Quote:
Originally Posted by emeritus
A number of the valves listed in the 1938-9 GEC-Osram valve catalogue were available in both screened and unscreened versions, both versions having the same type number.
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The MSP4 employed in the first generation EMI television receivers was supplied without metalizing to reduce the internal capacity between the anode and what would have been earth.
Screening cans were fitted to each RF section but were a safe distance from the actual valve.
The B7G EF91 is an unscreened RF pentode and will be discovered in pre ITA television RF and IF amplifier stages complete with screening cans to prevent instability.
The later replacement, the B9A EF80, was fully screened internally and did not require an external screening can. Pye continued to fit square screening cans to their receivers maybe as a belt and braces exercise but soon realized they were not necessary.
EMI painted their screening cans matt black in the notorious 1807 in a hope to reduce operating temperature but the cause of poor stability was eventually traced to nasty decoupling capacitors and leaky paxolin valve holders.
We all know what happens when the octal EF39 used as an I.F. amplifier in 1940's radios looses it's earth connection to the metalizing..
Much later tuner valves such as the PCC84 and PCF80 were not internally screened probably to prevent high interelctrode capacities but were screened externally with cans at a safe distance from the valves electrode assembly Regards, John.