Why remove horizontal output tube when using variac?

[Electronic M]

Some people also remove the vertical output tube...

You can't effectively do this on series heater sets without jumping heater leads back into circuit.

The set ironically where pulling the horizontal output matters most is the CT-100 that was designed so that even if the osc was running if the line voltage was around 90V the bias would be badly wrong on the output.

My approach depends on the value of the set. Something rare and desirable is far more likely to get this treatment than say a run of the mill monochrome set that's common and cheap.