Mesa 400 vs. 400+

[Jefenator]

Another toob noob here, just got the 400 and am happier than a pig in $#!&.

I'll be looking to try the 12AU7 mod for less dirt & more depth. Just found Psycho Bass Guy's original mention of it here:

http://www.talkbass.com/forum/showthread.php?t=387202

 

[B-string]

I predict you will like it! My old V4-B is now with a fellow TB'er, I used one in the channel one position for my active bass (actives were not around in 1973) it reduced the gain but in alot of ways I actually like the overall sound better. Just to much gain loss with a passive bass and I didn't want to mod a classic amp to use it in both channels. I install 12AU7's in twin reverb's driver positions for Jazz and blues players (12AT7's are a little harsh).

 

[traynorbassmate]

V3 is the driver tube which can be swapped for a different 12A*7 tube, correct?

I'm a Hammond tech and have worked on old tube amps before. What do I need to do to check the amp's bias value, and if it is off, what type of trim pot do I need to install, and where in the schematic should it go? (or is it more than just a pot)