FuzzniKator Push-Pull Tube Distortion/Preamp Box

FuzzniKator Outside View

FuzzniKator Inside View

FuzzniKator Schematic

FuzzniKator Push-Pull Tube Distortion/Preamp Box

(C) 2009-2015 G. Forrest Cook

Introduction

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 sounds. 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 nicely as a tube preamp stage and the distortion channel makes any amp, solid state or tube, sound warm and fuzzy. The version 2 circuit adds a mixer stage to the distortion channel, this allows the distorted and clean signals to be mixed together for a wider variety of sounds.

A Hammond AO-42 organ amplifier 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 AO-41 percussion unit can also be used. The original filament 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 AO-47 box. The Hammond AO-42 and AO-47 boxes are often sold together on eBay auctions, prices are usually low since the boxes have no power transformer.

Warning

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.

Connections

AC Power Input - grounded 120VAC
Guitar Input - High Impedance
Amp Output - Medium Impedance

Controls

On/Off (on the back)
Clean/Distort foot switch
Hunger (starvation voltage)
Distortion Drive
Distortion Tone
Distortion Mix Level for distortion channel
Clean Mix Level for distortion channel
Clean Signal Level for clean channel

Theory

The guitar signal drives V1b, a cathode-biased class A 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 triodes V2a and V2b. The plate circuits of V2a and V2b drive the output transformer in push-pull mode. The output transformer drives a 15 ohm resistor. The resistor gives a load to the output transformer which is important for producing good tone.

The output transformer's signal is routed back to the V1a input as a negative feedback high-cut Tone control. The distorted output signal also drives V3, a 6C4 triode wired as a class A amplifier. The 6C4 buffers the output and also inverts its phase so that the distortion and clean signals are sent to the mixer with the same polarity. Half of a common 12AU7 tube could be substituted for the 6C4.

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 distortion and clean signals are mixed together using two 100K potentiometers and 100K resistors. The clean signal level is adjusted with a third 100K potentiometer. The foot switch selects between the distortion mixer signal and the clean signal.

It is possible to substitude different tubes in the V2 distortion amplifier circuit. The 12A*7 tube family share the same pinouts but have different amplification factors. A low-gain 12AU7 produces a smoother sounding distortion and a high-gain 12AX7 produces more of a crunchy sounding distortion. The medium-gain 12AT7 produces a distortion sound that is between the other two tubes.

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.

Construction

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. One of the dual RCA jacks was drilled out of the AO-42 chassis and the hole was filed wider to allow the installation of the 7 pin 6C4 socket.

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 Electronic Supply.

The output transformer used in this version of the circuit is an EL1254 push-pull Universal Output type. A transformer with a 10K ohm center tapped primary and an 8 ohm secondary (or similar) should also work. The Hammond P-T125A or P-T291 would probably sound good in this circuit, but they tend to be expensive. It may also be possible to use an inexpensive 5VAC split-bobbin filament transformer with dual 120VAC primaries wired for 240V as the output transformer. You may need to adjust the value of the 15 ohm load resistor for best results.

Use

Plug an electric guitar or other musical device into the input, plug the output into an amplifier or mixing board. Turn the guitar volume control up high. Use the foot switch to select between the clean and distorted channels. Set the Starvation Voltage and Distortion Drive controls to the midpoint, play your instrument and adjust both controls for the best sound. Adjust the Clean mix and Distortion mix for the desired distortion sound. Adjust the Clean signal control for similar levels on both channels. Increasing the starvation control causes more compression.

Circuit Extensions

Brian White from UMass in Boston has extended the original Fuzznikator circuit by adding a spring reverb and associated driver circuitry. Take a look at his schematic and the photo of his prototype.

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