The frame timebase works at 50Hz and consists of 2 or 3 stages. Some valve sets use a sync pulse amplifier stage. Here is an example of a Grounded Grid Sync Amplifier.
The sync pulses are then used to control the frame oscillator stage. This generates a 50Hz waveform and an LC circuit is used to create a sawtooth to drive the frame output stage.
The Height control if on this stage acts to vary the amount of frame drive to the Frame output stage.
There are a number of capacitors here which affect the shape of the scan waveform via linearity controls. The scan must be uniform of course and the classic symptom of linerity problems is that the circle on Test Card F will be an irregular Oval (or Egg) shape. A scope check on the waveform will reveal the affect this has on the scan waveform, however my advice is to have a capacitor changing problem if linearity problems are experienced.
If these capacitors fail, the result is severe frame cramping at the bottom of the picture or in severe cases the scan comes up to the middle of the screen. Linearity problems are common on old sets as the capacitors in the frame oscillator stages are often in need of replacement. The sawtooth waveform becomes severely distorted as the capacitors, now low in value, reduce the time constant of the CR circuit or render the linearity controls ineffective.