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In Reply to: RE: ideas on variable filament supplies? posted by vinnie2 on July 23, 2015 at 12:30:12
Offhand, 20-25K sounds way too high, bit I have not done the math. It really depends on how much current is being drawn from the AC line.
One thing for sure-it will not boost voltage. If you have a 5V Xfmr, 5V is the most you will get when the pot is fully closed. You could use a 6V Xfmr, and dial it back slightly for 5V.
Follow Ups:
You were right about it being way too high a value; that was because I did not have a load on the transformer. Duh! With the tube filament connected into the circuit we need to get down to about 2.5k for the pot value to get a useful range of voltage. Trouble is then you get a hot pot after running it a few minutes. Going to have to break down and do a little math and figure what rating I need and theh shop around for a rheostat with the proper specs. It may not be the universal solution I first thought it could be, but it might still work for giving me an adjustable filament supply on my test mule bread board. Hopefully this will still be a simpler, cheaper solution than a $60 variac.
Edits: 07/24/15 07/24/15
Roger on that. Quite often you will find 5 and 6 volt filament transformers cheap at hamfests, and you could dial them back to anything from 5 volts on down. Very useful for some of the dht triodes that have the odd filament votages like the 26 and 27 tubes I am trying to work with now that have filament voltages of 1.5 and 2.5 v.
I tried several different values of pots, starting with a 100 ohms and working up. Didn't get any really useful range until I got up to 20k. Not sure if it will heat up too much; that is the next thing I want to try. I will let one run for a while under a filament load and see how it does.
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