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CV428

Sensibly equivalent to:
5B/251M 5B/254M
See also:
Beam Tetrodes and the 807
    
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The CV428 miniature beam tetrode has the same ratings as the classic 807. The difference in size between the 807 and this valve shows the extent to which valve technology matured during WWII and the effect of the introduction of the pressed glass base. The B8B base was the earliest to be developed (by Sylvania in the US) with eight equi-spaced pins and an external metal collar to hide the envelope seal and secure a locating spigot.
Each of the control grid's supporting rods is fitted with a blackened metal fin to assist with the cooling of the grid. This can be seen above the top mica.
The valve is also very similar to the STC 5B/251M, that was used as the prototype for the CV428, and electrically identical to the 5B/255M. The latter has the anode connected at the base and one less connection to the cathode.
With the pressed glass foot base the connecting pins became the direct supports for the valve electrodes. Contrast this with the 807 and the pinch construction that added greatly to lead inductance and thus limited the maximum frequency of operation.
With the eight pins equi-spaced around the the circumference of the pin circle, the B8B base required a locating spigot for correct alignment in the holder. In some valves this locating spigot was connected electrically requiring an isolated valve holder.
The wide glass tube envelope is 29 mm in diameter and, excluding the B8B base pins, is 88 mm tall.
References: Datasheet & 1040.

 

Pin Connections
B8B
1
2
3
4
5
6
7
8
tc
h
k,bp
g2
k,bp
g1
g1
k,bp
h
a

 

Absolute Maximum Operating Conditions
Beam Tetrode
Vh
Ah
Va
Vs
Vg
mAa
mAs
Pdriv
Pdiss
Pout
freq
6.3
0.9
600
250
-45
100
8
0.3W
25W
40W
60MHz
Thanks to Frank Philipse for supplying the above PDF datasheet.
Updated October 06, 2013.
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