Do you need matched output tubes? Is
it worth the extra bucks? Are unmatched
tubes just as good? There are at least
three major things to match in output
tubes that affect the sound of a pair of
tubes in guitar amps. “Matched” may
mean a number of things. And you may or
may not want perfectly matched tubes.
Today’s tubes vary a lot. The vagaries of
mechanical tolerances, materials chemistry
and perfection of vacuum translate to variations
in the DC bias point
of the tubes, the
AC signal gain of the tubes, and the distortion
of the tubes.
Most of us are familiar with biasing to
eliminate crossover distortion. We adjust
the grid bias voltage so a little DC current
flows with no signal. That current
lets the two tubes hand off signal cleanly
as the signal changes from positive to
negative, and it is what you set when
you DC-bias your amp. The tubes not
only need to have a little current flowing
with zero signal, but that current also
needs to be the same for both tubes.
That’s DC matching – selecting two
tubes which happen to have the same
current for the same bias voltage.
Some amplifiers have a bias adjuster for
each tube that can effectively DC-match
un-selected tubes. This is important
enough that I put per-tube bias controls
into the Workhorse amplifiers with LED
indicators so you can DC-match your own
tubes in the amp. If your amp doesn’t
have separate per-tube biasing, you may
need to pay the extra money for matched
Each tube is also responsible for amplifying
its own half of the signal smoothly.
The AC gain (or “transconductance”)
of the tube does this. If the AC gain of
one tube doesn’t match the gain of the
other tube, the result is distortion as
well. Unfortunately, most tube testers
can’t measure gain at full power, so gain
matching on a simple tube tester may not
reflect what you get in your amp. On the
bright side, this is asymmetrical or evenorder
distortion, and human ears perceive
a little bit of this as a “sweetening,” not
as distortion at all.
A high AC-gain tube on one side of a
pair will hit overload more quickly than a
lower gain partner will. A lower-emission
tube may simply run out of electrons to
pass, and may also overload earlier. Both
of these may cause a lower clean power
output from the amp.
Little quirks of the elements inside a
tube can cause another form of distortion
– little bends and twists of the signal
within what is otherwise a linear part of
its operation. This is the kind of difference
that helps make an EL34 sound different
from a 6L6. A lesser degree of this difference
exists between any two tubes of
the same type.
Distortion matching can be done, but this
is a very muddy area. I’ve talked to some
tube suppliers who claim to do this, and
they have all been reluctant to discuss this
area of matching. I’m guessing that most
of the “distortion matching” is really a
combination of DC and AC gain matching.
Some suppliers provide tubes matched by
a grouping scheme, the idea being that if
the tubes in your amp are from the “#5
group,” you can replace them with new
pair from the same group without re-biasing.
This is O.K.-ish, but since the tubes
in a group like this have a range of values
in them, don’t count on perfect matching
from set to set. And bias is not directly
tied to distortion, so I can’t see how the
grouping system can do anything about
overload or distortion.
Automated tube curve tracers exist that
can run the tubes at full power, and
record what the tube does at every point
over its entire range. This kind of testing
is great for hi-fi tubes, and is likely to be
the future of tube matching and testing.
Unfortunately, these are quite rare today.
Tubes also drift with heat and age. Tubes
that were matched when you bought
them can drift apart. You can see this
graphically in the Workhorse amps by
looking at the LED indicators after you
have adjusted the bias current. The tubes
will drift in and out of perfect bias slightly
– which is actually O.K.
After all that, the answer to whether you
need to buy matched tubes is a resounding
“maybe…” You need equal DC bias
currents per tube. If your amp is not one
that lets each tube be adjusted independently,
you need to buy matched tubes.
Beyond that, your ears may or may not
like the slightly sweetened sound of
imperfectly AC gain matched tubes better.
As a musician, you need to check it with
your best test equipment – your ears.