Quote:
Originally Posted by ukcol
Was adjusting the focus by electromagnetic means common in early sets
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Practically all early American sets pre war and post war sets, with magnetic deflection, used magnetic focus, with a focus coil. As did the 1939 English HMV904/5.
But in the post war period, most UK made sets used a permanent focus magnet, yet in the USA, it was practically always a focus coil.
As time went by, in the USA at least, many CRT's moved to electrostatic focus in the post war period. It was cheaper and saved on the focus coil assembly.
However, it was in fact never quite as good as magnetic focus, the reason being that the focus was more dependent on the control grid potential and the beam current. So with high beam intensity a tad more focus voltage is required on the focus electrode to keep the beam in focus. Where magnetic focus it tends to stay in focus with increasing beam intensity.
In short, a better focused image across a wide range of beam intensities is obtained with magnetic focus. Unless, dynamic focus (modulation of the focus voltage with beam intensity) is applied to the electrostatically focused CRT, as is done in some oscilloscopes and then the issue is circumvented.