When TAS reviewed the Fet Valve CF preamp, they did use premium NOS tubes. We were not too wild about that.
When they returned the preamp to me I asked for the loan of these tubes so I could evaluate them here.
I set up two identical Fet Valve CF preamps in our ABX switcher, one with our stock JJ tubes, the other with the NOS set that had been used in the review.
With levels matched nobody here could hear any difference at all between the two preamps on any program material or when driving different speakers and power amplifiers.
Both sounded great in all conditions of use here.
Because our tube preamps have separate regulated power supplies for each plate of each tube, circuits that don't drive the tubes to their limits, regulated heater supplies, and even isolated ground plains for each section of the preamps, high voltage, low voltage, and audio, the tubes are not worked nearly as hard as in other preamps. We suspect that when tubes are pushed really hard that there will be more difference in the way various brands behave than in an Audio by Van Alstine preamplifier.
We allow the tube to gracefully do what we what it to do, not what it does when asked to do too much.
No problem with tube rolling, its just that it won't buy you much with our equipment, the musical quality is built in, not something that will change much with tweaking.
Regards,
Frank Van Alstine
