This project is the second revision of the Hammonator 1 Hammond organ to guitar amplifier conversion project. Two versions of this amp have been built, the schematic has been verified. The Hammonator 1 circuit worked as a basic guitar amplifier, this version adds some very nice sounding vacuum tube audio effects. The 2RVT model number means: 2nd Generation, Reverb, Vibrato and Tremelo. The reverb circuit is a classic sounding tube circuit with a class A triode driving an 18" reverb pan. The reverb has both dwell (input) and output level controls. A pitch-shifting vibrato / volume shifting tremelo circuit is part of the signal path. The vibrato produces a variety of interesting phase shift sounds, from slow movement to warbling. Stereo vibrato can be easily achieved by chaining a second "clean cannel" amp to the second input jack, a practice amp makes an ok secondary amp. Stereo greatly enhances the vibrato effect. The Tremelo/vibrato switch changes the pitch shifting vibrato effect into a volume shifting tremelo effect. The wide range of LFO speeds produce some interesting tremelo effects.
One extra tube (A 6AV6 single triode) is required, as well as a stand-alone sine wave LFO oscillator board to drive the vibrato/tremelo. The 6AV6 also acts as a buffer to separate the reverb input and output, this allows the reverb signal to be higher without producing any feedback. The audio signal path is still 100% vacuum tubes, relatively modern digital circuitry was used to produce a high quality LFO signal. The LFO circuit is powered by the filament supply, rectified and filtered.
The 6V6 high voltage supply has been increased to 420V, giving the amp a higher power (18 Watts) and more headroom over the Hammonator 1 design. Most of the tube stages have been rebiased with new cathode resistor values to accomodate the circuit changes. The reverb return pot was rewired to allow the effect to be turned down completely. A side channel output was added for plugging in a secondary amplifier, this gives a nice stereo vibrato effect.
The amp's feedback circuit was changed to a three layer design. The 3.3K/5nF section cuts high frequencies above the audible range, the 1K/modem transformer section cuts sub sonic frequencies and the 750 ohm/5K pot section provides adjustable feedback in the audible range.
Another subtle change from the Hammonator 1 is the addition of 100nF capacitors to most of the 6SN7 cathode bias circuits. The 100uF caps work well at low frequencies, but have a lot of inductance at higher frequencies. The 100nF caps work well at higher frequencies. Paralleling the two caps produces a "super cap" that gives the amp a smoother frequency response.
The vibrato/tremelo sine wave generator is a fairly involved sub-circuit to build. This section can be left out of the amp to simplify construction, or built later once the amp is working. For a reverb-only amp, build the amp without the LVR, tremelo/vibrato switch and associated resistors. Be advised, the vibrato effect is worth the extra effort.
The 6HU6/EM87 ray tube and associated components can also be left out of the circuit, or added later. The eye-candy effect is certainly worth a little extra wiring. Other ray tubes may be used with the appropriate wiring changes. Russian made 6E1P/EM80/6BR5 magic eye tubes have been seen on eBay for good prices, they should work in this circuit by rewiring the tube pins.
This is a fairly advanced-level 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 420 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 120 VAC, 62 Watts Guitar Input 1 - High Impedance Guitar Input 2 - High/Low Impedance Reverb Send Reverb Return Side Channel Output - High Impedance Speaker Output - 8 ohms, 18 Watts
On/Off (on the back) Input Volume Bass Treble Reverb Send (Dwell) Reverb Return Feedback Vibrato/Tremelo Depth Vibrato/Tremelo Speed Vibrato/Tremelo Selector
A friend of mine loaned me his vintage Magnatone Lyric amplifier, the amp has an incredible sounding pitch-shifting vibrato circuit. The pitch-shifting vibrato effect could be described as liquidy or warbly. The type of varistors used in the Magnatone are difficult, if not impossible to find. The differential high voltage varistor drive circuitry also takes numerous tube stages to implement. This vibrato circuit uses a much simpler light dependent resistor (LDR) to produce the required phase shift. There is an excellent Geofex article on The technology of phase shifters and flangers that covers the Magnatone circuit and more. The phase shifting vibrato circuit used here was inspired by the transistorized Univibe circuit from that article.
The Vibrato circuit is quite simple. The 6AV6 tube is running as a simple phase inverter, similar to circuits that drive many push-pull amplifier output stages. The signals on the plate and cathode of the tube are 180 degrees out of phase. When the Tremelo/Vibrato switch is set to Vibrato, the Light Variable Resistor allows the output signal to shift from one signal toward the other, depending on the amount of light it sees.
When the Tremelo/Vibrato switch is set to Tremelo, one end of the LVR is connected to ground through a limiting resistor. As the light level on the LVR changes, the output signal is raised or lowered.
Power for the LFO board is pulled from the 6.3V filament supply and full-wave rectified and filtered to produce 7.5VDC. The Schottky diode bridge rectifier produces a higher output voltage compared to silicon diodes, this is important for making enough voltage regulator input voltage. The 7.5VDC is regulated by the 78L05 regulator to produce 5VDC. The 5VDC power is inverted with the ICL7660 chip to produce -5VDC. These three DC voltages power the rest of the LFO circuitry.
The 555 timer IC is wired for astable operation to provide a digital clock signal. The speed of the oscillator is controlled by the 5M potentiometer with switch. Ideally, the 5M speed control pot should be logarithmic taper. Unfortunately this part is very hard to find, it is even rarer with a built-in switch. If a more common audio-taper (reverse log) potentiometer is used for the speed control, it should be wired so that counter-clockwise is fast and clockwise is slow. This allows the LFO speed adjustment to have a normal feel.
The LFO circuit is basically a stepped digital sine wave generator. An adjustable square wave clock signal drives a CMOS 4040 binary counter IC. The lowest 8 bits of the 4040 counter are used to generate addresses in the range of 0 to 255. The addresses are fed into the 27C64 EPROM. The EPROM is programmed with 256 steps of a digitized sine wave. Here's the Intel Hex Format Data for programming the EPROM. The Digitized sine wave values are fed to the DAC0808 DAC, which converts them into analog currents. The op-amp on the output of the DAC converts the DAC output current into voltages in the range of 0-10V. The 100nF capacitor in the op-amp's feedback loop smooths out the steps in the sine wave. The LED/LVR optocoupler is driven by this sine wave voltage. The LED current is limited and adjusted by a resistor and the 25K depth potentiometer. A clever person could replace most of the LFO circuitry with a single microcontroller IC.
The negative side of the LED/LVR and the front panel LED are connected to an adjustable DC bias circuit which is set to around -1.1VDC. The bias is set so that at the zero point on the sine wave, the LED is just beginning to emit light. At the 10V point on the sine wave, the LED reaches the full brightness as determined by the intensity control. The LED bias insures that the response of the LVR is fairly sinusoidal, if the LED were not biased, the bottom of the sine wave would be effectively cut off.
The Tremelo/Vibrato switch is a DPDT 3 position center off type. When the switch is in the middle (off) position, the LFO enable line is pulled high by the 10K resistor, causing the 4040 counter to reset, this causes the DAC to stop generating waves and turn off the LEDs.
In addition to the new vibrato circuitry, a number of changes were performed on the Hammonator 1 to create the Hammonator 2. The black cardboard-coated electrolytic capacitor was removed because it vibrated and buzzed when exposed to loud sound from the speaker. Old electrolytics tend to emit funky chemical smells when they get hot, replace them with new capacitors. Additional front panel holes were drilled for the extra input (not shown in the photo) and the vibrato knobs and switch. The vibrato front-panel LED was mounted in an existing screw hole. The tubes were renumbered, the 6HU6/EM87 was changed from VT7 to VT8 and the new 6AV6 was numbered VT7.
The signal lines to the 6AV6 cross over the power amp circuitry, it is a good idea to run these through coaxial cable. Only ground the input side of the coaxial cables to prevent a ground loop. A 3.3K/5nF RC array was added to the power amp's feedback loop and the pad resistor on the feedback was increased to 750 ohms, these changes took care of an annoying RF oscillation issue. The R-L snubbers on the 6V6 plates were removed. A careful observer will notice the blue glow eminating from under the power transformer, this effect was created by hot-glueing a piece of blue plexiglass between the pilot light and the chassis.
The Hammonator 1 article explains most of the amplifier conversion process.
The vibrato circuit's optical coupler is constructed by placing a LVR and a medium brightness red LED inside of a piece of heat shrink tubing. The wires from the LED and LVR should be covered with pieces of wire insulation to prevent shorts.
The LFO circuit board was built on a 0.1" grid perforated board. Connections were hand-soldered to solder type IC sockets. Soldered wire-wrap wire was used for most of the wiring, some #22 solid wire was used for the ground connections. The board was built to fit on the side of the chassis next to the power transformer, it was a very tight fit. A piece of clear plastic from a soap bottle was cut to the same outline as the board and installed between the board and the chassis to prevent shorts.
This amplifier drives a 15" guitar speaker nicely. A more conventional 12" guitar speaker would also be a good match for the amp. The speaker was mounted in a heavy duty wood box that was made from 2x8 (top, bottom) and 2x10 (sides) dimensional lumber with plywood front and back plates. The reverb tank was mounted inside of the speaker cabinet to enhance the feedback to the springs.
The Hammonator 2 amp is mostly operated in the same way as the Hammonator 1. The Tremelo/Vibrato switch selects, oddly enough, whether the amp uses the tremelo or vibrato effect. The depth control selects the amount of effect and the speed control varies the rate of the effect. If the speed control is rotated all the way to the left, it switches off either effect completely. Unlike classic 1960s amps, the vibrato/tremelo speed covers a much wider range of frequencies and maintains a true sine waveform at all settings.