Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Chuck -
Those numbers are about what you see on a TV-7/D tester and are
equivalent to transconductance, or "gain" of the tubes.
The procedure used by Drake was to plug them into a test set that
duplicated the circuit in the T-4X final, set the bias for 35 mA, and
then plug in one at a time however many they wanted to test/match.
Their numbers, not related to any quantitative measurement, were
typically in the 35 - 40 range and were marked on the top of each tube
with a felt marker. Neither method has any direct relationship to their
performance at RF!
Matched tubes are not really _necessary_, but mismatched tubes can have
3rd order distortion products considerably higher than a matched set.
This can vary from a few percent up to tens of percent depending upon
the degree of mismatch. If you consider a pair of tubes in parallel,
say that one tube draws 30 mA at a given bias voltage, and the other
draws 40 mA. The plate meter shows 70 mA total, but one tube is
considerably different from the other. When drive is applied, one tube
will also draw more plate current than the other, so one tube may be
providing 60W of the output and the other 70W. Distortion increases
rapidly when tubes are driven past the optimum, which Drake measured at
175 mA per tube. So if one tube is drawing 150 mA and the other is
drawing the other 190 mA, distortion can be pretty bad! Your nearest
neighbor ham probably won't appreciate it...... Of course it gets MUCH
worse if you are driving an amp, since the amp contributes its own
distortion on top of that from the transmitter. This tends to widen
your circle of "fans"! :-)
Mixing "brands" of tubes is also not prudent, even if "matched" DC wise,
because different internal construction can make it difficult to
neutralize the final over all bands. It works ok a lot of the time, but
if the neutralization seems hard to optimize, or better on one band than
another, this could be the cause. Drake used both Sylvania and RCA, but
always paired, or tripleted(?) them by brand. Sylvania was the only
sweep tube manufacturer that published data for linear RF use.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>
Chuck Grandgent wrote:
looks like I found a "TR4 matched set" of 6JB6s in the garage that I
got back in '99, looks like from WB2LHP. Two Sylvania's and an RCA.
Slips of paper for each, two are marked 109, one is marked 110.
Those numbers are exactly what ? What is the typical variation seen
among NON-matched 6JB6's ? What bad stuff might one see when not
using matched sets ?
Would be fun to know what I paid for them back then. Of course those
dollars were worth more then than now.
BTW RFParts didn't have any Sylvania/RCA last week, but did have GE's.
Chuck, K1OM
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