[ARC5] Deaf BC-454

[Kenneth G. Gordon]

Both the 12SF7 and the 12SK7 are Remote-Cutoff Pentodes. 

However, pin 4 is the grid in the SK7 while it is the screen in the SF7.

Pin 6 is the screen in the SK7, while it is the plate in the SF7

Filaments are 8, and 7, in the SF7, while they are 2 and 7 in the SK7

Pin 8 is the plate in the SK7.

Pin 2 is the grid in the SF7

Pin 5 is the cathode in the SK7 while it is the diode plate in the SF7

The only pin other than one of the filament pins (7) common to both tubes is pin 3, and even 
then only partially. Pin 3 is the suppressor in both tubes, but in the SF7 that is connected 
internally to the cathode.

If a 12SF7 was plugged into a BC-454's IF sockets, either 1st or 2nd, or even the RF amp, 
unless the filaments were miswired, it wouldn't even light, but at least external components 
would not be damaged.

As I remember it, he said that all tubes were warm. That says to me that the filaments had 
been miswired by someone.

If that is the case, then plugging a 12SF7 into a socket wired for a 12SK7 would result in the 
signal being impressed on the screen of the tube, resulting in much reduced gain through 
that circuit.

Is this what has happened? I dunno.

The 12SF7 is ONLY used in the AN/ARC-5 receivers and NEVER in the SCR-274-N or ARA 
receivers.

Ken W7EKB

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