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OT design

4/20/1999 10:45 PM
OT design
I'm sorry to keep asking about OT's - this is the last thing I need to do before my amp will be finished.  
Can someone please tell me some typical values for the number of primary turns I'll need? I know my case will be different, but I need a ballpark.  
I'm running 2xEL34's, so that gives 7000 ohm plate-to plate. I'll want to play bass through it, so the inductance I need is 7000/(2*pi*41)=28H, maybe I'll try for 40H.  
I have the laminations, so that can't really change - I'm not sure what they're called, but they're a standard size with a 1.5in wide centre section.  
Parts of this are from RG's site, but his FAQ has an equation missing for this. So I'm stumped for the moment.  
(I designed and wound the PT, which wasn't too hard in the end.)  
Thanks a lot,  
PS - My amp is nearing completion, I couldn't have done it without the help of you guys, so I can't thank you enough. I'll tell you how it turns out.
And now, a word from our sponsors:

4/21/1999 6:46 AM

RG or Stephen can give a far more definitive answer, but AFAIK you actual turns are going to be a compromise between what you would like, and what will fit in the "window" (where the actual coil will be) What will fit is a function of wire gauges(big wire has less turns per layer, less layers to fill the window..), how much insulation (and how thick it is), etc. You need, I think, to figure out your current handling needs, & use that to figure out what gauge wire to use, and work from there. There is an excellent section of the RDH4 devoted to just this kind of calculating. As for primary H, it will be a joint product of your windings and your lamination material, so you're limited by the quality of the laminations you've chosen to use. Keep in mind that the H value you'll measure at low signal levels (like a DMM) will be different (a good bit lower) than the actual operating inductance. (that's normal)  
Actual # of windings will vary based on your own specs.. more=more pri H, but you have more DCR and also lose HF.. everything balances off of everything else. At least that's my understanding. You might also read the Williamson Amp article which goes into great detail as well, building an OT with nearly 7K of primary imp. GD LK!
4/21/1999 6:55 AM

Each set of laminations has what the industry calls an inductance constant, or Al ("A sub L"). This is the inductance of 1000 turns on a square stack of laminations. If you have your laminations fixed, can't change them, then all you have to do is to determine the inductance constant.  
Since winding 1000 turns is quite a task, wind 100 turns instead, and stack your laminations in the 100 turn bobbin. Then measure inductance for the winding.  
Inductance varies with the square of the number of turns, or in an equation, L = KN**2 where K is a constant with all the other junk in it. You have L and N (and therefore N**2), so you can figure out K, or more importantly L/(KN**2), which is the inductance per turn squared. You can then divide this into the desired inductance and get the turns squared and then the number of turns.  
Note that for a -3db at 41 Hz, you're assured to be losing half your power at the lowest notes, so you may want to double or quadruple the inductance to lower the loss. Shoot for 75 - 100H for primary inductance.
4/21/1999 9:13 AM
Bob S

Now I'm confused. I've been working a lot with ot.transformers and I swear I had 7000 to 8000 ohms on two EL-84's and 3400 to 4500 ohms on two EL-34's. I know you can put four EL-84's on a 3400-4500 ohm output trans but I'm curious about those EL-34's on that output trans. of 7000 ohms.  
Bob S.
4/21/1999 3:09 PM

Good point, Bob. I was focusing on the how-to and didn't read the p-p number.  
Most EL34's want about 4K plate to plate for max power.
4/21/1999 7:11 PM

4k p-p makes it a lot easier - I won't need as much inductance. Maybe I got it mixed up with EL84?  
Can someone tell me where I can find data on the laminations, like this A1 const?  
Thanks for the input - I'll keep you posted on what I find.  
4/22/1999 8:03 AM

That's really the problem. You may not be able to find it. Magnetic Materials has a web site, but the data on the laminations is all mechanical, they don't list Al.  
That difficulty is really why I suggested making a 100 turn inductor. You're in effect measuring Al for your laminations.  
The specific iron makes a difference as well, whether it's simply rolled and annealed, what percent silicon, cobalt, nickel, etc, and whether it's grain oriented or not. I'd say that if you don't already know the material, measure.

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