Hammonator 2RVT Organ to Guitar Amp Conversion

(C) 2008 G. Forrest Cook

Hammonator 2RVT copy 2

Hammonator 2RVT copy 2

Hammonator 2RVT copy 1

Hammonator 2RVT schematic

Hammonator 2RVT LFO schematic

Introduction

This project is a feature-added version of the Hammonator 1 Hammond organ to guitar amplifier conversion project. The Hammonator 1 circuit works 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 Tremolo. Two copies of the Hammonator 2RVT amp have been made (see photos), both have been working nicely for many years and the design has stood the test of time.

In year 2022, a few changes were made to this design. The bias circuit was changed to full-wave rectification and individual adjustments were added for the two output tubes. An output transformer snubber circuit was added to protect the transformer from the possibility of high-frequency arcing. A grid stopper resistor was added to the vibrato return circuit to reduce the chance of RF pickup. An optional balance resistor was added to the output transformer primary and an optional balance adjustment trimmer was added to the phase inverter stage. The schematic went through a number of improvements, capacitor voltage ratings and resistor wattage ratings were verified and updated where necessary.

The reverb uses 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 tremolo 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 achieved by chaining a second "clean channel" amp's input to the Hammonator's second input jack, or by driving the amp from the "Side Channel Output" connection. Stereo greatly enhances the vibrato effect, the sound appears to move around the room. A cheap guitar practice amp makes an ok secondary amp, a tube amp such as the Spartacus or the Howler Monkey will produce an even better sound. The Tremolo/Vibrato switch changes the pitch shifting vibrato effect into a volume shifting tremolo effect. The wide range of LFO speeds produce some interesting tremolo effects.

Compared to the Hammonator 1, some extra wiring is added to the 6AV6 triode tube. A stand-alone sine wave LFO oscillator board is added for driving the vibrato/tremolo. 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, the 6.3VAC is rectified and filtered to produce 7.5VDC.

Another subtle feature of this amp is the use of 100nF capacitors on 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.

Simplifications

The vibrato/tremolo sine wave generator is a fairly involved sub-circuit to build. It may be prudent to start with the Hammonator Model 1 amp and add the vibrato/tremolo circuitry later. Be advised, the vibrato effect is worth the extra effort.

The 6HU6/EM87 indicator tube and associated components can be left out of the circuit or added later. The eye-candy effect is certainly worth a little extra wiring. Other magic eye tubes may be used by making the appropriate wiring changes.

Warning

This is a fairly advanced-level project. It takes a lot of technical skills to deconstruct and reconstruct the ampifier circuitry. Also, there are 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.

Connections

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

Controls

On/Off (on the back)
Input Volume
Bass
Treble
Reverb Send (Dwell)
Reverb Return
Feedback (Gain)
Vibrato/Tremolo Depth
Vibrato/Tremolo Speed
Vibrato/Tremolo Selector

Inspiration

A friend of mine loaned me his vintage Magnatone Lyric amplifier, the amp has an incredible sounding pitch-shifting vibrato circuit that uses a varistor-based phase modulation scheme. The pitch-shifting vibrato effect could be described as liquidy or warbly and is similar to the sound produced by a rotating Leslie speaker. The type of varistors used in the Magnatone amps are no longer manufactured, they are difficult, if not impossible to find. The Magnatone's differential high voltage varistor drive circuitry also takes numerous tube stages to implement.

The vibrato circuit in the Hammonator 2RVT uses a much simpler light dependent resistor (LDR) circuit to produce the required phase shift. The VFO used here has a much wider range of frequencies than the Magnatone's simple Tube VFO, this allows it to produce some amazing slow motion effects. There is an excellent Geofex article on The technology of phase shifters and flangers that covers the Magnatone circuit and more. Another excellent source of Vibrato circuit information is Magnatone Vibrato Design by Tim Robbins.

The phase shifting vibrato circuit used in the Hammonator 2RVT was inspired by the transistorized Univibe circuit from the Geofex article, it is also similar to some Hammond organ vibrato circuits. The Liquidator Tube Phaser/Chorus Effect was an offshoot project that takes tube-based phasing and chorus to the next level, it has three phase-shift stages with individual mixers and a feedback control.

Theory

See the Hammonator 1 project for the theory of the basic amplifier.

The Vibrato circuit is quite simple. The 6AV6 tube is running as a simple raised-cathode 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 Tremolo/Vibrato switch is set to Vibrato, the Light Variable Resistor (LVR) causes the output signal to shift from one phase of the signal toward the other, depending on the amount of light coming out of the associated LED.

When the Tremolo/Vibrato switch is set to Tremolo, one end of the LVR is connected to ground through the variable intensity 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 2.5M potentiometer. Ideally, the 2.5M speed control pot should be logarithmic taper. Unfortunately this part is very hard to find. 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. The variable square wave clock signal from the 555 timer 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. The Digitized sine wave values are fed to the DAC0808 DAC, which converts them into analog currents for driving the 1458 output op-amp. The second half of the 1458 dual op-amp biases the negative side of the LEDs below zero volts so that the LEDs are just turning on at the low point of the LFO waveform.

Here are some Intel hex format files for programming the EPROM: Regular Sine Wave and Exponentially Weighted Sine Wave. The latter wave is recommended since it compensates for the LED/LVR non-linearity.

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 by the 2.7K resistor and the maximum phase deviation is adjusted with the 5K trimmer. The phaser effect's intensity is adjusted by varying the 100K pot that is in series with the light variable resistor (LVR).

A microprocessor-based LFO circuit such as my Arduino LFO V1 or my Arduino LFO V2 will also work in this amplifier. Those designs uses fewer parts and don't require an EPROM programmer.

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 -1.4VDC. 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 Tremolo/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.

Circuit Changes

In addition to the new vibrato circuitry, a number of changes were performed on the Hammonator 1 to create the Hammonator 2. Additional front panel holes were drilled for the extra input and the vibrato knobs and switch. The vibrato front-panel LED was mounted in an existing screw hole. The tubes were re-numbered, the 6HU6/EM87 was changed from VT7 to VT8 and the new 6AV6 was numbered VT7. In one of the photos above, a blue glow can be seen under the power transformer, this effect was created by hot-glueing a piece of blue plexiglass between the pilot light and the chassis.

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 and their value drops, causing hum. Alway use new electrolytic capacitors when re-building tube amps.

The signal lines to the 6AV6 cross over the power amp circuitry, it is critical to run these through shielded cables to avoid parasitic feedback issues. Only ground the input side of the shielded 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 control was increased to 750 ohms, these changes took care of an RF oscillation issue.

The high voltage filter circuit was modified from the Hammonator 1 circuit to include a standby switch for turning the B+ supply on and off, tube life can be improved if the tube filaments are warmed up before applying B+. My Vacuum Tube B+ Delay circuit was installed in both versions of the Hammonator 2 RVT amp, it replaces the standby switch and provides automatic power sequencing.

The bias circuitry for the output tubes was duplicated so that each tube gets its own bias adjustment pot. The bias adjustment circuit was inspired by an article from from The Valve Wizard. This helps the amp to work better with tubes that are somewhat mis-matched, which can help when using vintage tubes.

Construction

The Hammonator 1 article explains most of the amplifier conversion process. This photo shows the stripped inside and terminal strip locations of the Hammonator 2 RVT and this second photo shows the completed inside wiring, the LFO board can be seen on the left side of the chassis.

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. The wiring from the LFO board to the front panel potentiometers should be done with shielded wire to prevent pickup of the oscillator signal by the amp's input circuitry.

This amplifier drives a 15" guitar speaker nicely, this gives a baratone response. A more conventional 12" guitar speaker is also a good match for the amp and produces a brigher sound. The speaker was mounted in a heavy duty wood box that was made from 2x10 (top, bottom) and 2x8 (sides) dimensional lumber with a 3/4" plywood front and two 3/8" plywood back plates.

When building guitar speaker cabinets, it is a good idea to feed an amplified sine wave signal through the speaker to ferret out any cabinet buzzing points. If the cabinet buzzes at a certain frequency, you can poke and flex the cabinet corners and speaker mounting points to locate the source of the buzz. A little hot-glue can stop many buzz points, be sure to use nuts and bolts on all of the speaker mounting holes. In some cases, it can be helpful to add wooden ribs or braces to the inside of the speaker box.

The amplifiers were originally built on top of speaker cabinets and the reverb tanks were placed inside the bottom of the speaker cabinets to enhance the sonic coupling between the speaker and reverb springs. This setup was deemed too fragile, it was also too heavy so the amps were installed in their own stand-alone wooden boxes. The reverb assemblies were mounted above the amp on a small shelf that is part of the boxes. The reverb pickup coil should be placed on the opposite side from the amp's power transformer to prevent hum pickup.

Use

The Hammonator 2 amp is mostly operated in the same way as the Hammonator 1. The Tremolo/Vibrato switch selects the tremolo or vibrato effect, or neither in the center-off position. The Intensity control selects the depth of the vibrato or tremolo and the Speed control varies the rate of the effect. Unlike classic 1960s amps, the vibrato/tremolo speed covers a much wider range of frequencies and produces a more accurate sine wave response across those frequencies.

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