When replacing the 12AX7 and 12AU7 preamp tubes, do any of them require balanced and/or low noise options to ensure proper operation of the amp? If not, would there be any benefits or drawbacks to using them? I didn’t find any information in the manual and just wanted to confirm.
Thanks!
The phase inverter and drive stages will benefit from being as symmetrical (balanced) as possible in terms of typical triode parameters: \mu, R_k and I_a. The most significant stage in terms of noise is V1 because any noise introduced here is amplified by the gain of the whole amp!
using a V1 with a lower noise figure will definitely lower the inherent noise of the amp. but that’s not to say you absolutely must use the lowest noise tube or even a tube which is claimed to be “low noise.” additionally, we’re talking about a power amp, not preamp, so any noise introduced by V1 will likely be tiny compared the noise coming out of the preamp. I personally would probably only consider tone/timbre/dynamic response when selecting V1 here, under the assumption that most modern tubes you’re trying have a “small enough” noise figure in this application.
and I fully admit I don’t know the topology of the 2/90/2, but simply looking at the datasheet, it seems that V1/V3 are dual use input stage/phase inverter for channels A and B. I’m assuming that means that only half of each tube is used for the actual phase splitting. i.e. that the phase inverter is of a cathodyne form as opposed to a long-tail pair (which would use both halves of the tube). which means that using balanced triodes would have no impact. again, this is presumption on my part, and I might be off-base, so expert input is warranted.
