Build Your Own Tube Distortion
    By Ron Black

(Taken from an article in ‘Guitar Player’ magazine, October 1981, p 52-53)
 

NEARLY ALL AMPS ARE capable of producing a certain amount of distortion, even if it means cranking them up to 10. But getting that smooth, singing edge popular in much of today’s electric guitar music isn’t always that easy – especially at a low (or at least reasonable) volume. In recent years, the inclusion of a master volume control (allowing the preamp to overdrive the power amp section) on some amps has made controlled low-volume distortion easier to produce.

Unfortunately, many older amps have no master volume controls. Furthermore, some amps (especially solid-state models) are designed specifically for tremendous power, but minimal distortion. This means that some high-powered transistorized models must be turned up to 8 or 9 in order to introduce any edge to the sound, and even then it may be too brittle for the taste of some guitarists. Powerful tube amps, as well, may require great volumes to distort.

In the past few years, effects manufacturers have begun marketing electronic devices that use FETs (field-effect transistors) and other state-of-the-art integrated circuit technology to mimic the “warm” tonal character of tube distortion. And while the sound obtained can be awfully close to that created by a tube, the solid-state approach still doesn’t please everybody. Guitarists tend to be very fussy about their sound; if they want tubes, they’ve got to have tubes.

If you desire tube-style distortion but as yet haven’t been able to obtain it, the Vacuum Tube Distortion may be just the project for you. With about $30.00 worth of parts and a little electronics knowledge, you can put together this tube distortion preamp, which plugs into the signal path between your guitar and amp and requires no modifications to your amp.

Like most solid-state distortion units, this circuit has an input volume control (for regulating the amount of distortion) and an output volume control (for varying the volume being fed to the amp without affecting the amount of distortion). The Vacuum Tube Distortion has a low output impedance, which makes it compatible with guitar amplifiers, tape recorders, and effects. Out of necessity, it has its own power supply, because the vacuum tube’s filament draws too much current to make battery operation feasible.

The actual design is quite straightforward. There are two integrated circuits (ICs) and one vacuum tube. The integrated circuits are an LM340K 12-volt regulator and a 747 dual op-amp. The other op-amps – such as the NE5532, TL084, LM353, 4558, or 1458 – may be used; however, the pins of these substitute op-amps do not correspond directly to the 747’s, so check the manufacturer’s recommended pin connections. Also, two 741 op-amps or their equivalent can be used instead of a single 747; again you will have to make your connections to other pins.

One half of the op-amp is used to provide enough gain to guarantee that the tube circuit will be adequately overdriven. The other half is used to convert the high output impedance of the tube circuit to low impedance. The vacuum tube is a 12AU7A dual triode, and both triode portions are incorporated into the circuit for optimum sound quality.

The best approach to building a relatively simple project such as this is to hand wire it on a perforated circuit board. This method is very economical, and if you have a little electronic experience it shouldn’t take more than a few hours to construct. The total cost will depend on where you buy your parts, and what kind of box you house the project in.

The arrangement of parts on the circuit board is flexible – as long as everything is wired in accordance with the schematic diagram. The photographs illustrate one layout that demonstrates the relationship between the schematic and the physical circuit. Keep in mind, though, that there is no one correct way to build it. The best layout is the one that create the lease confusion for the individual building it.

Unlike many solid-state projects, this one uses 115-volt power from the wall – plenty to give you a good shock, or even kill you. Therefore, be very cautious; if you aren’t sure of what you’re doing, either don’t attempt this project, or get help from someone with a greater knowledge of electronics. Make sure that all connections are soldered, and that stand-offs (small posts that raise the circuit board off of the chassis) are used. Wires going from the power plug to the transformer must be well insulated; keep them away from the metal chassis. Also, secure the transformer to the chassis to prevent it from flopping around and causing damage to the other components, as well as to prevent it from pulling its wires loose.

When building the Vacuum Tube Distortion, as in other electronic projects you should be meticulous in your work. If you foul something up because you rush through the job, the results may be no only disappointing, but dangerous. First, never glob on the solder. Use just enough to create a good solid connection. When cool, the soldered connection should appear shiny. A dull appearance to the solder’s surface often indicates an improper connection. Also, don’t touch the soldering iron to components for any longer than necessary. If you’re not sure just how long to make such a contact, practice on a few spare part or wires. Too much heat, especially when applied to integrated circuits, transistors, or diodes, cam be the kiss of death for them.

Use a socket for the dual op-amp and for the tube. A socket makes it much easier to replace and or test such components. Also, soldering a socket into the circuit and later plugging in the IC eliminates the chase of ruining the IC with too much heat.

Note that the power supply diagram is separate from the circuit schematic. By keeping the two sections apart, it is easier to construct and, if necessary, to troubleshoot. Also, it separates the high voltage (115 volts) from the relatively low voltages (12 and 6 volts). The transformer will have four or five wires. Two of them will have black insulation; these are the primary wires, and should be connected to the AC power cord. The remaining wires will be two or one color (both green or both red, etc.), and if there is a fifth wire, it will be yet another color. The two wires of similar color are to be hooked up to the full-wave bridge rectifier; the other wire (called the center tap) won’t be used, so it should be wrapped with electrical tape so that it can’t short to anything. After soldering the wires of the transformer to the bridge rectifier or to the power cord, make sure that the connections are then covered with electrical tape, in order to avoid accidental short-circuiting.

If you have a voltmeter, check the output voltage of the power supply after you finish construction. Touch the negative probe to the ground (labeled GND), and the other first to the 6-volt terminal, and then the 12-volt terminal. Slight differences in the voltage measured (say 13 volts at the 12 volt point) shouldn’t affect the unit’s operation. If large differences are present, unplug the unit and recheck your connections. When wiring the device, don’t forget the power supply connections to the 12AU7A tube’s filament (points 4 and 5 of the socket).

Finally, recheck all your connections against the schematic. Place the completer circuit in a ventilated metal box. Unlike solid-state effects, this one produces heat because of its tube. If you can’t find a ventilated box, drill holes yourself (without the circuit in the box). Remember that the tube requires a warm-up time, so when you turn the unit on, expect a few seconds’ waiting before you can use the Vacuum Tube Distortion.

When plugging the unit in for the first time, make sure that all volume controls, including those on your amp, are turned down very low. Turn up the device’s output volume and the amp’s volume control just a bit. Next, strum a chord on your guitar and raise the device’s input volume control until the desired amount of distortion is achieved.

Naturally, you’ll find load of uses for the Vacuum Tube Distortion – from biting tones at low volumes to controlled feedback and absolutely filthy fuzztone when your amp is cranked up. And while this unit obviously affords less control over the quality of the distortion than a multi-tube preamp, for its size, low cost and ease of construction it’s hard to beat.