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In Reply to: RE: What the ?????? posted by Triode_Kingdom on January 25, 2016 at 15:36:40
Could be I guess. That would explain the simple hum pot working ok. I hope not though, because I sure like the fine tuning it gives. I will probably have to wait until I build the final version and have good lead dress to figure out what pot will work best.
Edits: 01/25/16Follow Ups:
My suggestion is to use a one-turn pot and add equal value resistors in series on either side of it to accomplish the total resistance needed. That will provide a finer adjustment window but without the problems of an inductive 10-turn pot.
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Buy Chinese. Bury freedom.
Good call TK. I tried your suggestion with a dale 20 ohm aluminum wire wound in series on each side and it worked very nicely. Hum is basically non existant now and I have more range for adjustment.
Thanks
Edits: 01/27/16
!
The Mind has No Firewall~ U.S. Army War College.
funny you should mention that ..... I was just going to post the following and ask if anyone had tried it. I think I will give it a shot. Does the resistor from pot end to wiper on each side sound as stated below right to you?"If the filaments of dht's were perfectly symmetrical, a ct filament would be the perfect solution, however, they seldom are, so enter humpots, and other remedies. I prefer the use of a combination of precision resistors and humpots, using the resistors to establish gross balance, and the humpot as a fine tuning device. For 2a3, I will typically use a pair of 22r mills, and a 50-100 ohm pots for the fine adjustment."
"Are your 22r resistors wired from each "end" of the pot to its wiper (in parallel to the pot's resistive element sections)?"
Edits: 01/26/16 01/26/16 01/26/16 01/26/16
"Does the resistor from pot end to wiper on each side sound as stated below right to you?"
That's not how I would do it, but yes, it would work. Bear in mind that adding resistors in parallel increases power consumption from the filament transformer. When they're wired in series, the values of the resistors and the pot can be juggled so there's very little additional draw.
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Buy Chinese. Bury freedom.
nt
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: 01/26/16
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
See attached, I have had very good results using this scenario
Edits: 01/26/16
What about what I asked TK about pot end to wiper on each side? Less complicated if it works.
I think its better if you wire it per the schematic. Wiring it to the wiper will simply change the value of your pot but could bypass some of your reactance.
I prefer using resistors and a smaller value pot for fine tuning.
You never know until you try it.
Edits: 01/26/16
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