|[AMRadio] Gates BC250L|
K9AF at aol.com
K9AF at aol.com
Wed Sep 1 10:27:01 EDT 2004
I have bought a 1955 Gates BC250L AM broadcast transmitter which was
originally on 640 KC. Upon delivering it, and setting it back up, and hooking it to
a dummy load, it made full power on 640 KC. I plan to put this on 1.885 MC,
so I removed the crystal oven, and changed the crystal to that frequency. All
intermediate amplifiers tuned up perfectly. However the output circuit uses a
6 section Pi-L network, where the capacitors are fixed micas and the three
inductors are either roller type (input and output) or a tapped coil. My problem
is I canot make full power on 160 meters, and the tuning is very critical. I
suspect the tank Q is very high at this frequency. The Gates manual lists all
frequencies in the AM band, with the associated tank Q and inductor values.
Using 1.000 MC as an example, the Gates engineers designed for a tank Q of 10,
a quite reasonable figure. Working the math backwards, it would reflect a
plate load impedance of about 3180 Ohms-- again a quite reasonable figure. The
input mica cap would also agree with these two figures. Their chart at 1600
KC claims a tank Q of almost 16, so I think it is safe to assume at 1885 KC
the Q would be even higher. The tuning on 160 meters IS quite touchy, and the
output amplifier MAY need to be reneutralized.
Does anyone have any thoughts and/or experience in converting a broadcast AM
transmitter such as this? The BC250L uses a pair of 810s in PARALLEL, not
push-pull. Normal plate voltage under load is 1200 and normal plate current,
loaded is 300 mA.
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