Ampeg B-12N Tone Modification
Adding ultra-low and ultra-high circuits to an Ampeg B-12N amp
I made this modification to channel one (guitar/bass) of my 1968 B-12 with excellent results. The amp has an even warmer and clearer sound. I play 4 and 5-string electric and upright basses through this amp in small combo settings and theatres. I pretty much always use the amp with the low and high tone circuits on. The amp has been working with this modification since 1992 without a problem.
I took this tone circuit design from a B-15 amp which is very nearly electronically identical to the B-12. I'm not sure if Ampeg ever produced the B-12 with this option - I have seen B-15's with and without it. If you try this, I hope you enjoy it as much as I have. Oh, please be careful doing this. These amps can be extremely dangerous to work on because of the voltages and circuits used. If you have any doubts, take this job to a professional.
Capacitor - 500PF
Capacitor - .01MF
Capacitor - .02MF (I used a .018)
Capacitor - .1MF
Resistor - 39K
Resistor - 1M
Switch - single-pole, single-throw (2)
PC board - I used a small project board from Radio Shack (#276-159A) with a few printed circuits to connect components
Medium gage wire
Soldering iron and solder
Shielded audio wire - 1 conductor plus shield
L-bracket - I made one from a strip of metal I had - about 1" wide x 2.5" long, bent half way
2 small nuts and bolts to attach pc board to bracket.
My B-12's Schematic (1.3MB) for reference. You should check this against the schematic in your amp if you are thinking of making this mod. When I searched, I didn't find any other preamp design for a tube-based B-12, but to be safe...In the electrical schematic below, I have added the parts in blue in place of the one capacitor marked in red. The physical schematic shows the rough placement of the components on the pc board.
Unplug the amp! Be extremely careful! There are capacitors in this amp that will be holding up to 500 volts after you unplug the amp. This means you can get shocked even after you unplug the amp. They will drain slowly or you can short the capacitors to ground to discharge them (only with the amp unplugged!).
The pc board I used has enough generic printed conductors to make assembly fairly straight forward.