I did some calculations. The capacitor value can be calculated from the relationships:
Q=IT and Q=CV where Q is the charge in coulombs, I is current in Amperes, T is time in seconds, C is capacitance in Farads and V is Voltage.
Therefore CV=IT,
C=IT/V
In the circuit with the TR20 rheostat set to minimum (maximum charge current) and the maximum charge voltage to maintain a linear ramp at about 9.7 volts:
I= 5/1.2K = 4.2mA
One field duration is 1/50Hz = 20ms = 2x10-2 seconds
C= 4.2x10-3 x 20x10-2 /9.7
C= 8.7x10-6 = 8.7uF
So 8.7 uF is the minimum value of the capacitor for the 1.2K minimum TR20 emitter series resistance value.
Note: I see that you have the ideal ramp charging to 14.5 volts. That is not correct. It should only charge to a maximum of 9.7 volts since this is the point when TR20 saturates. Therefore something is wrong with the charging circuit such as TR20 being shorted?
I would suggest changing the capacitor to 8.2 uF or 10 uF to prevent the ramp becoming non-linear with the range of the rheostat adjustment.
Good luck: I trust you will find the problem.
Cheers,
Terry