The photographs (courtesy of Tom Withers G3HGE) show the first TW VFO. This unit used a parallel line 72MHz oscillator followed by a pair of EF80 valves operating in class A providing sufficient output to drive a doubler to 144MHz. The oscillator had 'chirpless' and 'clickless' keying, this being achieved by applying a keyed DC voltage to a coil wound round a piece of ferrite inserted into the Kalitron oscillator box. NBFM could be achieved by applying audio to the same inductor.
The keying was FSK with the 'idling' frequency being many kHz away from the selected operating frequency. Keying was via a 'trigger' neon which can clearly be seen on top of the VFO box (right hand photograph above). Drift at 72MHz was negligible by virtue of a wire ended ruggedised ECC81 valve mounted inside a copper chimney.
Due to the amount of work involved in its manufacture, only two units were ever produced. (I wonder where the other one is ? )
The TW VFO in the photographs above was found in a barn covered in pigeon droppings and straw (although the underneath section looks almost pristine). According to Tom, when he powered this particular unit up, the calibration at its 72MHz free running oscillator was within a few hundred cycles at 144Mhz, 145MHz and 146Mhz (as monitored on a 2m receiver).
The photograph to the left also shows the underside view of the valve TW VFO.
This particular VFO was shown on display at the R.S.G.B. International Radio Hobbies Exhibition that was held in November 1961 at the Royal Horticutural Society's Old Hall in Vincent Square, London. The exhibition was opened by Henry Loomis (Director, Voice of America).
This was a new TW product at the time and a brief article about it appeared in the December 1961 RSGB Bulletin.
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