This amplifier is a variation on the Hammond AO-44 Reverb Amp to Hi-Fi Amp Conversion project. The goal is slightly different: the Spartacus amp has more gain and higher output power, making it better for use as a guitar amplifier. Unlike the previous project, the Spartacus amp involves a complete rewire of the Hammond AO-44 chassis. The hard to find 6GW8/ECL86 tubes are replaced with more common and higher power 6BQ5/EL84 tubes. Output power is around 12 Watts, that may not seem like much, but the amp can get quite loud. A cool looking 6U10 compactron triple triode is added to the amp's chassis, it provides the first two gain stages and the phase inverter stage.
The name "Spartacus" was inspired by the ancient slave who broke free from the Romans and wreaked havoc for many years. Like its namesake, the amp makes an excellent stereo guitar slave amp when used in conjuction with projects such as the Hammonator 2RVT and Lil' Tiger amps. The Spartacus amp can also break free and function as an independent amp, it works nicely with a 12" guitar speaker.
The Hammond AO-44 amplifier chassis can be found on eBay for reasonable prices. The Spartacus design is simple and clean. The wiring is dense, but not overly difficult to assemble. The controls include the basic volume, bass and treble adjustments as well as a gain control. The gain (negative feedback) control is more than just a second volume control, it adjusts the the character of amplifier's sound from compressed and tight to open and loud like the popular Fender 5E3 amp.
This is a fairly high-level project. It takes advanced 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 315 VDC. The project should only be taken on by someone who has experience working with high voltage circuitry. The power cord 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.
Power Input - grounded 120VAC Guitar Input - High Impedance Speaker Output - 8 ohms
On/Off Volume Bass Treble Gain (Negative Feedback)
The guitar input jack feeds directly into preamp VT1b, the single high gain triode section in the 6U10. The preamp output drives a Baxandall tone stack, which feeds the volume control pot.
The output of the volume control drives VT1a, a low gain triode section in the 6U10. VT1a acts as a gain stage and summing point for the Gain control's negative feedback signal. The 100pF capacitor on the grid of VT1a and the 47K/220pF network on the plate of VT1a reduce the gain of the amp above the hearing range to eliminate unwanted radio frequency sensitivity.
The output of VT1b feeds VT1c, a low gain triode section in the 6U10. VT1c is wired as a floating cathode phase inverter. The two out of phase signals from the phase inverter drive VT2 and VT3, the 6BQ5 pentodes, which are wired in a push-pull configuration and drive the output transformer. The 6BQ5 pentodes are configured with a fixed-bias cathode arrangement.
The configuration of the power amplifier (VT1a, VT1c, VT2 and VT3) is based on the Hammond AO-43 amp design. Some of the cathode and plate bias resistors were changed to accomodate the different gain characteristics of the 6U10 triodes compared to the AO-43's 12AX7 triodes.
In the power supply section, the transformer high voltage winding is sent to a center tapped full wave rectifier consisting of two 1N4007 diodes. The high voltage DC is dropped through a totem-pole array of resistors and capacitors to produce the voltages used in the amp. The 6.3VAC filament supply is rectified by four 1N4001 diodes and filtered by a 470uF capacitor to produce around 7.5VDC. The DC voltage drives an emerald green (505nm) LED via a 220 ohm current limiting resistor. A standard incandescent pilot light could also be used here if you can find enough space to mount it on the front panel.
The original 6X4 rectifier tube can be used in this amp, eliminate the 82 ohm 3W resistor and 100nF 400V capacitor then connect the 6X4's cathode + line to B1+. The solid state diodes are more efficient since they don't need power for a large filament, the power transformer will run cooler and last longer. Also, unlike the hot-running 6X4 tube, the 1N4007 diodes should never need replacing. The 82 ohm resistor simulates the rectifier tube's internal resistance and removes a bit of sharpeness from the amp's sound.
With the exception of the filament wiring and the ground wire, almost all of the original AO-44 wiring should be removed. The bakelite terminal plate on the back of the amp should be drilled out and replaced with an aluminum plate. Drill holes in the plate for the speaker output jack and AC power wire. The AC power wire goes through a protective rubber grommet and is secured to the chassis with a panduit tie. Holes were drilled in the chassis for the 6U10 compactron socket (12 pin), the four potentiometers, the input jack the power switch and the LED.
The 120VAC wiring requires the addition of a chassis-mounted fuse holder, a terminal strip and a power switch. The 1N4007 diodes can be wired to the back of the 6X4 rectifier tube socket or placed on their own terminal strip.
Two conductor coaxial wire was used for the gain control pot's wiring. Only one end of the coax shield should be grounded to eliminate ground loop hum. The AO-44 amp has a bare ground wire that links the ground pins on the tube sockets and terminal strips together with the capacitor and transformer center tap from the power supply. This ground should be extended to any additional terminal strips and the guitar/speaker jacks.
The majority of the amplifier wiring was done in a point-to-point fashion between the tube sockets and the nearby terminal strips. One of the original terminal strips was left in place and the original parts were unsoldered. A second original terminal strip was removed to make room for the 6U10 socket, a smaller terminal strip was installed in one of the vacated screw holes. The tone control wiring was done on the back of the tone control potentiometers.
A wood case was constructed by sawing three pieces of recycled oak flooring to size using a table saw. A piece of galvanized sheet metal was cut to size for the bottom plate. Stick-on rubber feet were applied to the corners of the bottom sheet to prevent scratching.
Plug the amp into a guitar speaker. Plug an electric guitar into the input. Tweak the knobs for a good sound. Play the guitar. Enjoy the "spartacular" tube sound! The Fuzznikator push-pull tube distortion circuit makes a nice companion for this amp.