I do remember the output tubes
(KT88s) being sold in matched pairs.
They were supplied to us by Dynaco
already "matched." There were no test
sheets included; the boxes were glued
together indicating they were a
matched pair.
Maybe the fact that they were
being sent to a service center and
would more than likely be used in a
customer repair instead of being sold
over the counter to the owner directly
had something to do with it.
As a service tech, I soon realized
that matching the tubes could
become compromised if certain
components in the circuit associated
with the output stage were off value
or not exactly matching — especially
in the MKVIs when comparing the left
and right channels. No matter if the
tubes were "matched" or not,
components that were off value were
more of an issue to performance
quality and tube life.
In my opinion, the best way to
make sure that matching the tubes is
maximized is to verify the
components — especially any carbon
resistors and capacitors in the
associated circuitry. Adjust the bias
properly and always use eight ohm
load resistors paralleled to the inputs
of a dual trace scope with the input
signal coming from a sine/square
wave generator to see if — all
combined — the circuitry end product
is truly matched. I use this technique
when servicing Marshall and Fender
heads to this day, and it always gives
me good results.
How did the tube supplier "match"
the tubes? I’m not really too sure, but I
would assume that they used some
sort of jig setup where they measured
certain parameters of each tube from
a large batch of tubes and then put
them together as pairs as close as they
came to having similar
values/readings. It would be
interesting if any of the readers could
shed some light on this matter, and
maybe describe the kind of setup that
tube manufacturers actually use for
matching vacuum tubes.
David Asselin
September 2013 49
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