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EVERYTHING FOR ELECTRONICS
December 2013 7
Getting (Tube) Testy
I've been looking at Bryan
Bergeron’s Developing Perspectives
from last March and letters in Reader
Feedback from Steve Borsher (June)
and David Asselin (September) about
tube matching. I've been in electronics
since the tube days, when I built four
different DIY tube amps. More
recently, I've repaired or rebuilt several
tube guitar amps — both vintage and
newer — and I think I can provide a
little information about tube matching.
First, regarding Mr. Asselin's
question of how manufacturers match
tubes for the sets they sell. According
to the 2013 catalog of Parts Express
( www.parts-express.com), Sovtek and
JJ tubes are tested for equal plate
current on a "state-of-the-art
computerized machine" after a 24 hour
burn-in. Electro-Harmonix matches
plate current after a 24 hour burn-in.
Svetlana matches plate current and
transconductance. Ruby Tubes are
matched for plate current and
transconductance, with the values
marked on each tube.
According to The Tube Amp Book
( 4.1 Edition, 2002) by Aspen Pittman
(founder of Groove Tubes), they use
actual amp circuits in a computerized
test to match their tubes.
Regarding Mr. Borsher's Gold Lion
tubes, Mr. Pittman says that Gold Lion
was a line of premium tubes. Their
KT88 was factory installed in the
McIntosh 75, which many consider
one of the best classic tube amps.
Most tube testers can be used for
tube matching. There were two types
of tube testers. The more expensive
type — such as your Hickok —
measured transconductance. At that
time, I couldn't afford those so I used
the cheaper type, called "emission" or
"cathode emission " testers. These
measured the cathode current of the
tube. Since the cathode current is
primarily plate current, this test would
be similar to the plate current test used
by several present-day tube
manufacturers. Mr. Borsher's "good-bad" tester was probably this type.
Usually, they had a numeric scale
in addition to the good-bad scale, or
Continued on page 73