. The Mesa tube may possibly add a touch, but again, not enough to find significant fault with.
Conclusion:
Except for some extra microphonics, I gotta' say the stock Chinese tube is not a bad one. It has great tone and gain. If I'd known this before I bought these tubes, I'm not sure the swap woulda' been worth turning all those screws. Yeah, the Tung-Sol is a better tube, but again, it's not a night/day thing.
Anyway, FWIW.
