The DET22 is a physically small disc seal triode valve, with co-axial connections. The heater and cathode are opposite the main envelope and the emitting surface is a flat topped thimble. The grid has a circular connection to the flat internal metal mesh. The anode is copper and has a flat surface facing the grid and positioned very close to it.

A grounded grid configuration would allow for a two cavity mechanical design.

This valve is rated at 600 MHz but its extended maximum frequency of 4 GHz made it a natural choice for microwave designs. The DET series of planar valves evolved during WWII.

The planar envelope is 15 mm in diameter and, overall is 60 mm tall.

References: Datasheet & ¹⁰⁴⁰. Type DET22 was first introduced^¶ in 1954.

 
Absolute Maximum Operating Conditions^¶

Triode	Vh	Ah	Va	mAa	Pout	Pdiss	freq	f.max
	6.3	0.4	350	40	3	10	600	4000

Thanks to Frank Philipse for supplying the above PDF datasheet

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