The Hammond AO-35 is a tube amplifier, chiefly used to amplify the Hammond M-100 organ. The amplifier is integrated into the organ itself. The AO-35 is noted for its reverb, which is generated inside the amplifier by a powered reverb unit. Whether you own a Hammond M-100 with an integrated AO-35 or have an AO-35 that has been separated from the organ, you can convert it into a guitar amplifier with a few minor modifications.
Unplug the AO-35. If you have been using it, leave it for 20 minutes so that the vacuum tubes have time to cool. Moving or jolting hot tubes is likely to make them shatter.
Open the rear panel of the amplifier to expose the tubes and the amplifier chassis. Remove the tubes to protect them during the modification. Slide the chassis toward you, but do not completely remove it. Unscrew the chassis lid and melt the outbound wire connections with a soldering iron.
Place the chassis on your workbench. Study the schematic and examine the physical circuit inside the chassis. Identify the input jack on the schematic and then identify the input jack inside the chassis. Use a colored pen to color-code the two for easy reference. Find the connection marked “C6” on the schematic and use a different colored pen to code it with the physical connection in on the circuit.
Cut the wire connection that is marked “C6” on the schematic. This disconnects the input signal resistor and renders it redundant. Because the input needs to be modified to suit a guitar signal, this resistor is of no use. You don’t need to remove it, disconnecting it is sufficient.
Locate the instrument input on the schematic and color code it by marking the physical input location with in the same color. Disconnect the input jack from the input wire and connect the existing input wire to a 1/4-inch jack.
Install new resistors. Because the guitar output is a high impedance signal and the amplifier has a low impedance input, you need to add one 27k-ohm resistor to each of the three speaker terminals. To install them, cut the wire running across each speaker terminal and solder the two loose ends to the connector legs of the resistor, effectively putting the resistor into the wire. Then solder the side of the resistor to the chassis to keep it in place.
Replace the tubes. Slot them into their respective tube sockets. It’s smart to wear latex gloves to keep the glass clean.
Only solder in a well-ventilated area.
Performing modifications to your AO-35 amp might invalidate the warranty.