There are zillions of guitar distortion circuits on the Internet, many with funny names. The FuzzniKator is somewhat unique among the lot. The circuit was inspired by the great distortion sound that tube amps give a guitar during the seconds after the power has been shut off and the signal is fading out. The effect is also similar to a cranked tube amp driving a resistive speaker attenuator. The circuit uses a special starved push-pull tube output stage driving an output transformer which drives a resistive load. The push-pull output tubes are run from an adjustable (and lower than normal) B+ voltage in order to "starve" the tubes and produce distortion. The push-pull configuration and transformer output produce a more harmonically pleasant distortion compared to most diode-clipped distortion effects.
The starvation voltage and drive signal are adjustable for a wide variety of distortion effects. The FuzzniKator also has a clean tube preamp channel that is similar to a classic Fender amp's tube front-end circuit. The clean channel works as a tube preamp for a solid state guitar amp, and tube distortion makes a solid state amp sound warm and fuzzy.
A used Hammond AO42 organ percussion unit was used as the platform for this circuit. This is optional, but it makes a good chassis for the device and saves a lot of trouble drilling holes for the tubes and terminal strips. A Hammond AO41 percussion unit can also be used. Some of the original wiring between the tube sockets and breadboard can be re-used. The FuzzniKator's companion project is the Liquidator Tube Phaser/Chorus Effect, which uses the Hammond AO47 box. The Hammond AO42 and AO47 boxes are often sold together on eBay auctions, prices are usually low since the boxes have no power transformer.
This circuit uses high voltages including 120 VAC and 250 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 device. 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 circuitry.
AC Power Input - grounded 120VAC Guitar Input - High Impedance Amp Output - Medium Impedance
On/Off (on the back) Clean/Distort foot switch Hunger (starvation voltage) Drive Tone Distortion Signal Level Clean Signal Level
The guitar signal drives V1b, a typical cathode-biased triode amplifier. The output of V1b is attenuated through the Clean Level control and sent to the SPDT foot switch as the Clean signal. The output of V1b also goes to the Distortion Drive control and on to V1a, which is wired as a floating cathode-biased phase inverter.
The two out-of-phase drive signals from V1b are fed to the grids of output tubes V2a and V2b. The plate circuits of V2a and V2b drive the output transformer in push-pull mode. The output transformer drives a 10 ohm resistor. The resistor gives a load to the output transformer, this is important for producing good tone. The transformer's output also goes to the Distort Level control and on to the Distort signal on the foot switch. The output is also routed through the negative feedback high-cut Tone control. The distortion comes from the push-pull starved V2a and V2b array driving a loaded transformer. The push-pull stage runs with low B+ values and high grid drive.
The power transformer has a standard fused and switched line input. The 6.3 VAC output powers the filament circuits for the two tubes. It also drives a bridge rectifier and filter circuit to provide DC for the LED pilot light. The transformer's high voltage output is full-wave rectified and R-C filtered to produce B+1 (around 250V). B+1 is used to drive the clean preamp and phase inverter stages. The B+2 voltage is adjustable and it drives the push-pull output stage. A multi-pole Starvation switch selects different dropping resistors to produce a variable B+2 voltage. A 2W 100K variable resistor in series with a 15K 2W pad resistor could be used in place of the switch to give an analog starvation control.
An "M" shaped metal framing section was used for the chassis, this allows the AO-42 box to be slightly recessed. The power transformer was mounted on the back side of the chassis top and holes were drilled for the jacks, switches, potentiometers and power cord. A U-shaped metal frame should be constructed to cover the bottom and sides of the chassis. Most of the components on the breadboard should be unsoldered or clipped off. The rectifier circuits and first high voltage capacitor are mounted in the main chassis.
The power transformer was pulled from some old electronic equipment, a Hammond P-T269EX transformer is roughly equivalent, these can be purchased from Antique Electrical Supply. A small push-pull output transformer (EL1254 Universal Output) was used for the output transformer. Any small push-pull tube output transformer should work in this circuit, the Hammond P-T125A or P-T291 would be good choices. It may also be possible to use a 6.3VAC filament transformer with dual 120VAC primaries as the output transformer, the 10 ohm load resistor would probably need to be replaced with another value.
Plug an electric guitar or other musical device into the input, plug the output into an amplifier or mixing board. Use the foot switch to select between the clean and distored channels. Use the Clean and Distort Level volume controls to equalize the two levels. Set the Starvation Voltage and Distortion Drive controls to the midpoint, play your instrument and adjust both controls for the best sound then re-adjust the two level controls as desired.