Most amps have cathode followers and on the early
ones there are voltages up to 220VDC on the second
cathode. Not all 12AX7 types can stand that voltage.
I know the Chinese ones have no issues with that.
The Chinese ones sounds best too me. It´s only
hard to select a non microphonic one for V1 and V2.
Laura from Valve Queen got some samples from
me and she knows what to send out and sell.
There is no problem if you like other brands of tubes,
the problem is to find good ones. The manufactures
don´t will improve the quality. It depends on the
distributor to take care and do a big effort on selection.
The Chinese 12AX7B has 1% to 2% of useable V1 pieces
in a bulk of 100.