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EF50 / VR91 / VR91A / 63SPT / ARP35 / CV1091 / VT250 / 10E/92 / 10E/92b / 10E/287 HF pentode

Size The valve measures 73x37mm and has a B9G base

The introduction of the EF50 was important as it enabled use at higher frequencies. Its construction differed from 'traditional' valves in that there were no long leads from the base pins up to the electrodes. Valves of the time had difficulty working above 30MHz due to their construction. After a lot of experimentation in the early 1930s, RCA produced the Acorn valve, the first production one of these being the 955 in 1934, then the 954 in 1935. Acorn valves were later produced in the UK but companies found them difficult to make, and thus after further research and development the EF50 was produced.

Heater voltage 6.3V
Heater current 0.3A
Max anode voltage 300V
Max screen voltage 300V
Max anode dissipation 3W
Max screen dissipation 1.7W
Mutual conductance
(Va=Vg2=250V; Ia=10mA; Vg1=-2V)
Max working heater-cathode voltage 140V
Max operating frequency 125MHz
Cae 5.6pF
Cge 8.3pF
Cag 0.007pF


Pin Function
1 Heater
2 Screen grid
3 Anode
4 Suppressor grid
5 Internal screen
6 Cathode
7 Control grid
8 Internal screen
9 Heater


An improved variant of the EF50 known as RL7 (VR136) was produced. This was later redesignated as EF54.



This Mullard EF50 has "MB1 B2F2" stamped on the edge of the base.

The EF50 had numerous aliases including VR91 and CV1091. However the VR91A (see below) is equivalent to a different CV number, though the data and test data are all the same so it is included here.

This Cossor EF50, also showing the 63SPT type number, has "MB1 B0H" stamped on the edge of the base. EF50
Cossor 63SPT
Mullard CV1091

VR91 Air Ministry logo

CR91 Sylvania logo

In addition to the above logos, this valve also has:


marked on the side, and on the end of its box.

VR91 box


Sylvania VR91, above, and VR91A below

VR91A / 10E/287

This VR91 has no manufacturer information but carries the number 10E/92b. VR91 / 10E/92b

VR91 VR91 rear

Rogers VR91 / 10E/92

VR91A donated by Wim de Rotte

This file was last modified 11:01:21, Thursday April 14, 2022