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So I'm breezing through the book "Handbook of 630-type TV Receivers", which covers these early sets plus spends some time talking about converting the chassis' for use with larger picture tubes. (among other things, you can convert a 620ts to a 27" 90degree crt) Anyhow, here is what it has to say about the issue at hand:
"Tubes with this construction (17HP4, 21YP4, 21XP4) provide sharp focus over a wide range of line voltage variation. When using the electrostatic focus type tubes on the 630-type chassis, remove the focus coil assembly from the neck of the tube and place it at the rear or bottom of the cabinet. (I think the ZP4 uses a permanent magnet? So you could just leave it out entirely and ignore this next part) Do not remove the focus coil from the circuit since it serves as part of the low-voltage bleeder network and its removal would cause a change in voltage divider values as well as a possible burnout of the focus coil and associated components. If desired a resistor equal to the resistance value and power rating (247 ohms at 4 watts) of the focus coil may be substituted in the circuit and the focus coil may then be removed. Pin 6 of the new kinescope must be connected to a voltage of from -100 volts to the value of the B+ boost; the exact value to be used is that voltage producing the best focus."
So I would say it could be done by merely finding a voltage to apply to pin 6, experimenting to find the best picture. I can't make any promises but thats how it reads, anyhow.
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Bryan
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