Jean,
Here’s the answer from my friend, Tom Singletary.
“It would be possible (to substitute an OZ4 for a 6X5 in a table radio) since the heater pins on a 6X5 are 2 and 7, which are unused on an OZ4. Also, as both the OZ4 and 6X5 have cathodes separate from the heaters (indirectly heated), this would be an easy interchange to make. I had never thought about that interchange.
“The OZ4 gave trouble for years in GM cars. I remember the owners would drive up to my shop and say ‘My radio won’t play unless I race my engine’ and I would grab an OZ4.”
Before making the tube swap, Jean, you might want to check a tube manual to make sure the OZ4 will handle your radio’s B+. I also checked some radio sites I follow and learned that the OZ4 has its champions and detractors — like just about every other tube type, it seems.
Also good to remember is that the inside of an automobile is a hostile environment for electronics, especially vacuum tubes. It is quite possible that an OZ4 in a table radio will happily rectify away for hundreds of years in a temperature-controlled, dirt and pothole-free living room.
Eric Strasen