6JB6 test results - LONG

[Al Parker]

The following LONG info was posted elsewhere last week, but thought more
folks might like it.  If you've seen it, just delete it now, there's no new
info in this post.  It's formatted to hopefully be readable as text for
most email recipients.  If anyone requests, I can attach it to an
individual reply as either a *.txt text file, or MSWord *.doc file.
  I will be out of town 9/9 - 9/19, so don't expect an early response.
73, Al, W8UT

====================
There has been lots of speculation recently concerning 6JB6 sweep tubes,
which are used in the Drake “4 - line” transmitters.  With the
encouragement and editorial advice from Dexter Francis and Jim Lockwood, I
have  characterized a total of  19   6JB6 tubes for power, efficiency, and
interelement capacitance.   One reason for this test was to evaluate the EI
brand tubes from Czechoslovakia, which, along with 2 others, were supplied
by Dexter Francis.
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CONCLUSIONS
·- “EI” brand tubes will not work reliably in Drake equipment that requires
3 such tubes.  
·- All other tube brands work fine in 2X or 3X configurations, however.
·- It is possible that brand “EI” tubes would work satisfactorily in 2X
configurations, at least on the lower frequencies.   
·- Tube matching may be necessary, but only if they are more than 10% apart
in plate current.  
·- Cathode resistors and screen resistors should be within 5% (they were a
mile off in my TR-3), so check them accurately, and replace as needed. 
·- Tubes may be matched by juggling cathode R if the tubes are not too far
off.  (I don’t know how far) 
·- Brand used seems to make little difference, except for the “foreigners”.
·- It is easy to measure actual current by voltage across the cathode
resistor.   
·- You can’t tell how good a tube is by looking at it.  
·- Measuring transconductance in a tester may do the job, but doing it
under actual operating conditions, as above, is probably the only reliable
indication.  
   -  however,
·- TV-7 test readings are indicative of tube “good-ness”.
·- High Cgp is a definite no-no, but all domestic tubes measured were very
close, and low, across all brands.
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METHODOLOGY
The details of how I performed my measurements and the raw test data
follow.  The Drake T4XB was used as the “fixture” for testing 19  6JB6
tubes for power output.
The T4XB had not been aligned or modified for the tests, and was
used to feed a 50 ohm dummy load through a URM-120 wattmeter.  Both cathode
and screen resistors matched within + - 3%.  The two original tubes,
Sylvanias, were good, and the better one (more cathode current drawn), was
retained as the “standard”, and the tubes to be tested were mounted in the
other socket.  The baseline tubes could be loaded to over 300 ma total
current, and would put out over 150 watts.  For testing, the indicated
current was held to 300 ma by maintaining plate loading at just under 50%,
while tuning for maximum output.
It was found that some tubes could be loaded to more plate current, 320 -
340, by increasing the load control beyond “5”, however actual output power
dropped when that was done.  Drake’s tuning
procedure calls for tuning for max. output power, and states that this will
occur between 250 and 320 ma plate current. Specification says input of
200w.  Little if any adjustment was required on the driver loading for the
different tubes


Tests were performed at 14.0 mc.  Filaments were allowed to heat for
approximately 2 minutes.  Voltage across the individual cathode resistors
was measured, and the current flow was calculated using the separate
voltage and resistance values.  Power per tube was calculated, using the
measured plate voltage of 650.  Output power was noted on the 0-500 watt
scale with 10w/div.  Only in a few cases was the reading interpolated to
half a div.  Current and power dropped slightly during measurement, and
key-up was used between each tube’s reading to equalize the amount of
“droop” for each tube after key-down.
The test setup will not allow separate measurement of tube output power.  
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DISCUSSION