Buy 12ax7 tube

OP, JAN is by far and away the best. If you listen to nonsense advice and purchase Chinese tubes, you’ll deserve all of the misery that will come your way.

Joe of Joe's Tube Lore did not favor the 12AX7 Telefunkens.  I still consider his opinions the gold standard.  That said, I have been purchasing the matched pairs of  Psvane 12AX7-TII from Grant Fidelity.  You pay a little more for Grant Fidelity supposedly picking the best pairs.

Can i use 2 brand or 3 in my amplifier in same time?
For example rca svetlana and chinese tube in same time?
Isnt harmful?

It seems that one of the tubes is the driver and the other two are the left and right channels (the gain stage).

The driver can be a different brand, but the L & R must be matched tubes.

I prefer vintage GE circa 60’s and 70’s. You can pick them up at real good prices. The Sylvania Baldwin green label is another one of my favorite 12AX7 type tube!