[morzh]

Herb

1. The information on the tubes is well available on the web, so instead of waiting for answers you could've simply do that:

Punch in "5V4G tube" in google and get this:
http://www.radiomuseum.org/tubes/tube_5v4g.html

Then do the same with 5Y4G and get this:
http://www.radiomuseum.org/tubes/tube_5y4g.html

Look at the pinouts:


You will see right away they are NOT the same.

5Y4G
   

5V4G
   

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Now you gotta be careful going back to original because God knows, the man who replaced it might've re-wired the circuit to accommodate the other tube and now the wiring might not work with the original.