The ATP4 is a screened RF output pentode with a total anode dissipation of 5 Watts. It functions as a single valve but is formed by strapping two sections in parallel.
A hand written note on the PDF data sheet says that the ATP4 was used as the the PA valve in the WS18 set. The valve line-up was VP23 (ARP12CV1331) and HL23DD (AR8 CV1306).
The AT in the designation stands for Army Transmitting and this valve was made for British Army use. The MO base indicates that this was supplied by Mazda.
The ATP4 is a low power device and the anodes are left bright.
The space between the pinch and the bottom mica is filled with connections and paralleling straps.
This image shows the central support welded to a U section strap that connects to the individual pentode electrodes. Three such assemblies can be seen as well as the grids in the right hand pentode.
The classic envelope is 39 mm in diameter, and excluding the MO base pins is 96 mm tall.
Reference: Data-sheet. Type ATP4 was first introduced in 1940. See also1940 adverts.
