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W148

Sensibly equivalent to:
7H7 W81
    
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The W148 is a variable μ amplifier pentode. It is a high impedance device with a medium mutual conductance. Variable μ devices are primarily designed for IF and RF amplifiers in radio receivers. The W148 in particular, was designed for use in the second IF stage of conventional domestic AC superhets, and in car radios. The 'W' prefix was used by the Marconi/Marconiphone/M-OV /Osram brands to indicate a vari-mu RF or IF pentode.
The W148 was copied from the US/International Type 7H7 but renamed W148 to conform with the house nomenclature. It also had the effect that the uninformed punter needing a spare for his set would have to buy one at BVA cartel prices rather than a 7H7 at a more competitive price.
The markings on this exhibit indicate that it was supplied through the Marconiphone organisation (HMV, etc.) rather than by MWT or M-OV. Dating of this sample is early 1950s.
The anode current is quite high and with the large electrodes, this suggests that the device may have been designed for low noise input stages of HF communications receivers.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 55 mm tall.
References: Private communication & 1040.

 

Pin Connections
B8B
1
2
3
4
5
6
7
8
h
a
g2
g3
s
g1
k
h

 

Absolute Maximum Operating Conditions
Pentode
Vh
Ah
Va
Vs
Vg
mAa
mAs
ra
gm
6.3
0.3
250
150
-2.5
9.5
3.5
0.8M
3.8
Updated December03, 2012.
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