The Hammond AO-44 reverb amplifier was designed to be used an a secondary reverb channel amp in Hammond A-100 and M-series organs. These units can easily be converted into a mono tube amplifier that is suitable for use as one channel in a stereo amp, aka a Mono Block amp. The audio power output is around 10 watts. The modified AO-44 is also suitable for use as a basic electric guitar amp. The AO-44 amps are currently available for reasonable prices on eBay. This project is a good introduction to working with tubes.
This is a non-trivial project. It takes a lot of technician skills to deconstruct and reconstruct the ampifier circuitry. Also, there are plenty of lethal high voltages inside of this amp including 120 VAC and 310 VDC. The project should only be taken on by someone who has experience working with high voltage circuitry. The power should always be removed when working on the amp, the circuitry is designed to discharge the capacitors when power is removed, but it's always a good idea to short out the electrolytic capacitors before working on the amp.
AC Power Input - grounded 120VAC, 40 Watts Input - High Impedance Speaker Output - 8 ohms, 10 Watts
On/Off switch Input Volume Bass Treble
The original Hammond AO-44 amp design is intended to have its input connected across the speaker output of the main organ power amp. There is a volume normalization circuit that uses a pair of small light bulbs and a fairly hissy preamp stage involving an ancient germanium transistor as the amplifier. All of these components need to be removed. Also, the 6X4 rectifier tube wiring should be completely removed and replaced with a pair of 1N4007 rectifier diodes on a new terminal strip. This frees up a 9 pin tube socket for a 12AX7 preamp tube, giving the amp an all-tube signal path. The speaker output wiring can be moved to a 1/4" jack for convenience. With the exception of the new output tube cathode capacitors, the majority of the wiring on the two ECL86/6GW8 tubes is left alone.
The audio input is fed to V3b, 1/2 of a 12AX7 triode wired as a standard class-A amplifier. The other half of V3 is not used. The cathod circuit of V3b uses two capacitors (100uF and 100nF) in parallel, the small capacitor improves the high frequency response of the circuit. It is possible to substitute a 12AT7 (lower gain) or a 12AU7 (even lower gain) for V3 if one wishes to adjust the total gain of the amp. The output of V3b is fed into a Baxandall tone stack which gives boost and cut for bass and treble. The output from the tone stack is fed through the volume control into V1a, another class-A amplifier.
Almost everything to the right of the volume control is the stock Hammond AO-44 circuit. Negative feedback is routed from the speaker output to the cathode of V1a to lower the overall gain of the power amp. V2a is wired as a phase inverter, the two phases of the audio signal drive V1b and V2b. V1b and V2b are wired as a cathode biased a push-pull pentode amplifier. The 10K/1W resistor and 1nF/2KV capacitor form a high-cut circuit that cancels out RF oscillations in the output stage.
The power supply high voltage circuit is a standard center-tapped full wave rectifier fed into an R-C totem-pole network. The 150/2W resistor is used to bring the B+1 down to 303V, which is similar to the voltage provided by the original 6X4 rectifier tube. All filaments are powered by the 6.3V transformer winding. Since only half of V3b is used, it is possible to only provide filament power to one half of the 12AX7 tube to save a bit of power, use pins 5 and 9 instead of 4/5 and 9 on the 12AX7.
The LED pilot light circuitry consists of four 1N4002 diodes wired as a bridge rectifier across the 6.3VAC filament circuit. The 470uF capacitor produces filtered DC, the 560 ohm resistor is wired as a current limitor in series with a high brightness amber LED.
Parts of the original amp wiring should be removed to make room for the new components. Before adding the new circuitry to the amp, it is a good idea to drill all of the holes. Holes should be added for the output jack, the power switch and control pots, the optional LED and the power cord. The power cord is installed in a 1/4" hole that was (carefully) drilled in one of the positions of the six pin wiring connector. It is important to use coaxial wire for all of the audio signal paths in order to prevent oscillations. The coaxial wire in the photo is white. All of the tone control components are wired directly to the two tone pots.
It is a good idea to verify the value of all of the power resistors and replace any bad parts, these may have drifted way out of tolerance during the decades of use. It may be necessary to fabricate a bottom for the amplifier, a piece of sheet metal can be cut to the desired dimensions and drilled so that the holes line up with the holes on the amp chassis. Rubber feet should be installed on the four mounting screws.
The original electrolytic filter capacitors are nearly 50 years old and have probably dried out. It is a good idea to solder some new electrolytics with the same values across the pins at the bottom of the original capacitor. This step should eliminate any hum that the amp produces, the hum will be audible with the volume control turned down.
The power supply dropping resistors on the bottom of the electrolytic capacitor should be checked for proper resistance and replaced if their values have changed, old carbon resistors tend to increase in value if they are run hot for many years. Also check the 10K 1W resistor that is across the output transformer.
Connect an 8 ohm speaker to the output of the amp and connect the input to a line level source such as a CD player or computer audio card's auxilliary output. Plug the amp into the power line, turn it on and adjust the controls for the desired sound. Enjoy the warmth and clarity of a tube amp.