Quote Originally Posted by duhvoodooman
I did three of the BillM mods on my Blues Junior and liked them all--

  • The "Simple/Adjustable" Bias mod: I e-mailed Bill and asked if he had instructions to do an in-between mod for tube bias. I didn't see a need for being able to adjust both power tube bias values independently, but I wanted more control than just soldering in a replacement resistor for R52. Sure enough, he had instructions for a single trimpot installation that lets you adjust the bias for both tubes simultaneously. Much simpler installation than the dual-pot mod! Since most of us buy power tubes in matched pairs anyway, this seemed more than adequate to me. If anybody wants a copy, PM me and I'll e-mail it to you. It's for the newer (mid-2001 and later) version of the BJr, with the cream-colored PCB.
IMHO, there really is no reason to match power tubes in a guitar amp circuit. Just buy a matched set from a repuatable dealer. I've heard some techs say that guitar amps sound better with slightly mismatched tubes (5-10mA). Besides, Fender used to just grab two or four tubes and stick them in the amp and called it good. They didn't worry about matching. Although at the time, RCA Blackplate tubes were readily available and they were made to much tighter tolerances than modern production tubes.

The only advantage I can see for the individual biasing scheme is if you have two tubes that are way mismatched.

Also keep in mind that the individual tubes do age differently, even ones from the same batch.

tung