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The 7RP4 looks like it can be made to work in a 621, however there are some important differences that will need attention.
The 7D is electrostatic focus. The 7R is magnetic. The 7R is aluminized, requires no ion trap and has no external aquadag coating. Overall dimensions, heater voltage/current and basing (except for focus anode) are the same. The 7R can handle 9KV vs 6KV for the 7D. I doubt that will make a difference in a TV set.
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