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In the vacuum tube era they were just a step up or boost wired autotransformer...There was no grid circuit change ( I don't think I have heard of that before).
They were rare by the solid state era.
Solid state era brighteners we're the same design but the transformers were different frequency.
Tube sets powered the CRT off 60Hz AC from a transformer or heater string, but most single board solid state sets powered the heater off a 15,750Hz winding on the flyback (so a 60Hz transformer wouldn't work).
In the last couple of years on a 2000's Sony HD CRT I needed to isolate the heater from ground, because I found an intermittent HK short that was causing shutdown. It was DC powered in the factory circuit, so I took a small 120VAC to 12V stepdown transformer, rectified it to DC, filtered that then fed it to an LM317 regulator configured as a 6.3V regulator (with the correct feedback resistors for that voltage) filtered that and fed it to the heater pins (I desoldered the socket pins from the board and pulled them out)...I figured out that a 6V relay I have draws around the same current as the CRT did so I hooked that relays coil to the factory heater supply (I think I have a reverse spike suppressor diode on the coil for safety) and used the relay contacts to switch on 120VAC to my home made supply. I'm still watching the set over a year later...
My scheme could be used as a brightener...All I need to do is adjust the feedback resistors that set the output voltage of the LM317 to increase the heater voltage and it would be a brightener.
You really should check what frequency of power your sets heater uses before you come up with a plan.
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