March 04, 2011
Hammond AO-39 Conversion to
Vox AC-15 Lite
AO-39 was used as the power amp for the A-100 series organs. It received a
balanced input from the organ's preamp and produced 12 watts to drive a 4Ω
speaker load consisting of two parallel connected 8Ω 12 inch drivers.
This unit is a good candidate for conversion to any of
several EL84 based guitar amps that use only one or two preamp tubes. The PT
will easily handle two extra 12AX7 tubes. Both transformers are very good
quality. The chassis is too small to accomodate more elaborate amps.
I chose to convert to a Vox AC-15 Lite circuit
with simple controls. I basically stripped the
chassis down to just the iron and tube sockets, added some extra sockets,
replaced the cap cans, relocated some terminal strips, and rebuilt from the
schematic. No layout is needed for this simple amp.
It's a fun little project especially when built using
point to point techniques. The sound is typical of the Vox AC-15 normal channel.
Note. I originally converted this
AO-39 into a Marshall 18 Watt Lite, but later replaced the preamp with the
Vox AC-15 EF86 preamp.
The original AO-39 is a typical push/pull power amp
with a few twists. (refer to schematic below)
- The "PI" is really just a two input dual phase
driver. The actual dual phase signals are developed using a transformer
located on the preamp chassis. The driver has
a signal balance adjustment (left end of chassis).
- The output tubes have a HUM BAL adjustment in the
cathode circuit that
actually balances (matches) the idle current through each output tube.
- The two NFB loops are taken from the plates of the
output tubes and loop back to the cathodes of the PI.
The original unit worked fine as is but needed a little mod
to make it guitar amp friendly. I ripped out the original dual driver and built
a Vox AC-15 LTP PI. I plugged in my Lightning preamp to test this unit. It's a
go! I hate to lose the vibe of that original power amp, but I need to move stuff
around a bit to make this a complete Vox AC-15 Lite.
I moved the LTP PI closer to the output tubes. The preamp tube now sits in the socket vacated
by the original PI tube. The preamp circuit is an EF86 with 6 position tone
switch. I used an NOS 5Y3 for lower B+ voltages.
I gutted the old power amp and power supply, then built the
Vox circuit using true point to point wiring. The power resistor mounted on the
front panel drops the B+ even more. (Click the image
for a closeup)
Download the original AO-39 schematic/layout and the AC-15
Lite conversion schematic
in a single PDF.
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