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The Valve Driver

A valve overdrive simulator for guitar or keyboards

★ Wide range of distortion effects
★ Mixable odd/even harmonics
★ Valve sound simulation
★ Low impedance output for driving long cables
★ Low noise in/out switching


Distortion effects have been around under various guises for many years now, all being descendants of the original 'fuzz box'. A feature common to all distortion effects is that they add harmonics to the original signal. The important question is which harmonics? Devoted valve amp users insist that the distortion produced by these warm-hearted beasts is somehow special to the valve, and cannot be reproduced by other means.

Many valve amps produce even harmonics in abundance due to the non linear transfer characteristics of their tubes, especially the stages operating in class A configuration. Furthermore, when the signal excursions are large, clipping is not nearly as hard as in transistor amps. The signal peaks become more 'rounded off' than clipped, producing odd harmonics.

The original objective of the design presented here was to simulate these two valve phenomena; but the wide range of adjustment available on E&MM's Valve Driver goes much further than this, allowing effects anywhere from a warm, gentle valve sound to a good 'rocky' overdriven amp sound to be produced. The project is housed in a neat footswitch box with an easy access battery compartment. A socket is also provided for a battery eliminator to be plugged in.

Figure 1. Circuit diagram.
(Click image for higher resolution version)


The circuit diagram, Figure 1 reveals that diodes play no part in the production of distortion. Instead, an operational transconductance amplifier (OTA) is used to dynamically 'bend' the waveform, rather than clip it using diodes, as is the case with most distortion units. After being buffered and amplified by IC1a, the input signal is passed to IC2a, whose gain is dynamically controlled by the current generated by IC2c. When the effect is switched in, SW1a allows the amplified input signal to be precision full-wave rectified by IC1b. Thus when RV1 is set clockwise, the normal rectifying action of IC1b & D1 causes the current generated by IC2c to be reduced as each half cycle of the signal approaches its peak value. Since the gain of IC2a is proportional to the control current, the signal peaks become flattened. This flattening of both the positive and negative peaks causes odd harmonics, to be added to the output signal, which appears at JK2 after being buffered by IC2b.

Figure 2. Typical waveforms:
(a) Sinewave input.
(b) With even harmonics.
(c) With odd harmonics.

When RV1 is set anticlockwise, IC1b operates in a completely linear manner, causing the amplified input signal to be fed to IC2c, whose output current follows the instantaneous signal voltage. The control current, and thus the gain of IC2a rises with positive going signal peaks, and falls with negative excursions. This non-symmetrical effect gives rise to even harmonics. The effect is amply demonstrated by the waveforms shown in Figure 2. 2a shows an undistorted sinewave input and 2b shows the effect of even harmonic distortion, where the positive half cycle is unusually 'peaky', whereas the negative half cycle is flattened off. Odd harmonics distortion is shown in Figure 2c, where both half cycles are flattened.

IC2d additionally introduces a square term by making use of the transconductance amplifier's multiplying ability. Without IC2d, the transfer characteristics would take the form:

Vo = AVin(B + Vin) with RV1 anti-clock-wise
Or Vo = AVin(B - Vin) with RV1 clockwise

IC2d, however causes the transfer function to take the form:

Vo = AVin(B + CVin + Vin2) with RV1 anti-clockwise
Or Vo = AVin(B - CVin + Vin2) with RV1 clockwise

Where A,B and C are arbitrary constants in each case.

RV2 allows the intensity of the effect to be controlled. Since SW1a acts only on the control path and not on the signal path, very low switching noise results when the in/out switch is operated.

If you have a guitar with an exceptionally high output level, then hard clipping distortion might be encountered, which can be remedied by reducing the value of R3. Try a value of 2k2 for starters. Similarly, if you have a low output pickup, then increasing the value of R3 will restore the proper distortion-inducing levels.

One other modifiable point is the value of R15. Its purpose is to always allow IC2a to operate with at least some gain, so that the sound does not break up completely. If the distortion is a little too tame for your liking, then R15 could be increased to, say 1M for a more 'crunchy' sound.


This project should present no problems, as most of the 'gubbins' is contained on the PCB, leaving little wiring to do. Since the standard footswitch unit comes with a jack socket and switch mounted on a PCB, the first job is to remove the PCB assembly from the case and de-solder the switch and socket. The PCB can then be discarded. If you don't have access to a solder pump, then the solder can be removed from the joints using the bared ends of stranded wire applied with the soldering iron to the joints. The assembly of the Valve Driver PCB starts by inserting from the track side, and soldering the 8 veropins. Next, insert and solder the IC sockets, but leave the ICs out till later. Now insert and solder D1 and all the resistors, some of which are mounted vertically. Mount and solder the capacitors similarly, taking care with the switch can now be soldered in place, making sure that they are pushed firmly down onto the PCB whilst doing so. Note that the socket removed from the original PCB is JK2. Having completed the PCB, it should be checked very carefully, preferably with an eyeglass before the ICs are loaded into their sockets.

Figure 3. Case preparation (a) control panel, (b) rear socket area.
(Click image for higher resolution version)

Remove the inner moulding from the foot-switch case by withdrawing the two securing screws. Now prepare the two mouldings as shown in Figure 3. Additionally make a small hole in one corner of the battery compartment, and thread the battery clip wires through it. Before fitting the LED in place, using a clip and collar, solder 100mm long insulated wires onto the leads and sleeve the joints. Identify the anode wire by bending over the free end.

Figure 4. PCB component overlay and wiring details.
(Click image for higher resolution version)

After mounting the pots on the inner moulding panel, they can be connected to the PCB assembly veropins using 100mm long insulated wires, along with the battery clip wires. Now drop the inner moulding back into the main moulding, guiding the LED wires through the hole in the inner moulding, and screw it in place. The LED wires can now be soldered to the PCB assembly, remembering that the bent-over wire is the anode connection. Feed the jack socket bushes into the appropriate case holes and locate the PCB on the pillars so that the switch lever drops into the actuator arm. All that remains now is to screw on the jack nuts (no need to secure the small jack), fit the control knobs, and screw on the baseplate.

Battery compartment.

Valve Sound

Internal construction.

Once you have popped a PP3 (or equivalent) battery into the compartment located under the footpedal cover, your Valve Driver should be raring to go. Although originally intended for use with an electric guitar, any electronic or electric instrument can be plugged into the input, although it may in some cases be necessary to keep the level down to prevent hard clipping distortion from occurring. Once the instrument jack is inserted the battery is switched on, so remember to remove the input jack from the Valve Driver when it is not in use. The output jack should then be connected to the guitar input of your amplifier, or similar lowish level input on your mixer or tape recorder. The output level is about the same as the input level.

While the LED remains unlit, the signal is allowed to pass unaffected. The status of the LED is changed by operating the foot-switch, which brings the distortion circuit into play when the LED is lit. The 'harmonics' control determines the mix of odd and even harmonics which are produced. The 'valve sound' is renowned for its prominence of even harmonics, so if it's valve sound you want, keep the harmonics control towards 'even'. Some odd harmonics are desirable though, to simulate the 'rounding-off' produced by a hard driven valve output stage. The 'Intensity' control, as its name suggests, simply affects the degree to which the sound is distorted; try the 2 o'clock position initially for valve sound.

The unit is by no means limited to valve sound. With both controls fully clockwise a good overdrive distortion is produced.


Jack and Power sockets.

The position of the Valve Driver unit in the effects chain can make quite a difference to the sound. If it is early in the chain, then any succeeding filtering effects, such as tone boosters, phasers, flangers, etc. will have a more profound effect. A compressor though, would be better placed before the Valve Driver. Indeed this combination will be found very rewarding. The valve purist would, of course, insist on placing the Valve Driver last in the chain, to best simulate an overdriven amp. As always, the best placement should be found by experimenting yourself.

One final point: If you wish to run the unit from a mains power supply then this should be of the regulated 9V DC variety.

So, if you like your guitar sound 'rocky' or your keyboard sound 'dirty' and hesitate to replace your tired, old valve amp in favour of that MOSFET amp, hesitate no longer. The Valve Driver may even allow the recording guitarist to DI his lead tracks.

The complete kit of parts including PCB for the Valve Driver is available from E&MM, (Contact Details). Price £23.95 including VAT and P&P. Please order as: Valve Driver kit.


Frequency response 8Hz to 22kHz (-3dB)
Output noise -82dBm (A)
Maximum input level -12dBm
Standby battery drain 9mA
Active battery drain 12mA


Resistors - all ¼w 5% carbon film
R1,2,13,15 220k 4 off
R3 4k7
R4,11,12 1k2 3 off
R5,6 820R 2 off
R7,8 100R 2 off
R9 8k2
R10,18 10k 2 off
R14 22k
R16 18k
R17,19,20,21 2k2 4 off
RV1 47k Lin pot
RV2 4k7 Log pot
C1 47n Polyester
C2 33p ceramic
C4,5 22u 16V radial electrolytic 2 off
C3 2u2 50V radial electrolytic
IC2 LM13600
D1 1N4148
JK1 ¼" PC Jack socket with make contact
JK2 ¼" PC Jack socket (supplied with case)
JK3 3.5mm PC Jack socket
SW1 Latchswitch (supplied with case)
Battery clip (PP3)
LED clip
Footswitch case (Rhino 801)
Knob 2 off
Knob cap 2 off
8 way DIL socket
16 way DIL socket

Previous Article in this issue


Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Dec 1983

Feature by Paul Williams

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