[benman94]

The change in tube is going to be an issue for you. The 10SP4 is a ~50 degree tube, the 23CP4 was probably between 70 and 90. (I don't have a datasheet in front of me.)

The degree of deflection experienced by the stream of electrons is governed both the strength of the magnetic field created by the deflection coils and the velocity of the electrons (which is itself governed by final anode voltage accelerating the electrons; we assume the electrons are essentially stationary when at the cathode.)

Whereas the final anode voltages required by 10SP4 and 23CP4 are likely similar, the magnetic field strength required to fully deflect the beam is going to be markedly different for both tubes.