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None of the RCA series string portables used a 6BK4 regulator...the earlier ones like CTC-22 and CTC-36 used a 17KV6 or 22KV6 pulse regulator tube, and a 3A3 rectifier tube. The later ones like CTC-53 have no seperate regulator tube...it must somehow regulate the bias on the HO tube with zener diodes or something. These too used a 3A3 tube.
The very last portables used the bias regulation on the HO tube along with a solid state tripler and high focus voltage (all other RCA portables used a low focus voltage CRT). Would like to see one of these as the performance would probably be just as good as the XL-100 sets as opposed to the softer, dimmer picture of the low focus voltage tubes...have not actually come across one yet. These sets were styled a lot like the XL-100 models too.
There were some 19" RCA sets that used a transformer powered chassis like CTC-31 (that is, CTC-19, CTC-27 and CTC-50), these did use a 6BK4.
Some 16" portables like Sears and some others, usually Japanese makes, did use a 6BK4 with a series string chassis...the CRT and 6BK4 used a 6-volt filament transformer.
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