The ARP35 is the UK Army, pre-WWII, name for the EF50. The valve is a high impedance variable μ pentode and was a workhorse of receivers for TV, radio and especially war-time radar.
This exhibit came with a detached aluminium screening can, and by chance the insides are not badly obscured. Note the screening between the lower mica and the pressed glass foot holding the pins. The anode can be seen to be open at the sides save for a narrow strip of metal.
More directional light shows how the anode side straps are spot welded to the main anode sheets.
A closer view of the screened lower components. Note the difference in the appearance of the glass between the hard glass 'foot' and the softer glass envelope.
The pointed and hard metal pins. The mid part of the pin is a different metal so as to match the thermal expansion of the hard glass foot. The third section of the pin is the part projecting above the glass to which the electrodes are spot welded.
The top mica components are harder to see due to the glass not being clear. The insulated heater can be seen and the tops of the grid supports. The control grid supports are topped with heat radiating fins.
One half of the anode seen full face. The visible grid is the suppressor grid. Note how the suppressor grid is wound with a tighter pitch than most pentodes. This grid exerts more influence on the electron stream and was used for modulation as well as to suppress secondary electrons emitted from the anode.
The wide glass tube envelope is 31 mm in diameter and, excluding the B9G base pins, is 57 mm tall.
References: Data-sheet & 1043. Type ARP35 was first introduced in 1938. See also1938 adverts.
