Dual Channel Blackface-style Preamp for Bass Guitar
|I've recently scratch-built this dual channel bass preamp for a band-mate. It's patterned after the Alembic F-2B, which borrows its basic circuit from the preamp section of the blackface-era Fender Dual Showman amp (AB763). The 1U x 6" deep rack enclosure is a Middle Atlantic Products EC-1, made from 1.5 mm powder coated steel. It is much more difficult to drill than aluminum. I spotted the potentiometer and jack holes in a drill press, and completed the holes with a #4 Unibit in a hand-held power drill. The holes required much clean-up with a deburring tool.|
| The layout and construction
technique follows Doug Hoffman's
style. The basic schematic (which has an error in V1b's tube pin
numbering) may be found at R.G. Keen's
Geofex website. The turret board is G10-FR4, 16" x 3.125" x
0.125". I made the L-brackets for the tube sockets from
1"x1"x1/16" aluminum angle stock.
|Both channels are nominally the
same, though I used tubular caps for
coupling and the tone stack in one channel, and orange drops in the
other. Each channel feeds a separate output for a stereo or
rig. If nothing is plugged into the channel out, the output is
fed to the middle jack, which mixes the two channels for a mono
is high impedance, so the power amp should have an input impedance of
at least 10K. It works great into my friend's bass rig with a
PB-2 power amp. The input of the two channels may be linked with
Channel #1 is dead quiet. A slight buzz from the power supply shows up on channel #2 when the gain and treble are turned way up. I will look into snubbing techniques to bring it closer to perfection. Update: 2004/04/07 - I isolated the output jacks from the chassis and most of the buzz disappeared. The noise level also dropped when I slipped shielding braid over the wires leading to the tone and volume pots. The preamp consumes less than 10W, so no holes in the box were needed for air circulation.
The hookup wire is #22 solid core teflon insulated. The total parts cost, using a mix of surplus and prime parts, was about $150.