FROMTHELAB
Dissecting Distortion
This month let’s take a look at our old friend
distortion and his little brother overdrive.
The ‘50s saw the beginning of an ongoing
attempt to make amps louder and louder,
and gentlemen like Mr. Jim Marshall and
Mr. Leo Fender were happy to oblige, making bigger and bigger amps in this ongoing
quest. Through the decade and into the
next crowds at rock n’ roll shows rapidly
increased, with things getting really out of
hand at Shea Stadium in 1965, when four
British boys played there, then again at a
little, muddy outdoor gig a few years later,
taking place about two hours north at Max
Yasgur’s farm.
saturation point is reached, so is the tube’s
maximum electron flow; it’s the tube’s way of
saying, “I can’t give ‘er no more, Captain!”
The resulting distortion is a function of overdrive. So, to achieve great tube distortion, all
one has to do is introduce a big whopping
signal into the preamp tubes. But this needs
to be qualified; the cleaner the signal here,
the better. Many amplifiers have two or three
tube preamp stages to kick the signal up
with an unfortunate side effect – noise.
From what I can see, two major con-
cepts developed from these fortuitous
events. First was the glorious sound
of an overdriven amp. The sec-
ond was the realization that
amp placement had been
almost an afterthought. Brian
Lionman and I have covered
amp placement in previous
articles, but the executive
summary is the typical back-
line is generally not the best
place to put amplifiers. Dig
around a little and check out
the previous columns online to
learn more.
Another aspect of gain comes into play with
respect to a particular circuit’s power supply parameters. You may have heard the
term “power rails.” This can be a somewhat
technical issue, but in a nutshell, rails are
the upper and lower limits of clean power
to the circuit. Push the limits and you will
be greeted with distortion. For tubes this
overdrive usually has good results. With solid
state circuits the resulting sound is usually
undesirable. A general way to think of gain is
to remember that you will only get louder up
to a certain point, depending on the capability of the circuit, after which it will add to
the amount of distortion, or pushing out of
bounds. In comparison, volume continues to
get louder and louder, regardless.
Now, what would happen if we were to
increase the size of the signal coming
into the power tubes, say through
an effects loop? In this case things
will get considerably louder. But
what about power tube distortion?
Power tubes pushed to their limit
can be extremely musical – think
of a cranked ’ 69 plexi or an AC30
all the way up. The power tubes are
now at or near the point of distortion
because they have been overdriven.
Make sense?
But back to the subject at hand; as
amps were turned up, the signals became
more and more distorted. This was due to
overdriving the preamp tubes into saturation.
Ironically, at the time amp manufacturers
were shooting for “distortion free” amplifiers, and although the larger Fender and
Marshall offerings came close to that claim,
few guitarists of the time could afford them,
instead relying on smaller combo amps.
Many of these diminutive tube monsters are
highly sought after today and sound great
without ripping your head off. A smaller,
low-wattage amp will overdrive very quickly,
giving you those distorted sounds easily. A
100-watt behemoth needs to be cranked to
ear-bleeding levels to even begin to breakup
unless it is equipped with a massive preamp
stage and possibly a master volume circuit to
keep the levels reasonable.
The old adage of “tubes rule” still holds true,
and we’ll cover more on this subject in the
months to come, as well as looking at other
ways to get distortion. We’ll also take a look
at some of the pitfalls inherent in this quest
and look at some their solutions. Until then,
remember to pick that guitar up and play,
and that if you can believe it, you can do it.
Gary “Sarge” Gistinger
President, Creation Audio Labs, Inc.
creationaudiolabs.com
As a preamp tube is given more signal
it begins to saturate. In this case, as the
As we have learned, overdrive causes distortion, but it can also be the source of unwanted noise. The introduction of gain into the
discussion would now be appropriate. Gain
is often incorrectly thought of as volume,
and in some cases, an increase in gain will
also have a resulting increase in volume. But
gain must be viewed in relation to where it
is in the circuit. In the majority of situations,
gain is near the beginning of the signal path,
found at the front of most pro audio gear
and as part of the front stage of most amps.
Gain must also be considered relative to the
ability of the corresponding stage where it
is applied. The capability of a preamp tube
to lift signal is certainly smaller than a power
tube, so an increase of the same amount of
gain to a preamp tube may cause that tube
to saturate, while the power tube may simply
get louder.
