The 5Z4 tube in my Zenith 5S56 has pin 1 grounded. The schematic for this set shows a ground connection to the rectifier tube, either 5Y3 or 5Z4. The ground connection appears to be made to pin 1 on the schematic though it is not identified as such. The tube pinouts on this site show pin 1 on these tubes as "NC" which I assume means no connection. The voltage indicated for pin 1 on the schematic is zero. Is pin 1 really the ground connection for the tube as indicated on the schematic?

Often in old radios they use N/C connections as a terminal lug for connecting components. Also pin one is often used as the shield connection for metal tubes.
EGO

I took a look at the schematic for your radio. It indicates the use of metal tubes (the outer circle is grounded).
EGO

Hi Will,

Pin 1 of the 5Z4 is a shield and should be connected to ground. The 5Y3 does not have this shield conneciton therefore the pin is marked N/C.

When replacing a 5Y3 with a 5Z4 I connect pin 1 to ground.

Gary

:The 5Z4 tube in my Zenith 5S56 has pin 1 grounded. The schematic for this set shows a ground connection to the rectifier tube, either 5Y3 or 5Z4. The ground connection appears to be made to pin 1 on the schematic though it is not identified as such. The tube pinouts on this site show pin 1 on these tubes as "NC" which I assume means no connection. The voltage indicated for pin 1 on the schematic is zero. Is pin 1 really the ground connection for the tube as indicated on the schematic?

:Hi Will,
:
:Pin 1 of the 5Z4 is a shield and should be connected to ground. The 5Y3 does not have this shield conneciton therefore the pin is marked N/C.
:
:When replacing a 5Y3 with a 5Z4 I connect pin 1 to ground.
:
:Gary

Thanks, Gary and E Centric. I'll just leave my set as is and not molest this connection. Thanks again,

Will
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::The 5Z4 tube in my Zenith 5S56 has pin 1 grounded. The schematic for this set shows a ground connection to the rectifier tube, either 5Y3 or 5Z4. The ground connection appears to be made to pin 1 on the schematic though it is not identified as such. The tube pinouts on this site show pin 1 on these tubes as "NC" which I assume means no connection. The voltage indicated for pin 1 on the schematic is zero. Is pin 1 really the ground connection for the tube as indicated on the schematic?