[Heathkit] More on HW-101 Finals

[Glen Zook]

It is not that simple!  The problems generally show up
first as neutralization.  However, the 6146B series
often require more bypassing and are more critical in
bias voltage.  The tube is not the same as the earlier
versions.  RCA originally said that the 6146B was
"backwards compatible" with the earlier versions. 
Unfortunately, this did not "pan out".

To get some idea how to change the neutralization
circuit you might look at the Collins S-Line including
the KWM-2 series.  The 32S-1, 32S-2 and the earlier
versions of the 32S-3 series used a particular
neutralization circuit which included a ceramic
trimmer in the final amplifier neutralization.  When
the military changed the specifications on the 6146W
in mid 1964 from the 6146A to the 6146B because RCA
had said that the 6146B was backwards compatible no
change was made in the nomenclature.  That is both the
6146A and 6146B equivalent tubes all "said" 6146W. 
Unfortunately, the ceramic trimmer burned up in
virtually all of the Collins transmitters when the
6146B types were used.

The result was that Collins had to redesign the
neutralization circuit on all transmitters that were
produced after they realized the problems with the
6146B.  Also, Collins came out with a modification kit
to "retro-fit" all of the transmitters that were
already in use.  The way to tell if the transmitter
can use the 6146B is that the neutralization capacitor
is an air variable.  If it is a ceramic trimmer then
the 6146B must NOT be used in the unit.  However,
there are also other changes in the neutralization
circuitry.

Yes, it is possible to modify most transmitters which
were originally designed for the 6146 and 6146A/8298
to use with the 6146B/8298A.  However, the amount of
modification required can range anywhere from fairly
simple to fairly complex.  Transmitters like the Heath
DX-100 require a substantial amount of modification
because there was no neutralization circuitry in them
to start with.

Many boat anchor transmitter owners wish to keep the
equipment as close to original as possible.  Others
are not that comfortable redesigning the unit.  Since
6146, 6146A/8298 tubes, and 6293 tubes are still
common most owners don't want to modify their
transmitters.  Remember that millions of 6146A/8298
tubes were manufactured to use in commercial two-way
FM units and are still available in large quantities.

Also, in many rigs, especially the Heath HW-100,
HW-101, SB-100, SB-101, SB-102, SB-110, and SB-110A
the heaters ("filaments") of the final amplifier are
wired in series so that they "work" on the 12.6 volt
system.  Those tubes can have their heater circuits
rewired in parallel and the 12 volt equivalent of the
6146 used.  That is the 6883 and the 6883A/8032. 
Those tubes are almost "a dime a dozen" since they
were used in even more commercial two-way FM
equipment.  Now the 6883B/8032A/8552 is the 12 volt
equivalent of the 6146B.

Since the 12 volt equivalent tubes are so plentiful I
have "converted" quite a number of units to use them
instead of the 6 volt tubes.  The only modification is
to move the heater wire and ground one pin on the
final amplifier tube that was originally on the "high
side" of the series string.  Of course I also put a
"sticker" on the final amplifier cage noting that the
tubes have been changed to the 12 volt equivalent
because if the 6 volt tubes are installed the heaters
will burn out VERY fast.

This also works on transceivers that use the Japanese
2002A tube which is the equivalent of the 6146B.  In
those cases I replace the tubes with 6883B/8032A/8552
types.

Glen, K9STH


--- Edward B Richards <zuu6k at juno.com> wrote:

It does not seem such a problem to me if
neutralization is the only problem. Why can't you put
a bigger capacitor from the tank circuit to the
neutralizing capacitor. It is easy to get to. Do you
have a suggested value as a starter? Thank you.









Glen, K9STH

Website:  http://k9sth.com

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