rjr@mgweed.UUCP (Bob Roehrig) (10/04/85)
Some time last year there was some interest in the results of my project of rebuilding/modernizing an old Johnson Desk Kilowatt. The basic renovations were removing the screen and bias supplies and converting the RF deck to a grounded grid scheme. I also added 160 meter capability (which has affected 10 meters due to added stray capacities to the point where it will not function on 10). Since I also completely rebuilt the control/power circuitry, I basically started over except for the main layout of the RF deck. Except for the 10 meter situation, results have met expectations and the unit performs well with the 100 watts drive from my 930S. Input matching is done with a low Q "T" network which replaced the old grid tank. For 160 meters, a coil is switched in series with the original rotary inductor and 300 pf added across the plate tuning capacitor. This coil is shorted out but still connected to the circuit on the other bands. The proximity of this coil to the shield cover adds the unwanted stray capacitance to ground which affects the upper bands. If I can find a suitable DPDT switch, that problem should be taken care of. Input power is about 950 watts (2500V at 380ma). On 160 thru 20 meters, output is 800 watts as measured on a Bird wattmeter. On 15 and 12 meters, the output is 600 watts. If anyone has any questions, ask away. I would like to know if anyone else on the net (or anyone you know) has one of these units. Bob, K9EUI