Loctal Tube Sockets in the Philco 40-120 Radio - Printable Version

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Loctal Tube Sockets in the Philco 40-120 Radio - t32above - 07-17-2024

The Philco 40-120 radio has 6 Loctal tubes.  Each of the tubes has a metal attachment that comes out of where the center keyed pin goes.  4 of the tubes, 7C7, 7A8, 7B7, and 7C6 are all connected via this center connection which then goes to a chassis ground.  The 35A5 and 35Z3 have to connection to this center pin.  Are they supposed to?  If not does it have to do with the fact that the 35Z3 and 35A5 are the rectifier and Output tube?

On the attached schematic, what denotes the center pin for each tube?  I do not see anything that comes out of the tubes and goes to a ground.

I am still learning so bear with my ignorant questions and thank you in advance.

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RE: Loctal Tube Sockets in the Philco 40-120 Radio - MrFixr55 - 07-18-2024

Hi T32,

There are no dumb questions, congratulations for pursuing this hobby!

The purpose of the center "Loktal" pin contact is to ground the metal shell of the Loctal tube. This is not important on the rectifier and audio output tube, but could be very important on the RF, Convertor, IF and 1st Audio tubes, especially those that have shields that fit over the tube but do not contact the chassis. These shields ground to the metal base which is grounded by that center pin.

Hope that helps.

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