Transatlantic Television Baird Success Part 2 – John Heys G3BDQ

The 2 KW Transmitter. Some Technical Details.

The transmitter circuit is basically a self excited oscillator employing a Mullard 0/150 valve which drives a power amplifier which uses a special high power valve which has a silica envelope. The choke (Heising) modulation employs a driver stage using a Mullard 0/150 valve which is followed by a pair of Mullard 0/250 valves in parallel. The quartz frequency control of oscillators was still in its infancy when this transmitter was designed.

transatlantic-television-baird-success-p2The 2Kw Transmitter Circuit
V1 –  Silica valve PA    V1 –  0/250 E oscillator
V3 & V4 –  0/510 audio modulator valves    V1 –  0/150  driver valve

The numbers which follow the ‘0’ of the valves denotes their power rating. Mullard power valves of that period had a glass ‘chimney’ on top which was where the vacuum evacuation pump connected during manufacture. To prevent the melting of the glass envelopes of high powered valves silica was employed instead. Pure fused quartz (silica) was introduced towards the end of WW1. This material does not expand or crack when heated and does not soften until it reaches a temperature of 1,500 C. Glass softens at about 450 C and may crack when suddenly heated or cooled.

The very high voltages for the transmitter were derived from DC generators driven by a 9 hp motor. The photo (last month) shows Logie Baird posed and pretending to adjust the filament rheostat for the big pair of 0/500 modulator valves. Ben Clapp is standing on the right and just behind his right arm stands the vertical housing for the fan cooled silica triode output triode valve. The valve between Baird and Clapp is the 0/250E valve used for the oscillator stage. The QSL card from the American station 1ARY was sent to Ben Clapp for their several QSOs across the ‘pond’ and is dated February 1925, and was sent to his Coulsdon address.

John Heys G3BDQ from Vital Spark March 2011.

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