George -
Decreased output IS a symptom of weak PA tubes. Usually it falls off on the high bands first, and
eventually affects the low bands as well.
A tube tester is essentially useless in this case, because of the difference in the test conditions
as compared to the actual operating conditions. About all the tube tester tells you in this case is
whether the filament is good or not! You can determine that by either looking or touching the tube
envelope. Is it hot?!?! :-)
Substituting known good tubes is the only way to really evaluate this sort of problem. Tubes further
back can have a slight effect on output, but usually the Driver and PAs are the culprits. Alignment
typically does NOT change on it's own, so if it was working and no one has been 'fixing' it, the
alignment is ok. Neutralization is the only adjustment necessary, and only when PA tubes are
changed. Even with the same brand, neutralization needs to be checked. Simply loading below @ 200 mA
of plate current, carefully dipping the PA, and then watching the output as you tune the PLATE
control either side is all it takes. IF the output decreases either side of the PA PLATE current
dip, neutralization is ok. If output goes UP, neutralization is needed.
Changing PA tube brands does necessitate neutralization for maximum output, but it's suspicious that
you get about the same output with either set. The fact that you have one tube 'weaker' than the
others is not good. You really should use matched tubes. Some argue that it's not necessary, BUT
these tubes are being operated right at their limitations. Overdriving them only increases
distortion produced by the PA, which is already within FCC specifications for Purity of Emissions
only because of being 'grandfathered' in. There are three tubes, hopefully sharing the maximum load
equally. If one is weaker, it's not carrying its share, meaning that to get the same output level
the other two tubes are carrying even more than their 'maximum' capability.
In fact, Drake issued a field change, coincidental with the FCC specification changes regarding
Purity of Emissions, recommending that the loaded plate current for the TR-3/4 should be limited to
a maximum of 350 mA on 80, 40 and 15M. 20 and 10M can be tuned for maximum output as described in
the manual.
I would suggest first checking the voltages from the power supply, especially if it still has the
original electrolytic caps in it. The AC-4R rebuild kit is an excellent idea for any of these
supplies, as they are ALL well past the expected lifetime of the caps. In this case, specifically
check the Screen voltage of the PA tubes, which is the +250V bus. Also, check the three Screen
decoupling resistors, R42, 43 and 44, (68 ohm, 1/2W,) the three Cathode resistors, R119, 120 and
121, (15 ohm, 1W,) and the meter shunt resistor R45, (1.5 ohms, 1W). If R45 is off value, check the
actual current vs the current shown on the meter. If it is incorrect, replace and select a resistor
for R131 to correct. If an early TR-3, <s/n 3000, R45 will be a selected resistor(s) equaling 0.8
ohms at 2W.
If the +250 is low, don't forget the multisection electrolytic in the TR-3. A replacement is
available from hayseedhamfest.com.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
George Pagels wrote:
My TR-3 has recently had a decline in output. When tuned for max output on a Bird Power meter,
Plate Current is about 300mA, and output is about 115 Watts on 80, 40 and 20 CW tune-up. It used
to be able to tune up to 450 mA Plate Current. On an Eico 667 tube tester, all final tubes (12JB6)
appear to have reasonable emission (one is a little less than the other two, but comparing to
another set, they are all a little light). I substituted a different 12BY7 driver with no
difference. I also substituted some sylvania 12JB6 final tubes for the RCA’s that were originally
in the transceiver, and got about the same output, although tuning up was a little difficult. I
noted a label in the radio stating that substituting a different brand of final tubes necessitates
realignment, and before I get into that I would want to know if that just entails the adjustment
of C76, and if it would be worth trying the sylvania tubes and realigning to see if that improves
the output.
I wonder what to look for next?
Thanks, George K9AAL
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