I sort of think Leo liked as much power as he could
advertise out of a small a radio as possible. The Champ is
about the size and weight of a DX 100 and put out about
double the power. So the AX 9909 fit that pretty well. Too
bad it was such a fragile tube.
Going to a 4CX 250B would have required a socket change.
Like you, I worked with what I had so the sockets stayed the
same and I used the 8643 rather than the 5894. I had
access to a lot of 5894s and a few 8643 since I spent many
years in the land mobile business. They were used by
Motorola in their Micor line. The only thing I had to
fabricate was for the 8643 plate pins. My friend Glen Zook
suggested getting a 3 ft. piece of aluminum stock 1 inch X
1/4 inch at the hardware store, cutting two 1" pieces and
drilling and tapping the holes for set screws. Gives a bit
of heat sink for the plates
I didn't like the fan set up either, mostly because of the
noise. I happened to have a small muffin fan that ran from
110 VAC. I mounted it on the other side of the shield from
the tubes and the opening in the muffin fan fit the hole for
the other. I don't remember the model number of the fan,
but it was about 2 inches square and 3/4 inch thick. I did
have to remove the shield and drill one hole for a mounting
screw since the pattern was different.
It works but you have a better margin for tube HV. I hope
my holds out for a long time.
Jim/W5JO
Kim et al:
More importantly, none of the tubes were spec'd for the
kind of service used in the 350A although clearly they did
work for others. Just about only common tube I found that
had published specifications for AM operation at 1000
volts on the anode was the 4CX250B, which I had a few of.
The interelectrode capacitance and so on are also
extremely close. So that's what I did and it's worked ever
since, even after I dropped the rig on a dirt floor in a
friend's barn when the steps I was carrying it up
disintegrated under me :(
It's a simple mod on the A, harder on the others. The fan
is removed and the tube socket sits in a tuna can mounted
vertically over the fan hole with the flat squirrel fan
mounted atop one of the transformers feeding a plastic
tube plenum. Electrically the filament has to be dropped
by .3 volts and the drive increased since it's a tetrode,
but as I recall that's all that's needed. A no-holes mod.
I should write this up but it's been a tough ten years...
73,
-Pete
WB2QLL
Somers, WI
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