The TJ Full Music 12AX7 is a box plate Chinese design that looks much like a vintage Mullard M8137 and completely unlike any other current new 12AX7 offering. The gold pins are a nice touch, and it looks like actual mica (?) was used instead of the whatever the heck is used for the Shuguang small tubes. Bulb size is slightly larger than my collection of old stock, a few mm difference in height and circumference..
Break-in was approximately fifty hours as the driver tube in my Frankenstein Eico ST70. This is basically the classic Mullard 5-20 circuit. It's been my experience that the driver tube - before the phase splitter - has a high impact on the overall amplifier sound, more so than the phase-splitter or even the output tubes.
Tonally the TJ 12AX7 was one of the better new production tubes I've heard so far. I was initially hard-pressed to hear any difference between it and a quality old tube like a 1950s Mullard. Bass definition and instrumental shading of the TJ was particularly good, but, even as the hours piled on, I detected a slight muffled sound. There was a lack - especially compared to the Mullard - of inner detail or instrumental/vocal bloom. That is to say the TJ had a flatter and smaller presentation. I'm reminded of an inexpensive cartridge or CD player; the magic "glow" or shimmer, whatever one prefers to call it, was pulled back on the soundstage.
The old Mullard, in comparison, sounded bigger and had better bloom. The music was more "filled in" with depth and soundstage width.
Do the TJs need more break-in? I really didn't hear large differences between hour zero and fifty so I remain skeptical. Nonetheless I did like the TJ tonally, especially compared to some other new production valves I have heard, but I was ultimately disappointed. I do want new tubes that can perform in the realm of the best of the old. At least with this tube I'm not hearing it!
Disclaimers: No tube is 100% perfect for all gear. The only way to be sure how a tube will sound in _your_ system is to hear it. Also a tube in a phono stage, for example, may have different audio characteristics than say line, input, or phase-splitter use.
Negative feedback - several dB in this circuit - also reduces the sonic signature of the tube. An example of this came apparent to me when I built a triode-connected EF86 linestage. The tube selection - GEC CV4085 and the Mullard "long mesh" were the best - played a big role in the sound quality. However the EF86 in my Eico HF60 monoblocks were less important, most likely due to the loop negative feedback or perhaps the total number of tubes in play with a more complicated circuit.
So take this review as a general guideline, not as hard fact.
