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Now this one really has me puzzled. As part of my effort to determine what parts I still need to order for the deuce26/45 build, I wanted to try some older 10 turn pots I have in my parts bins to see if they would work as hum pots.
I have a separate filament trans and hum pot on each of the 3 tubes I have bread boarded; two 26's and a 45. I have a r/c node off the main B+ supply for the 45 filament and another r/c node off the same B+ supply that goes to a rheostat that is used to reduce the voltage of the two 26 2.5vac filament trans down to 1.5vac. There is a separate cathode resistor and cap tied to the wiper of each hum pot and then to ground.
The very first hum pot is just a regular hum pot, not a 10 turn one. I took one of my ten turns and put it in place of the standard hum pt. Very loud hum resulted that I could not null out with the pot. Thinking it might be a bad pot I tried two more 10 turns of different brands and got the same results. Also tried one of the chinese 10 turns, no change. If I disconnected the ground wire of the cathode resistor/cap pair connected to the center wiper of the pot, the hum stopped. So I put the standard hum pot back in and hooked up the ground wire again and everything is back to normal. No hum. Disconnect the ground wire on this one though and it hums, hook it back up and it stops. Just the opposite of the 10 turn pot. And it only happens on this first 26 tube. I tried putting the standard hum pot on the second 26 instead and it still hummed with the ten turn on the first tube.
What could be different about the 10 turn pots that is causing this? I suppose I could leave the standard hum pot in place, but then I would loose the fine adjustment of the 10 turn. Anyone got any ideas?
Edits: 01/25/16Follow Ups:
I have been so involved with the hum pots on the deuce 26/45 that I forgot that the 26 preamp has the 10 turn pots in it and they are working just fine for hum cancelation. I am beginning to think I won't be able to really nail this down until I build the final version with no clips leads laying around and proper lead dress for everything. I will have parts for both the single turn and the 10 turn on hand by that time so I will try each and report back at that time.
I also took a good look at the back of the 10 turn pots I got from china and they say quite clearly "bourns" and "Mexico", so apparently they were not made in china afterall. See attached liink for more info on these. More study is required methinks, as these can be had at various places around the net for good prices. I really need to figure out why they are working on the preamp but not the amp.
Are you sure the 10 turn pots you got are linear? What is their actual value in Ohms?
I would use a pair of dropping resistors to reduce your filament voltage if possible rather than a rheostat.
How do you tell if they are linear? I don't see any marking on them other than their value (100 ohms) and the terminal markings (1,2,cw).
You are probably right about the rheostat, but I wanted to wait until I had some idea what values to use. I can measure it off the rheostat when the hum is cnaceled.
Edits: 01/25/16
You need to verify the pot connections first. There should be two lugs that don't change the impedance between them when you rotate the control. These are the outer lugs, and the incoming AC should go there.
Next, count the turns that the pot has, then set it approximately to center. Measure the resistance between each outer lug and the wiper. With the wiper approximately in the center, the resistances will be about equal. If it's a log pot, they will be way off.
It did occur to me that if you wired one leg of the AC to the wiper on accident, you might've ended up with some of the problems you're having.
I did all that previously and set them in the middle of the range at half the number of turns so they are linearI guess. The AC was hooked to the two outside lugs.
My guess would be inductive coupling into the pot. This is the wrong application for a 10-turn pot in my opinion.
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Buy Chinese. Bury freedom.
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: 01/25/16
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
Ghosts. Your ghost likes one pot over another. Sounds like a pretty cool dude. Some people have ghosts that simply hate them and they will not stop humming no matter what you do for them.
There are some odd ground paths in your pots.
That is possible considering all the clip leasds involved. I like the ghost theory best though.
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