silvertone roundie

[David Roper]

A very tiny amount of air will make the tune unusable. The ability to make an image is destroyed long before the filaments are.

[compucat]

When I was restoring my 1965 Zenith roundie the CRT was gassy but I did not know it at first. On my crt tester the tube showed good emission, etc. I restored the chassis and got 25 kV on the bench at the anode connector. I assemble the set, power it up and get no hv and purple glow in the crt neck. There was no picture and the crt filaments did not burn out. After I had the tube rebuilt, changing nothing else, I had a great picture with proper hv. It sounds like you might have a slightly gassy crt. Don't give up on it. These round tube color sets perform great even if they are a little maintenance intensive.