The 'real' catch is that these tubes were originally designed to work at
~16 kHz, NOT 30 MHz. So depending upon internal structure, a tube that
works at 15 kHz may not work at all at 30 MHz. The inter-element
capacitances are balanced out by the neutralization circuit, and these
may differ considerably depending upon who made the tube. These
capacitances have little or no effect at 15 kHz, but ...
A plain emission tester will tell you if the tube is 'dead' at DC, but
all that means is that there is still some coating on the cathode and
that none of the major elements are shorted. I don't know enough about
tube science to explain WHY, but in this service these tubes lose gain
at the highest frequencies first, and progress downwards. A tube can
produce 85W on 80M and 0W on 10M. But they weaken from the 'top down'.
The best thing you can do to prolong tube life is to put a small fan
either on top of, or on the back of, the PA cage, blowing UP or OUT. It
doesn't have to move much air to make a major difference in reducing the
heat in the cage. A small computer fan that runs silently is fine.
Good PA tubes will run for literally years as long as you don't take 10
minutes to tune them up because you have the wrong antenna selected. I
was able to get over two years in full power RTTY service back in the
60's. Typical SSB or CW should last 5 - 10 years. The PA tubes in my
'daily driver' T-4XB are over 10 years old.
Drake matched final tubes by selecting according to cathode current vs
bias voltage at idle. They can be matched in the transmitter by
installing ONE tube, and setting the Bias voltage for 35 mA of cathode
current. Then replace that tube with successive tubes, without
adjusting Bias, and note the current drawn by each tube. Pairs are two
tubes that are closest to each other in current drawn. If most of the
tubes tried draw 40 mA in this situation, install one of those, reset
bias to 35 mA and try again.
Obviously the meter in the T-4X(any) isn't all that accurate, (it DID
cost around a dollar new!,) so it's best to measure the voltage drop
across the 15 ohm resistor for the socket being used. Measure the
actual resistance and then measure the voltage across it for 35 mA.
From a practical standpoint, two new tubes from the same manufacturer
are probably close enough. A reasonable target is to find a pair that
are within 5% of each other at idle current.
Matching IS important, because these tubes are being run VERY hard. IF
one tube is only carrying 90% of the load, the other is doing the 110%,
adding insult to injury. And third-order distortion products go up rapidly!
Best brands seem to be Sylvania, Zenith, RCA and GE, in that order.
Other brands may or may not neutralize properly, depending upon who
actually built them.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-B, C-Line&
TR-4/C Service Supplement CDs
<www.k4oah.com>
Curt wrote:
Experience with finals on my T-4XB prompts some questions. Can a
simple emissions tester be used in any way to estimate 6JB6
performance in circuit? Can that tester be used to "match" 6JB6
tubes? Evidently these transmitters were pretty picky about what PA
tubes they would tolerate, with differences between manufacturers
being an issue.
Another thing I'm wondering is whether most of the 6JB6 tubes for sale
these days are used and maybe already worn out. One pair I bought off
the web were pretty useless in my T-4X. Any source of known-good 6JB6s?
Changing out the finals and then neutralizing them is a pain in the
rear I'd like to do rarely, would like to somehow know that the
replacements are better than the soft tubes being replaced.
73, Curt KB5JO
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