[Collins] 4D32 question

[Glen Zook]

Putting both sections of a 5894 (and similar tubes) in
parallel works very well on HF and the lower
frequencies on VHF.  Motorola used the 5894 in their
lowband (25 MHz to 50 MHz) 100 watt output Motrac FM
mobile units with the sections in parallel.  This was
done in MANY thousands of mobile units.

In the WRL Globe Champion 300, 300A, 350, and 350A the
final amplifier tubes are originally the AX9909 which,
unfortunately, have become pretty expensive whereas
the 5894 series was used in hundreds of thousands of
FM mobile units and can be often obtained for under
$20 each.  As such, many of these transmitters have
been converted to use the 5894 series.  The tube
sockets are the same and the combined rating when the
two halves of the 5894 are put in parallel they are
almost exactly the same as the AX9909.  You do have to
move the plate connection from the tube socket to a
plate cap arrangement which is basically a solid piece
of metal connecting both plate wires to each other.

The grid to plate capacitance of the 5894 is rated at
a maximum of 0.08 pf which, especially at HF, is of no
concern.  The 829B grid to plate capacitance is 50%
greater than the 5894 being a maximum of 0.12 pf. 
Again, this isn't going to make a "hill of beans
difference" especially on HF.

The total time required to make this modification to
the Globe Champion series is under 1 hour (there are 2
tubes which are operated in parallel involved).  When
the transmitter is modified the power output is
generally around 10 watts more at the same input power
using the 5894 tubes instead of the AX9909 tubes.  I
suspect this additional power is due to the fact that
the 5894 was designed for VHF operation and is thus a
little more efficient on HF.

I modified my Globe Champion 350A several years ago to
use the 5894 and it works great!  There was an article
in Electric Radio several years ago about making this
modification.

Again, there is no problem with with putting the
sections in parallel using the 832 series, the 6907,
the 829 series, the 5894 series, or the 8643 series. 
All of these tubes were designed for VHF / lower UHF
operation and are listed in order of increasing power.

Glen, K9STH


--- "Dr. Gerald N. Johnson"
<geraldj at storm.weather.net> wrote:

Paralleling pieces of 5894 may not work well because
it has internal neutralization capacitors set up for
push-pull operation and those would add to the
grid-plate capacitance when paralleled.









Glen, K9STH

Website:  http://k9sth.com


       
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