McCoy Mighty Midget Transmitter

Posted on Feb 5, 2014

It uses a 6GW8 triode-pentode in an oscillator-amplifier design that combines a curious misapplication of oscillator fundamentals with insufficient attention paid to issues of amplifier stability. The result is a transmitter that shouldnot be duplicated and used on the air without considerable circuit modification.

McCoy Mighty Midget Transmitter
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Figure 1 ” Schematic of McCoy`s "Mighty Midget" transmitter. That the design`s power amplifier (6GW8 pentode) is not neutralized and includes operating-frequency-tuned circuits in its grid and plate circuits renders that stage a tuned-plate, tuned-grid oscillator that ”if you`re lucky ”locks to the the frequency of its triode crystal oscillator and stays there. The Mighty Midget`s troubles begin with its oscillator, a common-anode triode Colpitts circuit that nonetheless operates with its anode held above RF ground by the imprecisely resonant circuit formed by one RF choke (40 meters) or that RF choke plus another choke in series (80 meters) paralleled by the capacitance to ground of the triode anode and pentode grid and their interconnecting wiring. The use of the Colpitts arrangement, the building-in of oscillator-anode parallel resonance, and the use of an RF choke in series with the power amplifier grid-leak resistor ”a usage more or less obsolete as of the mid-1950s ”suggest that in designing the Mighty Midget, McCoy had worked to avoid the power amplifier underdrive of the Boosted Pierce: Eleven years earlier, beginning on page 36 of October 1955 QST, he had modified an aperiodic-oscillator-based commercial design to overcome exactly that problem by adding oscillator-anode tuning and power-amplifier neutralization in "More Power with the AT-1. " The presence of oscillator anode (amplifier grid) resonance without the...

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