04-16-2014, 09:52 AM
As Arran said, "Though you can get away with using a resistor in a Pi filter arrangement in a 4 tuber like a model 84, and it will work fine if you use larger filter capacitors to compensate for the lower inductance".
Now, about increasing capacitances.
Your 80 tube can handle up to 32uF direct capacitive load, so you should not be exceeding that. Go say 20uF for starters.
This will 1) decrease ripple and 2) increase the output voltage some but then some of it will be lost on your resistor as you might want to go a bit higher.
Then the resistor. Go at least with the value of your field coil loss resistance which is 1.14 kOhm.
Then choose your second cap from the ratio
R*C >> 1/60, where R is in Ohms, C is in Farads, and the results are in seconds.
So if your resistance is roughly 1K, your output cap ideally should be 160uF.
This is a bit too much, as this kind of cap will be too large and expensive. However if you go with say 2K resistor and 47uF (50uF) 500V cap you will stay within size and fairly low cost. And the time constant is still acceptable.
Choose your resistance wattage so it could handle whatever the voltage drop you will measure, formula V-square/R for dissipated and two-three times that for rating of the resistor so it does not self destruct too soon.
Or if you know your current consumption in advance, go I-square * R for the dissipated (same as above for rated).
You will need to experiment a bit to see if your output voltage is the same or close to recommended; should not be much lower or much higher.
------------------
This said, the choke is still a preferred solution.
Now, about increasing capacitances.
Your 80 tube can handle up to 32uF direct capacitive load, so you should not be exceeding that. Go say 20uF for starters.
This will 1) decrease ripple and 2) increase the output voltage some but then some of it will be lost on your resistor as you might want to go a bit higher.
Then the resistor. Go at least with the value of your field coil loss resistance which is 1.14 kOhm.
Then choose your second cap from the ratio
R*C >> 1/60, where R is in Ohms, C is in Farads, and the results are in seconds.
So if your resistance is roughly 1K, your output cap ideally should be 160uF.
This is a bit too much, as this kind of cap will be too large and expensive. However if you go with say 2K resistor and 47uF (50uF) 500V cap you will stay within size and fairly low cost. And the time constant is still acceptable.
Choose your resistance wattage so it could handle whatever the voltage drop you will measure, formula V-square/R for dissipated and two-three times that for rating of the resistor so it does not self destruct too soon.
Or if you know your current consumption in advance, go I-square * R for the dissipated (same as above for rated).
You will need to experiment a bit to see if your output voltage is the same or close to recommended; should not be much lower or much higher.
------------------
This said, the choke is still a preferred solution.
