Dual Channel Blackface-style Preamp for Bass Guitar

Dual blackface-style preamp - covered
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.
Dual blackface preamp - front view
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.
The power supply starts with a Stancor LB-1220 dual-primary, dual-secondary flatpack low voltage transformer designed for PCB mounting.  The second primary winding is used as a high voltage secondary.  With 120 VAC line voltage, the second primary puts out about 100 VAC.  It feeds a full wave voltage doubler and two filter stages as shown in this schematic, produced by Duncan Monro's Power Supply Designer II.  The B+  under load is about 250 VDC.  Both 10 VAC secondary windings are connected in parallel, rectified by a full wave bridge, and filtered to provide about 12.5 VDC to the filaments of the two 12AX7's.
Power supply schematic
dual-channel blackface preamp - rear view
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 biamped 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 rig.  The output 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 Crown PB-2 power amp.  The input of the two channels may be linked with a jumper cable.

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.