[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.  

Many thanks,

Jerry  K9AF

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