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A random check of 10 schematics shows about 50-50 use of pin 7 or 2. Since *either* side of the directly heated cathode/filament can serve as the cathode connection, either pin can be used.
jr
Add: Looking at several 1B3s, there is an internal shield that appears to be connected to pin7, but that should not matter either, as there is only a volt (1.25) difference from one side of the filament to the other.
Last edited by jr_tech; 05-08-2014 at 12:56 PM.
Reason: add note about internal shield
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