On May 13, 2:13=A0pm, exray <radioex...@[EMAIL PROTECTED]
> wrote:
> To beat a dead horse...You have to match the bias/amplification curve to
> truly call them matched. =A0Equivalent at the "Hickok Parameter" is only
> one point along that curve.
> On the other hand, looking identical or having the same brand name is
> enough for many ebay sellers to call them "matched".
Mpfffff.... You are hardly beating a dead horse - you are making a
poorly understood point a bit clearer!
Transconductance is one parameter. And *some* Hickok testers are
capable of being used to match tubes. But not alone.
Tubes must match (typically to within <5% of each other) in:
Transconductance
Filament Current
Plate Current
Given bias voltage - which as it varies in equipment should also be
varied to verify that the match continues. i.e. a CURVE of matches at
various bias points.
Assuming a Hickok 539-series tester which has taps for Plate &
Filament currents and bias-voltage input, two ammeters and one
regulated, precision low-voltage DC power-supply are required for
proper matching. Yes, the Hickok has an internal bias supply, but it
is fiddly at low voltages and imprecise such that 5% at 0.5V (for
instance) would be near-impossible to discern.
The tube must be "hot" for at least 15 minutes at the proper filament
voltage, and under load for at least 10 minutes as well (The Hickok
testers listed have a Test Lock function for this purpose).
Note also that even a Hickok tester will not apply maximum B+ to all
tubes under all conditions, so even such matches are of limited
utility.
Those who match tubes in bulk will have testers set up for just that
purpose (assuming that they are actually doing the work vs.
pretending). These testers will apply full rated operating voltages in
order to determine a valid match. It's not rocket-science, but it is
not trivial either to get good results.
Note also that "matched" does not mean "especially good", only that a
given quantity of tubes matches on the parameters listed. Whether at
the high or low end of the "good" scale is not relevant to the Match.
Neither is brand, age, manufacturing date, serial number or much of
anything else along those lines. A Sylvania from 1960 can be matched
to a Svetlana from 2007 with perfect validity as long as the
parameters in question are met.
Note also that after some amount of use in the actual amplifier, the
tubes will age differently and that match will drift some. The amount
of drift will be a function of initial tube quality, actual use, and
the piece of equipment in which they are used.
Note that most equipment is pretty tolerant of some pretty broad mis-
matches as long as the tubes meet the minimal standards required by
the manufacturer. Manufacturers cannot count on the end-user to
"follow the rules" on these sorts of things and plan accordingly. Yes,
an amp may perform better with matched tubes, but that difference is
for the most part at the margins. With vintage radios, the performance
difference will be barely discernable as long as... and so forth.
Peter Wieck
Melrose Park, PA
|
