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.
|
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.
|
 |
|
 |
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.
|