[old_tv_nut]

Originally Posted by Findm-Keepm

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,

Not a good idea to calculate the resistance needed based on the DC (average) current. It might be right but probably won't be. The voltage drop actually depends on the instantaneous charging current, and that depends on the size of the filter caps and how much ripple is on those caps.