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.
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.
