Gentleman
The Johnson Desk Killowatt amplifier is a wonderfull piece of equipment.
It has 435 pounds of iron in it. But it lacks 160m coverage. When I
wanted to add 160m to the Desk I had to figure a way to measure the
extra L & C I was adding with all the components installed.
We all know that open air inductors change there inductance when objects
are in close proximity to them. I thought of using ferrite cores, but
with some simple calculations the cumlative heat generated in a 1KW
class C plate modulated tank circuit would have a disastrous effect on
the polymers holding the cores togehter( even the 5" cores would get
very HOT)
What I did was to figure the plate impedance and substitute the
tubes(4-400) with a pair of resistors from the plate caps to the grids
on the tube sockets. Then I used one of the antenna analyzers availible
from Autek. The RF1 is an invaluble tool for just such
a task. Not only does is show SWR Z and Frequency, it will also measure
L and C at the operating frequency. Back fed the tank circuit from the
antenna SO239 and this gem will tell all.
I set the analyser to the frequency in question and adjusted according.
I noticed that the values for the inductors and capacitors were exactly
double of those values for 80m.( 3.8mhz to 1.9mhz showed flate SWR and a
Z of 50 on the Autek RF1
analyzer when the C1-L-C2 were doubled from the stock values).
The logistics of switching the extra L & C in and out of the circuit can
add some stray values to the tank and I did not want the Q and
efficiency to go down. The Desk exhibits 73-79% efficiency on 80-15m
the 10m band drops to 60%. I used vaccum relays and
short runs of strapping for all my interconnects.
I know there are other ways to approch this modification with the same
success as I had.
I enjoy this reflector and the knowledgeable Gentleman on it.
Randy
KE7TV
I must thank W9FGJ(Mr AM Audio), WA7RAI (RAIbeam fame) and W3AM
(Broadcast Engineer)
For there help in the mods to the Desk.
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