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If it's a Dumont RA-103 doghouse then it's parallel heater.
On a parallel heater set you can run the CRT heater through a voltage doubler circuit (couple of 220uF 25V caps and a pair of 1N400x diodes) which will make around 12.6VDC from 6.3VAC (CRT heater shouldn't care if it's being fed DC) then you can use a LM317 or LM337 (and a pair of resistors to set its output voltage) to regulate that anywhere from 1.37V up to 12.6V...You can even use a pot to adjust the level on the fly.
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