Lil' Tiger Hammond AO-43 Organ to Guitar Amp Conversion

(C) 2009 G. Forrest Cook

Introduction

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 prime 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.

This amp has undergone a number of design changes since the original version was published. A second copy of the amp was constructed and the reverb driver tube was changed from a 6SN7 to a more period-correct 6FQ7/6CG7. The reverb driver circuitry was changed to include a Reverb Send control, this gives the amp a much wider variety of reverb sounds. The volume and reverb return controls have been wired in a mixer configuration which is somewhat unusual for amp designs. This makes it possible to listen to the reverb channel by itself or to have a mix with very heavy reverb. The vibrato circuit was also rebiased to produce a more intense effect. I liked the rev 2 modifications enough that I applied them to the original amp.

The output stage of this amplifier uses 6BQ5/EL34 pentodes wired in a cathode-biased push-pull configuration, audio power output is around 15W. In the Hammonator project, almost all of the original circuitry was replaced. In this amp, most of the original power amp circuitry was reused. All of the resistors that will be re-used should be checked to verify that they still have the correct value. Old carbon resistors that dissipate a lot of power tend to go up in value, they can also become noisy. Resistors that are out of tolerance should be replaced.

The vibrato is somewhat subtle when the amp is played by itself. To get the best vibrato effect, a second amplifier should be plugged into Guitar Input 2 to tap into the input signal. The resulting stereo signal moves around the room in a manner that sounds a lot like a rotating Leslie speaker. Beware, once you get used to playing through a stereo setup all other amps will sound flat.

One could consider this to be a left-handed amp, the signal path and knobs are mostly right to left. The chassis tube layout made this necessary.

Warning

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 120VAC and 350VDC. 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.

Connections

Power Input - grounded 120VAC
Guitar Input 1 - High Impedance
Guitar Input 2 - High/Low Impedance
Reverb Send
Reverb Return
Speaker Output - 8 ohms

Controls

On/Off (on the back)
Clean/Vibrato Volume
Bass
Treble
Reverb Send
Reverb Return Volume
Gain (P.A. Negative Feedback Adjust)
Vibrato/Tremolo Speed
Vibrato/Off/Tremolo Select
Vibrato/Tremolo Intensity

Theory

The guitar input jacks are configured in the typical Fender-style High/Low inpedance configuration. The input options are: high(1)-or-low(2) impedance, or high(1)-and-high(2) when both jacks are used. The combined inputs are fed to a standard 12AX7 triode preamplifier (VT2a). The preamp output drives a Baxandall tone stack.

The output of the tone stack is fed to a 12AU7 triode buffer amplfier (VT1b). The output of the tone stack buffer amp feeds the 12AU7 vibrato stage (VT1a) and the 6SN7 (or 6FQ7/6CG7) reverb driver (VT6).

The VT1a vibrato stage uses 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 reberb driver tube (VT6) can be either a 6SN7 octal tube or a 6FQ7/6CG7 9 pin miniature tube. The two tube types are very similar medium power dual triodes. The original AO-43 amp used a 12BH7 tube which would probably work here with appropriate pin changes, although it has not been tried. The two triodes of the reverb driver are wired in parallel and used in a single-ended triode (SET) power amp configuration.

The 2nF capacitor on the input of the reverb driver acts as a high pass filter for reducing boomy bass sounds. It can be switch-adjusted for a different reverb tone, values in the 1nF to 2nF range work well. The reverb send pot allows the spring drive level to be adjusted. The reverb driver tube drives the reverb transformer which feeds the input side of the reverb tank. The 30nF/4.7K rc across the reverb transformer's primary acts as a high cut filter and improves the reverb driver's behavior with large signals.

The six diodes and resistor across the reverb transformer's secondary act as a soft +/-2.1V limiter, this prevents large signals from slamming the reverb spring too hard and making unpleasant sounds. The limiter also adds another effect, if the reverb drive level is turned up, pleasant sounding distortion is sent into the reverb spring, while the main signal path remains clean.

The reverb spring output feeds the reverb return amp (VT2b), a 12AX7 triode preamplifier. The 2nF capacitor in the reverb return amp's output acts as a high pass filter to reduce undesirable low frequency reverb sounds.

The output of the vibrato/tremolo circuit and the reverb return amp are combined in a two channel volume/reverb mixer circuit. The vibrato/tremelo signal is reduced by the 1M resistor above the volume control pot, this balances the clean/vibrato level relative to the reverb signal. The mixer output signal drives the original Hammond power amp circuitry.

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 (negative feedback adjust) to the front panel. The gain control contours the amp's behavior, it adjusts the amp's gain and signal compression characteristics. Triode VT3b boosts the mixed signal and includes an RF supression network on the plate and a negative feedback input from the speaker output to the cathode.

The VT3b 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. This resistor also "softens" the sound of the amp a bit, just like a tube rectifier would. By eliminating the huge tube rectifier filament, the amp becomes more power efficient and the power transformer runs much cooler. The reverb driver B+5 line is tapped off with a separate RC filter near the high voltage end of the power supply totem pole.

The Arduino LFO Waveform Generator V2 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. 1N5818 Schottky diodes are used in the low voltage power supply instead of normal silicon diodes in order to produce a slightly higher DC voltage and give the 78L05 regulator IC more headroom. The above photos show the original prototype of the LFO. In the second version of the amp, the LFO circuit was reduced to a single circuit board and mounted inside of the amplifier chassis.

Construction

Note that the photos of the top of the amp do not show the Reverb Send control, it was added later. The first job is to install new 120VAC input wiring. 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 modified 120VAC wiring, this includes the 5V transformer winding and the varistor. An aluminum plate was cut and drilled to fit into the hole that used to house the square multi-pin connector. The power switch was installed in this plate.

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 for the transformer's primary. This makes the power transformer compatible with 120VAC and prevents the tube filaments from running at 7V, which would cause shortened tube life.

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 now be around 6.5V.

The Metal Oxide Varistor (MOV) on the primary of the power transformer absorbs power line voltage spikes that would otherwise be stepped up in the power transformer, potentially damaging the 1N4007 rectifier diodes.

If you decide to use a 6SN7 for the reverb output tube, all of the wiring on the octal 5U4 rectifier tube socket should be removed. The 5U4 socket can now be used for the 6SN7 reverb driver tube. If a 6FQ7/6CG7 tube is used instead of the 6SN7, the tube's socket should be located in an aluminum plate that sits in the empty capacitor hole next to the 12AU7 tube.

A three pin terminal strip was mounted under one of the power transformer's chassis screws, it holds the solid state rectifiers. The original electrolytic capacitors in an amp this old are very likely shorted, or dried out and lower in value. New capacitors can be soldered directly across the old capacitor terminals. It is better practice to clip off the old capacitor terminals and install a few terminal lugs to support the new capacitors. Old shorted capacitors can overload and destroy the power transformer.

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 should 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 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 occurring.

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 into 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 one or both of the holes where the old choke coil used to reside.

Use

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. Fifteen 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 the Spartacus slave amp can be plugged into the Lil Tiger's second input jack for an incredible moving stereo sound.


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