Using Vacuum Tube heathkit as Linear Amplifier for QRP

Gordon Gibby

Finally got a chance to try my idea of using a heathkit ssb rig as a power amplifier for a QRP rig.   I've seen "amplifiers" for these rigs going for many hundreds of $$$.  I happen to be swimming in ancient heathkits with 2x6146 linear finals and tuned plate circuits on the driver as well (all good for getting rid of unwanted harmonics.....)

Allison gave some much needed advice on this crazy idea months back, and I finally got the chance to try it:

1 watt output from an exciter [I used an icom 718 that was readily availalbe] -- my only "power measurements" are ancient SWR meters with built in diode detectors so these measurements may be fairly INaccurate.

coax over to a 49:1 homebrew balun our group made for matching resonant half-wave antennas:   ours is a simple autotuner with 14 turns total of #18 teflon wire around a FT-240-43 (or FT-140-43) tapped at turn 2 to give us 7:1 turns ratio, 49:1 impedance ratio, and that checked out roughly with an antenna analyzer and some resistors

I loaded the output with 2500 ohms (temporarily made out of 4 10K resistors in parallel because that is all i had)
output of that via 6 inches of small coax to a 0.01 1kv ceramic capacitor connecting to the PLATE circuit of the removed 6CL6 driver, and to a nearby ground.

Output tested only with very brief bursts of CW -- on 80 meters and 20 meters, got approx 40-50 watts output measured also with an ancient swr meter/wattmeter into a 50 ohm load, everything on the Heathkit peaked for output.   Plate current rose appropriately.   Tuning circuit all appeared appropriately.

No obvious signs of any instability in this pilot test.    
Did not make any measurements with spectrum analyzer.   (In due time......)

That heathkit has other problems elsewhere but the finals appeared in fine shape.   Earlier VERY CRUDE measuresments with a siglent oscilloscope and some math suggested that the drive required at the INPUT to the 6CL6 stage would be less than 100 milliwatts, and that the drive required at the output of 6CL6 would be in the single digit watts range.    The above test appears to confirm those crude measurements.   Since it appeared the two 6146's could be driven adequately with single digit watts I opted to temporarily just remove the 6CL6 driver and connect to its (DC powered!!!!!!!  High voltage!)  plate circucit with a 1kv .01 blocking capacitor.   

Quite pleased with this very very initial test.   
At some point I'll repeat with a uBitx, but that's all for right now.
Gordon Gibby

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