An audio equipment designer,
consultant, and long-time guitarist, 34-year-old R. Aspen Pittman is the president of Groove Tubes [13994 Simshaw, Sylmar, CA
91342], which supplies tube sets based on the philosophy that matching their functional
characteristics optimizes an amp's performance. He was the sales manager at Los
Angeles' Guitar Center for four years, director of marketing and artist relations for
Acoustic for seven years, and through his firm, Aspen & Associates, he has served as
equipment consultant to Gauss, Cerwin-Vega, Jim Kelley, and groups such
as Weather Report and the Doobie Brothers. Pittman's
consultant in the preparation of this article was J. Hunt Dabney of JHD Audio[1370 Logan
Ave., Costa Mesa, CA 92626]. Dabney developed the test methods and provided additional
technical support to the company by serving as its head engineer.
By R. Aspen Pittman
both today and yesterday are linked in many cases by one piece of equipment: a tube
amp. In fact, the tube amp is currently enjoying its greatest popularity with
musician, even though there have been great strides in transistor amp technology over the
past 20 years. Guitarists prefer tube amps.
Why do designs built around tubes sound different from those following the
solid-state approach? Simply, tubes work differently. What follows, then, is
an explanation of their construction, function, and applications.
Tube Construction And Operation
A tube is an electronic device consisting of
a minimum of four active elements: a heater (filament), a cathode, a grid, and a
plate. All of these are sealed in a glass enclosure with its air removed -a vacuum-
to prevent the parts from burning. The filament is heated in order to warm the
cathode. Once heated, the cathode begins to emit electrons, which flow from the
cathode (which is negatively charge) toward the plate (which is positively charged).
The grid's purpose is to control this flow. If the grid were absent, this movement
of the electrons would be uncontrolled, much like water rushing from the faucet that's
opened all the way.
Theory of Operation
When a small signal is applied to the grid,
it causes a larger change in the current that flows between the cathode and plate
accordingly. In effect, it acts as a valve. A portion of the amp's electronic
circuitry, the grid bias control adjusts the proper voltage setting of the
grid. The amount of bias varies from tube to tube, depending on its sensitivity, and
it acts to keep the tube "idling". When the grid bias is properly set,
the tube is balance to the circuit, and therefore produces a clean, powerful signal
(proper biasing also extends the life of the tube).
For optimum performance, the bias setting should be checked whenever power
tubes are changed-preferably by a qualified technician using an oscilloscope. A bias
adjustment is a relatively simple operation, and can be performed for a minimal bench
charge (typically $15.00 to $20.00). Some symptoms of improper bias setting include
the amp running too hot, excessive hum after it's been on for a short while, or distortion
that just doesn't sound right. The amp doesn't necessarily have to sound bad for its
tubes to be incorrectly biases, and these symptoms may indicate other problems.
However, if your amp is behaving in an extraordinary manner, a trip to the shop may head
off damage to it, regardless of the cause.
Tube Functions In The Amp
Let's look at a common example of how a tube
works in an amplifier. Imagine a small guitar amp with no volume or tone controls:
just a guitar input, one tube, an output transformer, and a speaker. The guitar's
pickup produces a small voltage, the result of the string vibrating in the pickup's
magnetic field. In general, this signal is applied to the grid, which in turn causes
a large current flow from the cathode to the plate. Thus, a correspondingly large
voltage now appears at the plate. This plate is connected to an output transformer,
which matches the tube's output impedance to that of the speaker. (Because there is a
great disparity between the impedance of the tube amplification circuit and the speaker,
the transformer must act as a buffer to interface the two components). Thus, a
small, low-power signal from a guitar's pickup can produce a high-powered signal to drive
Naturally, amps don't all sound alike. This is due to variations in the
type of tube that is used, the quality of the tubes, and the specific circuit design of
the amp. In other words, some tubes amplify more than other tubes under similar
conditions. Also, the amount of gain a tube produces varies with the circuit design.
This is why different makes of tube amps can sound very different, even though they
use the same tube types. Additionally, certain amps use completely different types
of tubes. A good example of this is the English-type Marshall using European EL-34 tubes in its
power amp section, compared to the U.S. type, which employs American-made 6550 power
tubes. The U.S. and English styles sound and play very differently, reflecting the
character of their power tubes. That is, the English EL-34 tubes yield more
distortion than their American counterparts, although they produce roughly the same amount
of volume. With internal bias modification (which mostly involves changing some
resistance values), any U.S. Marshall amp can use European EL-34s, and
Larger and more complex amps have many
stages of tube amplification: preamp stages, signal-processing stages, and power amp
The preamp stage is much like a mixer in a PA system, which must amplify an incoming mike
or guitar signal to line-level strength before the signal can be processed
with effects for tonal shaping. Likewise, a tube amp must preamplify a guitar's
signal so that it can be further processed. This is the first gain stage of the
stages. An example of a signal-processing stage is the reverb
section, where the signal is diverted through a reverb spring system and then returned by
another gain stage, and finally blended with the original signal. Tone controls and
second gain stages (often employed for an overdrive effect) are other examples of
Power amp stages.
The power amp section takes the preamp's signal and amplifies it many times to a level
that can drive the speakers. All tube amps with power rating of 10 watts or more
employ a push/pull power amp. This means that the power tubes work as a
team to amplify the signal and drive the speaker. (Practically all transistor amps employ
a push/pull configuration as well.) The output tubes all share in the sound, so for
maximum efficiency it is desirable to use tubes that operate as similar as possible.
Also, for efficiency use power tubes of the same make-manufacturers' specifications for
tubes bearing the same stock number may vary over a broad range. And, if one power
tube is bad, it is advisable to change all of them. Having one fresh,
powerful tube and three old ones, for example, can create an imbalance in the push/pull
effect, resulting in inefficient operation. The power amp section is only as strong
as its weakest link. So, if one tube out of four is faulty or varies from the
others in its performance character, the overall sound of the amp will be limited.
The process of output tube matching dates back almost as far as tube amps.
The military began matching certain properties of tubes to produce longer field
life and higher performance. Later, top audio companies such as McIntosh developed a
system to match power tubes for use in their audio amps, and would guarantee performance
specifications only when their matched tube sets were used. Unfortunately, it is
impossible to specifically manufacture matched power tubes because of the
mechanical nature of the device and the extreme operating temperatures that exist within
the tube (around 700 F). However, once the tube is made, it can be performance
tested for various parameters and matched into sets with identical characteristics.
Since a tube is a mechanical assembly of
parts that forms an electronic device, it is subject to some mechanical problems and
limitations. Tubes wear out in direct proportion to how hard they are worked (due to
the circuit design) and how often and loud you play your amp. Vibration and jarring
shorten the useful life of the tube as well. Ideally, a tube could be built so that
no vibration existed between its mechanical elements. However, adverse tube microphonics
can be a big problem when the elements of the tube rattle or ring, producing a signal all
by itself. A tube with this problem is unsuitable for use in music amps, much like a
faulty guitar pickup or a bad microphone is undesirable for most musical purposes.
When To Replace Tubes
So, when should you change your tubes?
Chances are, your power tubes are worn out when your amp starts sounding
weak, lacks punch, makes funny noises, has its power fading up and down, or loses highs or
lows. If your amp squeals, is excessively noisy, loses gain in one channel, hums,
lacks sensitivity to touch, or generally feels as if it's working against you, a preamp
tube could be malfunctioning, and is in need of replacement. In both cases, though,
the tubes may not be at fault. Unless you are skilled in specific troubleshooting,
regard the high-voltage circuits found in amplifiers as extremely dangerous. Take
the amp to a professional for diagnosis and repair.
Unfortunately, you can't simply pull your tubes out and take them to the
drugstore or local electronics outlet and evaluate them on one the tube-testing machines
designed for TVs. This is because of the high voltage levels at which guitar
amplifier tubes are driven. Amp tubes can be powered with 450 volts or more, whereas
the testing machine provides only about 150 volts. This difference can completely
foul up a diagnosis. Tube-for-tube replacement and a before-and-after comparison is often
the most reliable test.
Good-sounding, non-microphonic preamp tubes are the exception, not the rule.
Quality preamp tubes along with matched sets of power tubes are a little harder
find, and you may par more when you do locate them. However, you can expect improved
sound and longer life, so there is a payoff.
Tubes, Transistors, and Distortion
No tube primer would be complete without an
explanation of how tubes distort in a way that is different from transistors. Tubes
distort uniquely because as the signal emitting from the plate approaches its maximum
potential, the tube gradually begins to react less and less to the original input signal.
This results in a types of compression of the signal, and produces a soft clipping.
Clipping occurs when the input signal increases but the maximum power has been reached.
Thus the signal becomes cut off, or clipped. Transistors, on the other hand,
react exactly the same to the input signals rights up to their maximum power; then
they stop quickly, creating a sharp clipping. These different types of clipping
produce different series of harmonics (overtones). When the transistor amp clips, it
produces more odd-order harmonics (and in its worst case can sound hollow and dry),
whereas tube distortion produces even-order harmonics. Tube distortion sounds
warmer. It should be mentioned that various types of transistor and tube distortion
are possible, depending the amp's design.
In the case of a tube amp, preamp and power amp tubes have different
distortion characteristics due to the difference in both their tubes and their circuit
design. For example, relying on a master volume distortion circuit by itself will
yield less sensitivity to variations in a player's touch than if the amp is
attenuated (has its volume limited) after its power stage (that is, with a power
attenuator). This is due to the contribution of the output transformer to the sound
of the amp and also to the difference in sonic qualities between different power tubes
compared to preamp tubes. Leaving some of the distortion to the power amp section
rather than relying mainly on the preamp section gives a broader range of sensitivity.
In addition, the nature of the tube allows the player to vary his touch, producing
different tonal responses from the amp according to the manner in which he plays.
There are many variables in tube amp design, and each has its characteristic
sounds and quirks. Regardless of what type of amp you use, you will find that like
strings on guitars or oil in an automobile, tube do wear out. Amps are not
maintenance-free, and as they age, they undergo changes. The tubes are subjected to
wear and tear, some of the electronic parts lose their initial properties, and pots and
jacks get old. Bad tubes can cause premature failure of other parts, such as the
output transformer, speaker, and other vital components. If your amp sound bad,
weak, or otherwise not up to par, don't just hope the problem will go away. Get it
fixed. Keep on top of the maintenance, replace the tube when necessary, and get the
most form your amp.