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I check the RCA manual on my CTC-5, it has the filament up to 300v which puts it practically right at the cathode level on my 21AXP22.
it was not until the CTC-12 did they drop it down to about 180v using the divider (before that there was just a 10k resistor to the power supply lower B+ supplies.
Not sure why the change unless it was just a compromised voltage to better suit that IF tube (cathode voltage on in was in the 160v range) I did check and the CTC-11 also used the IF the same way with the 160v cathode but still had the filament at 275v, maybe they started having a lot of HK shorts on that IF tube?
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