The Millen Variarm Electron-Coupled Oscillator made its appearance in the mid-40's.
It is the commercial version of a unit presented in the January 1941 QST (cover
article) designed by Henry Rice W9YZH. The 90700 was designed to operate on the
amateur 160, 80 and 40 meter bands. Two wiring flavors were offered, that could
be changed by the user, with the 90700 wired for 80 and 40 meters and the 90701
for 160 and 40 meters.|
The Variarm was a stable transformerless oscillator capable of roughly 2 watts RF output. It uses a 6K7 oscillator and a 25L6 buffer. No power line transformer is used, so the above tubes along with a 25Z5 rectifier have their filaments wired in series across the AC input power line. The name variarm comes from the fine frequency tuning arm that is attached to the side of the 6K7 oscillator box.
The unit pictured above belongs to Glen Reid K5FX (picture taken by N5EM). He has had the unit on-the-air on 80 meters barefoot once, but has some concerns about the lack of a power transformer for safety reasons.