“A randomly chosen NOS 12AX7/ECC83 is unlikely to be as quiet.”
Maybe so but that shouldn’t deter anyone to not experiment, right? It’s a calculated gamble with any NOS tubes…well regarded NOS 12AX7s or ECC83s (like Mullards, Telefunkens, or Amperex) may not be low noise as Sovtek but what they have in spades is harmonic complexity and tonal richness.
The Sovtek 12AX7LPS is indeed not a separate “variant” of the 12AX7 but rather a Sovtek-branded 12AX7 with the “LPS” designation, which stands for long plate spiral. This spiral filament design helps reduce hum in applications like phono stages, where low noise is critical. Its long plates can offer a bit more detail and tonal richness compared to short plate designs.
I had an extensive dialog with the manufacturer prior to my purchase, the Model 4 supports all 12AX7 variants. My particular piece is supplied with JJ ECC803S and it is dead quiet.