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Sensibly equivalent to:
31 ESG250 SG250 VT-31
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The CV1031 transmitting valve was based on the commercial SG250. This CV1031 looks as though it has never been used (and probably never will) it is a power tetrode with a filament requirement 8V at 11.25A and an anode voltage of 5 KV the valve dissipation is 250 Watts. As with other valves of this type the construction has a pinch at either end and the electrodes are held in place by clamps fixed round the pinch.
The left hand end has a pair of clamps around the pinch. The control grid and the screen grid are supported by these clamps and the stout supporting rods fixed to the clamps. The filament support is fixed within the glass of the pinch.
The right hand pinch also leads to the evacuation seal. The clamp holds the anode and the single connection is to the anode. Note also the filament fixing and tensioning spring. Although not clear it is probable that the filament is a single V of pure tungsten wire.
This image shows the grid supports and the filament fixings to the pinch. Both grids extend beyond the anode cylinder and the outer screen grid extends beyond the control grid to afford maximum screening.
It can be seen that the anode is quite short within the envelope.
The ends of the envelope are about 50 mm in diameter.
The envelope has etched markings for identification. The glass is clear with no signs of gettering. It was common practice to evacuate to a hard vacuum on the pump as hand made transmitter valves were expensive to produce and the price could justify a prolonged evacuation time. Also as the envelope glass became lossy at RF when hot, hence the large balloon shape, gettering would increase the risk of losses.
At the right hand end of the envelope one can see that the grids are supported by an insulator connected to an end ring. This also supports the filament tensioning spring.
The ruler shows that the screen grid is about 1.25 inches in or 32 mm in diameter. The screen grid is wound to a closer pitch than the inner control grid that appears as a gentle helix. The two tapering strands of the filament can also be seen.
Close-up of filament, control grid and screen grid.
This advert was found in Wireless World January, 1956. pp 113. The advertiser was Electronic precision equipment Ltd.
The valve came from radio amateur 'Bob' Cook G3UZL. He probably got it from the Plymouth naval stores where he (legally) obtained a large amount of early radio gear. He used to run a broadcast station somewhere in South Africa for a few years when he was young and when he came back to this country he got a similar transmitter for amateur use. He moved to Exmouth and worked for East Devon District Council until he retired. He passed away in about 2004. Bob Maynard G4YRM, who donated these pictures, obtained the valve from a silent key sale. The valve was sold again on ebay in December 2007.
The balloon envelope is 120 mm in diameter, and is 410 mm overall.
References: Data-sheet & Wireless World.


Absolute Maximum Operating Conditions
Thanks to Frank Philipse for supplying the above PDF datasheet.
Updated June 09, 2020.
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