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IIRC, Seleniums have a forward voltage drop of about 5 to 9 volts, and a silicon rectifier has a forward voltage drop of .7 to .9 volts.
Georg Simon Ohm's little contribution says that E=IxR, and we need to drop approximately 7 volts (somewhere in the middle of the selenium voltage drop range).
I'm assuming a 21-CT-55 would draw about 450 watts from the power line at 115/117V, so the current draw would be about 3 Amperes. Not all of that is B+ current, so let's say (worst case) the B+ current is 1 Ampere. It is more likely in the 550-800mA range, I suppose.
Now that we know the current and voltage, getting to the resistance needed to drop that voltage is simple Algebra 0.5:
R=E/I, so 7/1 gives you about 7 ohms, well within the adjustable range of Bob's 7.5 ohm resistor.
My calculations/explanations are full of assumptions, and are VERY open to correction.....I just did the same math for an old 6L6-based amp, and it worked well. Your mileage may vary. Fuses are a must, I might add.
Cheers,
__________________
Brian
USN RET 22YRS (Avionics/Cal)
CET-Consumer Repair and Avionics ('88)
"Capacitor Cosmetologist since '79"
When fuses go to work, they quit!
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