(C) 2024 G. Forrest Cook
Spartacus Max AO-35 amp front, click for a larger image.
Nearly original Hammond AO-35 amp underside, click for a larger image.
Stripped and Drilled Hammond AO-35 amp underside, click for a larger image.
Spartacus Max AO-35 amp underside, click for a larger image.
Schematic of the Spartacus Max Hammond AO-35 amp conversion
The Spartacus Max amp (A.K.A. Spartacus Maximus) project involves the conversion of a Hammond AO-35 organ reverb amplifier into a 5 tube, 15 Watt guitar amplifier. The Spartacus Max can also be used as a monoblock Hi-Fi amp or a keyboard amp by using its line-level input. The amp produces a very clean output signal and the active tone controls provide a wide range of adjustability. The amp also includes an adjustable gain control which allows the response to be varied between tight and linear to loud and punchy.
The Spartacus Max conversion is the ninth Hammond amp conversion project that your author has undertaken. It borrows much of its preamp, tone control and phase inverter circuitry from the Basserator Bass/Guitar amp, while having a feature set that is similar to the original Spartacus amp. It produces more output power than the original Spartacus amp.
This is a fairly high-level project. It takes advanced technician skills to deconstruct and reconstruct the circuitry in guitar amplifiers. The Spartacus Max is a fairly dense 3 dimensional build with a lot of components packed into a small space.
There are numerous potentially lethal high voltages inside of this amp including 120 VAC, 390VDC and 540VAC. The project should only be taken on by someone who has experience working with high voltage circuitry. The power cord should always be unplugged when working on the amp. The circuitry will eventually discharge the capacitors when power is turned off, but it is always a good idea to short out the electrolytic capacitors before working on the amp.
Before building the Spartacus Max circuit, it will be necessary to remove most of the original parts from the tube sockets, terminal strips and plug-board. This process takes a lot of time and patience. See the photos above to see the before and after images. Be careful not to damage the tube socket pins when removing the original parts. All of the wiring to the internal plug board was clipped off and the board was removed from the chassis to unsolder the parts.
The plug board flux residue was scraped off with a dental tool and the board was cleaned with alcohol and a toothbrush. The liquified flux residue was then soaked up with a tissue. The electrolytic can capacitor was removed from the chassis and its hole was enlarged with a chassis punch to make room for a second octal tube socket. The new socket should be centered between the two transformers. All of the wiring was removed from the original octal rectifier tube socket.
The external bakelite lug strip and the RCA jack were removed by drilling out the rivets. An aluminum plate was fashioned and installed in place of the lug strip. Holes were drilled in the chassis for the various jacks, controls, switches and power cord strain relief. All of the sharp edges and corners on the chassis were filed smooth to prevent cuts and scratches while working on the amp.
An 8 pin terminal strip was added to the inside of the chassis by the power transformer, a 2 pin terminal strip was added between the new octal socket and the power transformer and a 3 pin terminal strip was added to the back panel. The filament wiring was mostly left in place, one of the 9 pin sockets was re-wired to support a 12AU7 and wires were added to the octal sockets and pilot light. The 0.1 ohm filament voltage dropping resistors were wired between two of the terminal strips.
The AO-35 chassis includes many small ground wires that connect between the terminal strip ground tags and the tube sockets. These improve the overall ground system and insure that the grounds do not rely on just the terminal strip rivets, which can become oxidized over time. A few extra ground wires were added between some of the previously unused terminal strip ground tags. The power cord ground lead should have a direct connection to the grounding network.
The power transformer's unused 5V rectifier winding was added in series with the primary winding, it is phased to provide a voltage boost on the output windings. In several of my other Hammond amp conversion projects, the 5V winding was wired to buck the output voltage to correct for high filament voltages when running on modern 120+ VAC line voltages. In this amp, the boost configuration increases the B+ supply voltge in order to squeeze a bit more output power out of the oversized power transformer. Two dropping resistors were added to the 6.3V winding to correct the filament voltages.
The high voltage supply uses two 1N5408 1KV/3A rectifier diodes to feed a string of capacitors and resistors in a typical R-C series arrangement. Each R-C stage reduces the voltage and lowers the AC ripple, ending with the preamp stage. Resistors with oversized wattages were used to insure long life. A 2.2M resistor across the first filter capacitor discharges the capacitor bank when the power is shut off.
The power amp section consists of a pair of beam-power pentode tubes which feed the output transformer. A variety of beam-power tubes can be used in this amp including the metal 6L6 and the glass 6L6G, 6L6GB and 5881 types. The 6L6 screen grids are supplied by a pair of 470 ohm grid-stopper resistors and the control grids have 1.5K grid-stopper resistors. The grid-stoppers help to tame the tubes by lowering the response to radio frequencies on the control grids and eliminating negative resistance effects on the screen grids. A pair of 1.0 ohm resistors were installed between the 6L6 cathodes and the bias current regulator, they are used for monitoring the individual cathode currents.
Each of the 6L6 cathodes is bypassed to ground with a 100uF capacitor. A regulated cathode bias current is provided by an LM317T regulator IC, this allows the bias for both output tubes to be set to a range of levels. The 50 ohm bias balance pot allows the bias to be matched between the two output pentodes. The current regulator circuit is a variation on the typical R/C cathode bias circuit, it keeps the cathode bias steady across changing line voltages and allows the bias to be adjusted. Note that the LM317T regulator should be mounted on a smal aluminum heat sink that is insulated from ground.
The PA Gain control acts as a variable negative feedback circuit by feeding the speaker output back to the phase inverter. The green output transformer secondary wire is connected to the negative feedback circuit and the black wire is connected to ground.
The phase inverter uses a 12AU7 dual-triode in a current-mirror long tailed pair circuit. An LM317T adjustable voltage regulator is wired as a constant-current source for the two 12AU7 cathodes. The constant-current circuit improves on similar phase inverter designs by insuring that triodes are run at the same idle current and gain. The master Volume control feeds one side of the phase inverter and the Gain control feeds the adjustable negative feedback signal to the other side.
The tone control circuit was found on Max Robinson's site (see below), the treble circuit was modified to affect lower frequencies which makes it more useful for adjusting the tone of musical instruments. The tone control uses an active negative feedback circuit to produce a wider range of buck and boost control compared to passive tone control circuits. The active tone control is followed by a 12AU7 gain-recovery stage. The 12nF cathode bypass capacitor on VT4a boosts the upper audio frequencies to provide a flatter tonal response. The 4.7K/330pF low-pass network on VT4a rolls off the frequency response above the audio range.
The guitar input preamp is a standard design, it uses a 33K grid-stopper resistor to lower the response to high frequency (RF) signals. The 25uF cathode capacitors in the first two stages increase the AC gain of the preamp and tone stages.
The optional line-level input preamp uses the spare VT4b 12AU7 triode as a low-gain buffer amplifier. The input select switch applies the B+Pre voltage to one or the other of the preamp stage plates and connects either output to the tone control circuit.
There are a few optional components in this build. The 12 ohm series resistor in series with the brown wire of the output transformer primary is used to compensate for differences in the primary winding resistance. This gives a minor improvent to the balance of the push-pull output stage. The 10nF capacitor on the B+Pre line improves the return path for the low-pass network on VT4a for better attenuation of frequencies above the audio range. This capacitor is in parallel with the 22uF power supply filter capacitor.
The VT4b line-level amp and selector switch are optional if you don't need a line-level input. The unused 12AU7 triode section is essentially a freebee, so a few more components allow the amp to be used for more than just guitar amplification.
Alignment of the amp should be performed whenever a different set of output tubes are installed. Measure the idle current of the two 6L6 tubes by inserting DMM probes across the two 1 ohm 6L6 cathode resistors. Set the overall cathode bias by adjusting the 10 ohm trimmer resistor in the LM317T current regultor circuit. The DMM should be set to measure volts and the current will be the same as the displayed voltage.
The idle current should be somewhere between 35mA (0.035V) and 55mA (0.055V). Higher values will produce more output power and less cross-over distortion while lower values will allow the tubes to run cooler and last longer. A 45mA idle current is a good medium value to start with. Set the idle current of both tubes to the same value by adjusting the bias balance control. The balance control can be fine-tuned for minimum output hum. That completes the alignment of the amp.
As with all new amp builds it is a good idea to get the amp up and running with used power tubes. Once you are satisfied that the circuitry is working properly, a new set of tubes can be installed. The Spartacus Max amp was originally set up with a pair of new old stock (NOS) metal 6L6 tubes, these worked fine but produced a lot of foul-smelling paint outgassing when they warmed up. A pair of used and matched Tung-Sol 5881 tubes were substituted for the 6L6 tubes, they provided a slightly higher output power level and don't smell bad.
It should be possible to use 6V6 output tubes in this amp, although this has not been tried by your author. The 6V6 tubes would probably produce less output power and more distortion, which may be desirable for guitar amplification. The 6V6 cathode bias should be set to a lower level, 35mA is a good starting place. It may be necessary to increase the 6.8 ohm current regulator resistor to 10 ohms to lower the bias current adjustment range.
All of the preamp tubes were used, they were verified to be in good condition using a Gm-reading tube tester. It is a good idea to pick a 12AU7 with equal gain figures on the two triodes for the phase inverter. It is also important to select a 12AX7 with low microphonics for the preamp stage. Just tap on the 12AX7 while the amp is running to determine if it picks up mechanical noise. Low-noise 7025 tubes are another good choice for the preamp tube. Europen ECC83 tubes can also be used.
The Spartacus Max is a loud and clean amp with good low and high frequency response. The amp works quite well with an electric guitar and can get very loud. A 12 inch speaker is recommended for guitar, although smaller speakers will also work as long as they are rated above 15W.
Start with the standby and power switches turned off. Turn on the power switch. Wait 30 seconds or more for the filaments to warm up, then turn on the standby switch. Set the gain control to its mid range, lower levels are good for a clean and tight sound while higher levels are good for a punchy loud sound. Plug in your guitar, turn the volume up to a comfortable level and adjust the tone controls for the desired tone. Play your instrument and enjoy the warm tube sound.
The Spartacus Max can also be used as a monoblock Hi-Fi amp, just send a line-level signal to the line input and adjust the controls for the best sound. A pair of these amps would work nicely for a Hi-Fi stereo setup.
Going full-circle back to its roots, the Spartacus Max has also been used to amplify a Hammond B3 organ and power a 15 inch Leslie speaker. It handles the wide range of sounds produced by the organ with ease. The B3 organ's output signal should be fed into the amp's line-level input.
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