1929 MOPA 80 meter transmitter using a pair of 27 triodes to an 865 tetrode

Posted on Feb 5, 2014

This transmitter had its origins as a conventional shunt fed Hartley oscillator for 80 meters using a pair of 27 triodes connected in parallel; with 300 VDC on the plates of the 27`s, I was able to get about 4 watts output ( with decent stability ) for about 18 watts input, about 23% efficiency. Here is a schematic of the oscillator by itself: Thi

1929 MOPA 80 meter transmitter using a pair of 27 triodes to an 865 tetrode
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s circuit will work fine as shown as a stand-alone transmitter. Circuit parameters are as follows: 15K grid leak; tank circuit: 12 turns, 1/4" copper tubing, 2. 5" diameter, about 4. 75" long, with 540 pf padding capacitance ( tub micas ) and a 60 pf tuning capacitor. For an antenna coupler, I used a 250 pf variable capacitor in series with a coil consisting of 40 turns 18 Awg insulated wire wound on a 2. 5" diameter black PVC pipe. I put taps at 10, 20, 30, 35, and 40 turns. This will tune a 1/4 wave end fed antenna ( marconi antenna), which for 80 meters, is about 67 feet. Incidentally, I use the same tuner, in parallel configuration, to tune this same antenna as a 40 meter "half-wave". The coupler is simply placed in inductive proximity to the tank coil- don`t couple the antenna too heavily or the "note" will suffer with this circuit. There is an optimal coupling point whereby stability is good and yet useful output power is developed. Trial and error required here. Inspired by a George Grammer article in a February 1931 issue of QST magazine, "More power with better frequency stability", I decided to build a MOPA ( Master Oscillator Power Amplifier ) using the two type 27s in parallel to drive a single 865 tetrode. Both tubes were available in 1929, and both are still relatively easy to find and inexpensive. Here is a schematic of the 865 power amplifier: The Hartley oscillator is coupled to the grid of the 865 thru a...

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