[Johnson] Not only no but HELL NO!

[Glen Zook]

What is usually not "right" in the circuit is that the
transmitter was NOT designed for the 6146B!  Now,
there certainly are individual examples of
transmitters that were designed for the 6146 / 6146A
that work fine with the 6146B.  However, identical
models of a transmitter will not often work the same
with the 6146B.  For every transmitter that works with
the 6146B without modification there are numerous
others that will not work correctly!

The 6146B is NOT the same tube as the 6146 / 6146A /
8298.  It is unfortunate that RCA decided to call the
6146B by that nomenclature.  The 6146B does have the
same base configuration but definitely does not have
the same electrical configuration.  RCA originally
said that the 6146B could directly replace the 6146 /
6146A with an increase of 33% in input power. 
However, although most transmitter manufacturers
originally put out information that this was "OK" to
do, they soon had to rescind this information.

The "usual" problem with using the 6146B in place of
the earlier versions is that it often "takes off". 
That is, the 6146B puts out all sorts of VHF
parasitics which often cause the tube to overheat and
eventually self-destruct.  Even if the tube doesn't
self-destruct, the transmitter usually puts out all
sorts of parasitics.  This is often evident when the
transmitter is checked on a dummy load and then on an
antenna.  Since a dummy load is not frequency
dependent, all will seem to be fine.  Power output
will be very good, etc.

Then, when the transmitter is hooked to an antenna
often the reflected power ("SWR") will be "way off". 
This is because the antenna is resonant in a narrow
frequency range and the transmitter is putting out
power at frequencies that are well removed from the
desired frequency.

A very good check on whether or not an individual
transmitter will work OK with the 6146B is to
neutralize the transmitter (unfortunately, not all of
the older "boat anchor" transmitters have this
capability).  If the transmitter will not neutralize
properly, then the 6146B tubes should be immediately
replaced with the non-"B" versions.  If the
transmitter does neutralize then it needs to be
watched for at least several hours of actual
operation, if not for several days of operation.  If
the neutralization changes, then again the 6146B tubes
need to be replaced with the non-"B" versions.

If the neutralization "holds" for several days, then
there is a very good chance that the particular
transmitter is "happy" with the 6146B tubes.  Now, I
have seen individual model transmitters that had
serial numbers within 10 of each other.  One was very
happy with the 6146B tubes and two others wouldn't
work at all without all sorts of parasitics, etc.

Towards the end of their production, Heath's primary
supplier of 6146 tubes stopped production of the
earlier versions and went only to the 6146B.  Heath
originally said not to use the 6146B in their
equipment.  Then, they found a single supplier that
was willing to test individual 6146B tubes.  Those
specially "culled" 6146B tubes were provided by Heath.
 However, Heath definitely said NOT to use any 6146B
tube produced by RCA in the equipment designed for the
6146 / 6146A.  The RCA tubes were "top of the line"
and they did not "tend" to work like the earlier
version tubes.

In 1964 the U.S. military changed the standards for
the 6146W from the earlier versions to the 6146B. 
However, there was nothing in the nomenclature to
indicate that the change had been made.  Those 6146W
tubes made with a code date of 1963 and earlier are
definitely 6146 / 6146A versions.  Those made with a
1965 or later code date are pretty much 6146B
versions.  Those made in 1964, some are one version
some are the other!

Collins originally said that it was "OK" to use the
6146B in the "S-Line" equipment.  Then, when the
military started using the 6146B equivalent 6146W
tubes, the neutralization circuitry in the 32S-1,
32S-1, 32S-3 series, and the KWM-2 series equipment
started "burning up".  Since the military wanted to be
able to use the later version tubes Collins had to
make a production change in the equipment and make a
retrofit kit available to modify the neutralization
circuit.  After making the modification, either the
older versions or the 6146B can be used in the
equipment without any problems.

Now, even though the transmitters designed for the
6146 / 6146A appear to operate correctly with the
6146B I am very hesitant to use the 6146B.  Even if
the transmitter neutralizes fine, I still use a GDO in
the wavemeter mode to check for VHF parasitics.  More
often than not, the 6146B tubes are putting out
parasitics even though the transmitter "appears" to be
operating normally.

Anyway, use the 6146B tubes if you must.  But, please
be aware that the use of the 6146B in transmitters
that were not originally designed for them can, and
often does, result in all sorts of problems.  Some of
them are immediately evident whereas others (like
parasitics) can be waiting to bite you in your
anatomy!

Glen, K9STH


--- Gary Schafer <garyschafer@comcast.net> wrote:

I have used 6146B's in many transmitters that
originally had 6146s and have never had a problem.
Problems usually originate from something not right in
the circuit. Poor ground, bad component etc.

=====
Glen, K9STH

Web sites

http://home.comcast.net/~k9sth
http://home.comcast.net/~zcomco

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