Solidstate bias control for tube amp

[Bruce / Mission Amps]

Well, if I need to swap tubes in a hurry, I just twiddle the bias pot until the meter reads something sensible. And you can tell bad tubes if the meter either starts reading more than it usually does at idle, or doesn't read high enough when you crank it to full volume.

I argue that a cathode current meter and an easily accessible bias pot is all you need. And a pair of oven gloves to make tube swaps even quicker!

And being really lazy, I'll submit just buying a quad of matched power tubes from a reputable dealer for a two power tube amp and just use the other two later to replace that "once in a while bad one" and don't even bother looking at the idle current! Plug and play.

[Old Tele man]

...computers to the rescue?

...I recall reading about a VERY high-end audio vacuum-tube system that used an embedded microprocessor with built-in A/D converter that constantly "monitored" and "adjusted" the bias by selectively injecting (via isolation resistor) a ±voltage that added to or subtracted from the "standing" bias whenever the amp was in quiescent state.

...can't recall if it was done "collectively" (same ±voltage to all tubes) or "individually" or what, but the advertisement guaranteed it to work with ANY 'approved' octal power tubes.

...does anybody else remember seeing this?