Mike -
I haven't had to do that to one, but I would start with 5 pF, then 10 pF
if necessary. Be sure they are at least 2 kV caps, which is going to be
the hard part. They also need to be NP0 or 0 TC.
The trimmer cap is 2.7 - 8.4 pF, or a delta-C of 5.7 pF. The T-4XC had
a 6 pF parallel cap when new, so depending upon what you can find in an
NP0 you could go with either a 5 pF in addition to the 6 pF or replace
the 6 pF with a 10 or 12 pF. It's probably not going to need more than
that.
The Sylvania/RCA 6JB6 takes about 10 pF typically to neutralize. I
don't have any data for GE 6JB6's, but the formula is Cn (Neutralization
cap) / C (Input Bypass cap) = Cgp (Grid - Plate cap) / Cgf (Grid -
Cathode cap) - or - X / 500 pF (C50 in T-4XC) = 0.4 pF (2 x Grid - Plate
for RCA 6JB6) / 30 pF (2 x Grid - Cathode for RCA 6JB6) - or - X = 500
(0.4/30) - or - X = 6.66 pF. There are relatively large stray
capacities that are not included here, such as tube socket and wiring,
but if you can find cap data for a GE tube you should be able to get in
the ballpark.
Perhaps someone else has direct experience and can get you closer.
When you are experimenting, I would suggest checking the neutralization
first on 20M as the null is considerably easier to find (broader) than
on 10M. Once you get neutralized on 20M, it will take only a very
slight adjustment to refine it on 10M.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs
<www.k4oah.com>
WA3WOM wrote:
A Drake technical bulletin issued when Drake could no longer get
Sylvania 6JB6s and they had to begin substituting GE brand ones, says
the following:
/The T-4X series transmitter uses a 6 pf @ 2KV padder capacitor (C-48)
across the neutralization capacitor (C-49). You must experiment with a
small value capacitor across the existing padder C-48 to achieve
proper neutralization./
I have several sets of matched GE and RCA brand 6JB6s that I am having
trouble neutralizing. Anyone have any idea what "_small value
capacitor_" means in this context?
TNX ES 73 DE MIKE WA3WOM K
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