March 04, 2011 

Hammond AO-39 Conversion to Vox AC-15 Lite

The 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. hammond_ao39.pdf (1MB)
 

Return to Index