F2B Preamp

Here is a clone I built of an Alembic F2B preamp, which itself used the ’60s Blackface Fender Dual Showman’s preamp circuit. It’s a versatile preamp which found much favour for bass, keyboard and guitar, with the two identical channels being able to be used for true stereo or a mono mix; or they can be daisy chained together for very effective higher gain guitar sounds with the two sets of treble/middle/bass tone section and volume controls providing flexible shaping for EQ and tailoring the overdrive character.

 

Technical description

I constructed a fairly straightforward layout based on the Fender’s audio circuit, hardwired on tag boards (the Alembic actually used a pcb); it’s adequate to prevent oscillation when used in the higher gain scenario. I designed a power supply using various parts I had available; an EZ80 valve rectifier feeding CLC filters in both positive and ground legs, followed by another LC filter section to get the HT ripple down to levels adequate for high gain preamp use. Each channel is then decoupled by its own RC filter. Since the power transformer I used didn’t have a centre tap for the HT, I made a compound full wave bridge using UF4007 diodes with the EZ80. Heater filaments for the 12AX7s are ac run at 12.6V to minimise current fields, with a hum balance pot with centre tap elevated to about 40V relative to ground using a voltage divider on the HT. There is no hum audible when the channels are daisy chained and with everything on ’10’.

The case creates separate bays double decker style for each audio channel, with the power supply isolated from them in its own compartment at the rear. All jacks are at the front for convenient access, since the amp is designed for rack mounting (occupies 2U).