This project follows in the footsteps of the Hammonator and Hammonator 2RVT organ amplifier to guitar amplifier conversion projects. A variety of Hammond tube amps are available on eBay for reasonable prices and they make a good platform for a guitar amplifier project. The Hammond AO-43 chassis is a good candidate for conversion, it is large enough to build a full-featured amp with reverb and other effects and has a bare side that can hold many knobs and jacks.
The output stage of this amplifier uses 6BQ5/EL34 pentodes wired in a cathode-biased push-pull configuration, audio power output is around 12W. In the Hammonator project, almost all of the original circuitry was replaced. In this amp, most of the original power amp circuitry was reused. It is important to replace all of the electrolytic capacitors with new parts or at least wire new capacitors across the old ones. All of the resistors should be checked to verify the correct value, old resistors that are run hot tend to go up in value and should be replaced.
This is a fairly advanced-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 400 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 1 - High Impedance Guitar Input 2 - High/Low Impedance Reverb Send Reverb Return Speaker Output - 8 ohms
On/Off (on the back) Input Volume Bass Treble Reverb Gain (Negative Feedback) Vibrato/Tremolo Speed Vibrato/Off/Tremolo Select Vibrato/Tremolo Intensity
The guitar input jacks are configured in the typical Fender-style High/Low inpedance configuration. The input options are: high(1)/low(2) impedance, with high(1)/high(2) when (1) is plugged in. The combined inputs are fed to a standard 12AX7 triode preamplifier (VT2a). The preamp output drives a Baxandall tone stack and branches to the reverb driver amp (VT6) via a high pass RC filter.
The output of the tone stack drives an intentionally low-gain 12AU7 triode buffer amplfier (VT1b). The output of the buffer amp feeds VT1a, a 12AU7 triode wired as a floating cathode switchable phase shift (vibrato)/volume shift (tremolo) circuit. The effect is switched between tremolo, off and vibrato with a selector switch. The other part of the switch enables/disables the LFO. The programmed wafeform from the LFO drives the LED and modulates the light variable resistor. The LVR modulates the effect and the intensity control adjusts the amount of effect.
The output of the vibrato/tremolo circuit is fed to the volume control. The reverb spring output feeds VT2b, a 12AX7 triode preamplifier. The reverb preamp output is fed through a high pass filter and mixed with the vibrato/tremolo output. This signal drives VT3b, the input to the mostly unmodified Hammond power amp circuit. The reverb circuitry connections to the rest of the amp are somewhat unconventional, but the configuration sounds really good.
The rest of the amplifier is the standard circuit from the Hammond AO-43 amp. Differences include a few additional cathode capacitors and movement of the Gain control to the front panel. The gain control contours the amp's behavior, it adjusts the amp's gain and signal compression characteristics. Triode VT3b is the front-end of the Hammond power amp. It boosts the signal and includes an RF supression network on the plate and a negative feedback input from the speaker output to the cathode. The VT2b output goes to VT3a, the floating cathod phase inverter. VT3a provides the two out-of-phase signals which drive the two 6BQ5/EL84 pentodes VT4 and VT5.The 6BQ5/EL84 tubes are wired in a standard push-pull configuration. The 4.7K/1nF low-pass RC network across the output transformer's primary is common in this vintage of Hammond power amps, it supresses high frequency parasitic oscillations (snivets) from the signal.
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 220 ohm/10W resistor drops B1+ to around 338V, which is about what the tube rectifier produced. The reverb driver B+5 line is tapped off with a separate RC filter near the high end of the power supply totem pole.
The Arduino LFO Waveform Generator is used to generate the Tremolo/Vibrato modulation waveform. The 7.5V DC Power for the LFO comes from a bridge rectifier and 2200uF/16V capacitor that is fed by the 6.3VAC filament line.
All new 120VAC input wiring should be installed. A grounded power cord should be installed and a fuse and switch should be added. The round black four pin connector on the back of the amp can be used as a terminal strip for the new 120VAC wiring.
When this amp was made, power line voltages were around 115VAC, they are now 120VAC. By switching to solid state rectifier diodes, the power transformer's 5V winding can be freed up and used as a buck winding. This makes the power transformer compatible with 120VAC and prevents the tubes from running too hot. The 5V winding needs to be phased correctly when connected in series with the power transformer's primary winding. Measure the 6V filament voltage, then try both polarities with the 5V wires, use the orientation that produces the lower 6V winding voltage. With all of the tubes installed, the final filament voltage should be around 6.5V.
All of the wiring should be removed from the octal 5U4 rectifier tube socket. This socket will now be used for the 6SN7 reverb driver tube. A three pin terminal strip was bolted to one of the power transformer's mounting screws to hold the solid state rectifiers.
The plug board that contains most of the parts should be relocated toward the center of the chassis to provide room for the front panel controls. The plug board's mounting screws should be replaced with longer screws and 1" spacers should be installed between the chassis and the plug board's mounting flanges.
With the exception of the filament wiring, the rest of the circuitry around VT1 and VT2 should be removed. The original VT1 and VT2 circuitry should be removed from the plugboard, the plugboard can be refilled with parts for the new circuitry. The tone stack parts can be soldered directly to the back of the tone pots. One of the leads of the original gain trimmer pot can be left in place and used as a connection point, this connects to the front panel's Gain control (negative feedback) pot.
Coaxial wire should be used for guitar input and the audio lines that go to and from the tremolo/vibrato and tone stack circuits. Only ground one end of the coax shield to prevent ground loop hum from occuring.
Holes were drilled in the chassis for mounting the various input/output jacks and control potentiometers. Use a drill press and start with a small pilot drill before making the larger holes. Sharp drills and cutting oil are essential for making good holes. Be very careful not to drill through the metal and rip through the internal parts. Remove the choke coil and the electrolytic capacitor that is covered with black cardboard. Fashion an aluminum plate to fill the hole where the electrolytic capacitor was located. The reverb transformer can be placed in the holes where the old choke coil used to reside.
Plug the amp into a guitar speaker. Plug an electric guitar into input 1. Tweak the knobs for a good sound, play the guitar, enjoy the great tube sound. Twelve watts may not seem like a huge amount of power, but this amp can get very LOUD. The Fuzznikator push-pull tube distortion circuit makes a nice companion for this amp and a Spartacus slave amp can be plugged into the Lil Tiger's second input jack for an incredible moving stereo sound.