Mike -
The 6JB6A is identical to the 6JB6 with the exception that the "A"
version is spec'd for 11 second filament warmup. This was a spec that
came along because many TV sets were series filament strings, (which is
also where the 3, 16, 19, etc. filament tubes came from,) and tubes
warming up at different rates stressed the filaments of other tubes.
Either is suitable for the Drake 4 Series transmitters.
I don't think Sylvania ever made an "A" version, the only ones I've seen
are RCA and GE among the 'preferred' brands. One caveat, some of the
later manufacture tubes were too tall to fit into the PA cages of the
Drake transmitters. Despite the fact that the 6JB6 was specified as
2.91 - 3.17" seated height, and the 6JB6A as 2.875 - 3.125" my
experience has been that just about all of the non-A tubes were toward
the smaller size of their spec, while the "A's" seemed to be toward the
high end of their spec. Just be aware that some tubes are too tall to
fit. I don't recall what the exact height of the cage is, but it's
right at 3 ".
Mutual conductance matching is good, but the final test is the bias
voltage vs cathode current. Just the idle current exceeds the rated
plate dissipation rating of the tubes, so you want them to be as close
as possible. With a fixed bias voltage select two that draw between 32
and 38 mA each.
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>
m coffey wrote:
Although I have been a member of the list for a while now, this is the
first time that I have tried to post to it, so I hope that I have done
this correctly!
My question is that I have seen 6JB6A (vs. straight 6JB6) tubes
listed for sale. I am wondering what difference(s) there are between
the two and if eigther is suitable for use in the T4X series (or
TR4s). Also, is a fairly close reading in mutual conductance ( ie. on
my Heathkit TT1) consider as indicating a true match on the tubes?
Mike, KB1EZL
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