AUDIOCLINIC (Q and A) (Aug. 1976)

by Joseph Giovanelli

High Fidelity Amplifiers and Musical Instruments

Q. I have several, old mono tube basic amplifiers of undistinguished lineage. As they are of little use for hifi or stereo use, but are still capable of a fair amount of power (12-25 watts) I have been wondering if they could be used for instrument amplifiers, such as for electric guitar and bass.

I have been variously told that it would require a preamplifier or that the input from a guitar would be too strong, requiring cutting into the circuit somewhere or adding some complicated additional circuitry. What is your advice?

-Steven Lindblom, Stratham, N. H.

A. Many musicians buy amplifiers which can produce 100 watts of power and up for use with their instruments and your amplifiers are not close to that figure. However, there are any number of small amplifiers designed for musical instruments which do not have any more power than those which you wish to use, so from this standpoint you should be fine.

With the volume of a guitar or electric bass turned up fully, I would estimate that we would get a approximately 0.1 volt of output signal. The actual output will vary over a wide range, depending on the physical effort the performer uses to play the instrument. While this 0.1 volt is more volume than is produced by most microphones, it is not as much volume as is produced by a tube tuner of the sort probably used when your amplifiers were popular. Thus, the musical instrument might not drive the amplifier to full output. The input sensitivity of your amplifier is, therefore, too low.

It all resolves itself to what input voltage, with the amplifier's volume control turned up fully, will produce full output power? If this output is around 0.1 volt, your amplifier may be suitable just as it is. However, if you play the instrument very softly, then perhaps you won't have enough signal to drive the amplifier to the volume level you might like.

In any event, you will not need any sort of circuit which would provide gain reduction, requiring you to cut into the amplifier circuit at some later stage.

What you probably need is some sort of voltage amplifier which will make up any difference between what your amplifier can do and what it has to do in order to be driven to full output. Such devices are available as "power boosters," sold by dealers who sell musical instruments and amplifiers. These devices are small, battery-powered boxes, generally employing one or two transistors. They are much smaller than "fuzz" or vibrato generators, envelope modifiers, octave boxes, and the like.

If these amplifiers do not have their own tone controls, these circuits must be added. The makers of the power boosters also make simple units designed to boost either the bass or treble ends of the spectrum but not both at the same time. These inexpensive elements can be useful to you. Both booster amplifiers and tone controls are sometimes available for as little as $10.00 each.

High-fidelity amplifiers do not have vibrato or reverb built into them as is the case with many guitar amplifiers. Therefore, you might need to add these devices externally, and units are available for this application.

You may find that these amplifiers do not sound just as they should when driven into distortion but this aspect of music is often useful to performers to create an effect. The character of this distortion is different in tube amplifiers from that in solid-state amplifiers. This is the reason that some performers still use tube amplifiers rather than solid-state units.

From this standpoint you may be well served by the tube amplifiers you have.

(Audio magazine, Aug. 1976, JOSEPH GIOVANELLI)

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