On Saturday we spent about an hour at Mission Amps' shop tuning up AC Blue's Bassman reissue amp. The amp is late 80s reissue with lots of patina from being vigorously gigged, first as a guitar amp and for the last couple of years as AC's main gig rig for blues harp. It was sounding a little tired so he brought it to Mission Amps for a check-up, and I hung around and took notes and photos.
First, Bruce Collins -- the owner of Mission Amps -- re-wired the rectifier socket so it would accept a tube, and replaced the solid state rectifier with a new old stock 5U4GB tube. Next, he found that the 6L6 power tubes were mismatched an one was failing. Bruce replaced them with a matched pair of new Sovtek 6L6 tubes, and biased them at 15 watts each. The amp produced 48 watts of power when bench tested.
The improvement in tone was impressive. I tried it with both AC's EV RE-10 mic and my Front & Center crystal bullet mic, and the Bassman just ripped. Lots of texture, lots of beef. The tube rectifier gave it a nice touch of sag. It's now a very good-sounding harp amp.
The last task was a line out. Bruce removed the "Ground" toggle switch (which did nothing) and replaced it with a 1/4 inch phone jack. He stepped down the speaker tap with two resistors, 270K and 27K, shunted to ground. The unbalanced line out delivered about 700 millivolts with the volume on 4, when the amp was just barely tipping into distortion. When we plugged it into Bruce's Peavey PA the Bassman sounded big and bad-ass. AC is a happy harp guy.
NOTE: I played AC's EV RE-10 mic for several minutes and really liked it. It has a big sound and a deep tone. The mic itself is a bit too small for me to hold comfortably when cupping the harp, but the sound was excellent, very similar to my F&C crystal bullet mic. I can see why some players are so devoted to the RE-10.
New 5U4 rectifier and 6L6 power tubes:
The line out jack where the "Ground" toggle switch used to be:
The line out originates at the speaker tap: