Zesto Andros PS-1 Tubes

I'm pretty sure that there's no need or way to change the bias on the Andros. I've never had any tube gear that called for biasing of the small signal tubes, like Mechans said.

Sorry to hear your Brimars got tired-out prematurely. I think that my Cary line stage can be hard on tubes (6SN7) but I haven't noticed any of that with the Andros.

I do like what I'm hearing with the M8137 tubes now on the inputs and the Tele ribbed-plates on the outputs.

The Teles are very durable they seem to last forever. Did you get your ECC83 12AX7 = M8137 equivalent NOS NIB. If so it should also have a long life. I am wondering why your Brimar gave out so quickly even if it had a limited lifetime by measurement.
Did you try the New Sensor Gold Lion ECC83? I heard they were sort of Mullard like, sweet and melodious If you don't need all the gain of the 12AX7 you might want to try a 5157. I like them a lot, and now that the NOS and OS (Old Sh*t)12AX7 are getting hard to find.

@Mechans: I bought the M8137s lightly used from a fella who bought them NOS from an online dealer.. I think he had tried them in a Marshall amp and decided he was looking for something else. One NOS tube dealer that I've worked with called the M8137 "the un-Mullard like Mullard tube." The Teles I got from a seller here a while back so I would say they were ANOS, but tested really strong in my B&K 707.

Sorry to hear that Toddc's Brimars didn't hold up for some reason :(