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In Reply to: RE: What the ?????? posted by vinnie2 on January 26, 2016 at 06:18:50
For 2A3s, I use a precision 10 Ohm Vishay 1280G 27-turn bulk foil pot with a 22 Ohm resistor between each socket filament pin and each end of the pot. The only resistor I run from the wiper is the cathode R. That is, filament pin -> 22R -> one end of the 1280G; repeat for other pin and end of pot.Allows very fine, step-free tuning that can be easily seen on the multimeter. Need to think about mounting though.
Get 'em from Michael Percy or Mouser.
Cheers.
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 01/26/16Follow Ups:
Very interesting. Mouser only has the 100 ohm units though. I wish the wattage ws a little higher. Ever had them over heat?
I think silicone would work for mounting.... a little dab will do ya
Vinnie, have not noticed any overheating and have been reliable for nearly two years of frequent use. Also, I reckon that if cathode Rs have an affect on sound quality, I will take a bulk foil resistive element over cement or inductive WW.The formula for calculating the dissipation is: P = V^2 / R
For my 2A3s, the whole circuit dissipation is: 2.5 * 2.5 / 54 = 0.115W; the 10 Ohm pot only a small portion of that, I think.
With 27 turns at your disposal, for the 26es you could get good results using the 100 Ohm version without dropper Rs. The 100 Ohm pot dissipation = 1.5 * 1.5 / 100 = a massive 0.0225W... so you would only have a pot rated at 30 times the required dissipation. :^)
Edit: The 1280G pots are essentially non-inductive. They seem ideal for use as hum pots but are used rarely. Maybe the low Wattage puts people off because they are used to traditional higher Wattage parts, which are unnecessary in this application.
Hope that helps!
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 01/26/16 01/26/16 01/26/16
nt
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