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Interesting comparison between NOS RCA 12ax7, Chinese 12ax7a

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I recently swapped in a pair of NOS RCA 12ax7's I found laying around in place of the Chinese 12ax7a's I had in the first stage of my phono preamp ever since I built it and heard some surprising differences to the NOS tubes' advantage, IMO.

I should first describe my phono preamp general topology. It's set up for MM (I use a Shure V15 Type 5 MR). The input stage is 1/2 the 12ax7a w/o feedback. The other half is used to provide an inverted version of the signal with feedback. Then I sum the output of the inverting stage through an air variable capacitor and the feedback-less 12ax7a into the grid of the second stage which has RIAA compensation in its feedback loop. The second stage is a cascode amplifier using a 6BK7B driving a third stage which is a White follower using a second 6BK7B.

With this approach, the summation of the signal through the air-variable cap with the signal from the input stage forms a first order all pass filter which can be set to reduce the HF group delay error due to the MM resonance. This definitely helps tighten up the top octaves and reduces the audibility of ticks and pops. The main RIAA feedback cap in my design is 100pf which I made myself out of copper foil and teflon film with bare copper leads soldered with tin-silver solder and with teflon shrink wrap over the whole thing. They've worked fine for over ten years, although I think about the possibility now and then of trying to make my own stacked plate 100pf air dielectric capacitors for the RIAA feedback.

Anyway, after all that description, I was really surprised how much more tuneful and much more body the bass had with the NOS RCA input tube compared to the Chinese tubes. OTOH, the highs were definitely somewhat harder with the RCAs, but I think that is likely just a reflection of their greater accuracy. I suspect that the Chinese tubes 'diffuse' some fine detail which makes them appear a little smoother on the top end but robs their bass of punch, at least in my preamp application. I could be talking out my hat here, but I wonder if the open plate construction of the Chinese tubes possibly adds a little dynamic nonlinearity to the signal as the plate voltage and current change. Just a thought.
New vs NOS

I have yet to audition any new manufacture small signal tubes that comes 1/2 way close to NOS sonics. I have not tried Sovtek 12AX7XLS or the Groove tube Mullard copy. Most new manufacture 12AX7s only measure 1200 gm whereas any NOS measures 1600 gm or better. Something is wrong here, is it not?

Some new manufacture power tubes perform well, and especially so with the Shuguang 845. This plant in China has been making tubes for many years & has experience. BTW- The Shuguang 12AX7 measures 1600 gm.
I have been running old GE/Wurlitzers long plates for some time now in a couple of my guitar amps. They also sound darn nice in my little RH-84 I just built.
I had no idea prior to trying these just how bad current production tubes are.

According to several guitar player magazines I glanced through, Mullards & Tung Sols reissues are owned by Electro-Harmonix?

Thats New Sensor correct?

How true this Is I have no idea. But I do own a couple sets of 12AX7EH's. They have a really unusual brittle kind of sound that just doesnt cut it in my amp.

J&J EL34s I own seem pretty solid, But then again, I have no NOS EL34's to compare to.
Hi, Sy -

The 12ax7 itself is outside any feedback loop or equalization circuit. The only effect I could imagine is if there is a substantial difference in effective output impedance which might modify the input resistance to the feedback summation point slightly. All the nonresistive (RIAA-related) elements are in the feedback leg. But the LF rolloff point could be affected slightly. However, the feedback runs out definitely under 10hz in either case.
Home entertainment vs guitar amps

I find a difference regarding the application. I strive for precise & clear sonics in home entertainment amps. For the most part, the opposite is true in guitar amps. I prefer a lively harmonic structure. Although many NOS preamp type tubes are still best, I like some new manufacture power tubes a lot.
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