Hi: 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
