>> In a transistorised scope the cathode and filament would be at - a few
>> hundred volts wrt. the main transistor chassis ground (the one that
>> powers the X and Y amplifiers).
>
> The filament may or may not be electrically connected to the cathode.

Yes but you are usually not allowed to put more than ~150V between 
filament and cathode. Often much less than that.

Peter
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