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The DLS1 is not a valve but a thermal delay switch.
The filament is clearly visible with a flat plate to the left. The filament would begin to heat as soon as the apparatus was switched on, as wood the heaters of the rest of the valves. At a pre-set time the heating effect of the filament would cause a bi-metallic strip to bend to the extent that the switch contacts would be made. These devices were usually used to prevent HT voltages being applied to mercury vapour rectifiers until the heater was up to temperature.
The heater ran at 3.0 Volts and 0.6 Amps. The delay time was around 30 seconds and not more than a minute. The current carrying capability of the switch was under 200 mA and the voltage across the connections had to be less than 2,000 Volts.
Any valve requiring such protection could similarly be used with this type of switch.
The classic envelope is 43 mm in diameter and, excluding the base pins, is 104 mm tall.
Reference: 3002.


Updated April 20, 2013.
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