Let's say you just designed something and it turns out you can't find an off-the-shelf transformer to use. What do you do? I'd redesign it, unless I: (a). don't feel like paying through the foot for expensive iron, or (b). really want to build the design.
I haven't done much winding... too lazy... however I have wound a few chokes, and they work just fine. One is in Revision 3, another in Hept'AU7.
<-- Choke in Revision 3
Choke in Hept'AU7 -->
Today I thought I'd start on something a bit more serious. I'll write a page about the project later.. long story short, I need a 2kohm 80 watt PP output transformer. Hammond sells a 4k 100W (or so, I forget) OPT, and I could use it for 2k, putting an 8-ohm load on the 16-ohm tap, but that screws up the frequency response and phase shift stuff. Besides, I'm just feeling unusually constructive all of a sudden. :) So, here's my design:
Target impedance 2,000 ohms anode to anode (500 ohms per plate) to 8 ohms secondary.
Frequency response 16Hz to 50kHz -3dB; maximum power level (saturation limited) at 16Hz, 30W; saturation limit -3dB (45W) at around 25Hz. In the upper range, leakage inductance should be very low given the amount of interleaving; however, parasitic capacitance may be high depending on whether I use paper or plastic for the insulation between layers. I should still be able to get 50kHz no problem.
Windup will be 4 x 500T each, 28AWG for each quarter of the primary, secondary sections 43T 17AWG per, in order of P1-S1-P2-S2-P3-S3-P4, primary wired P1-P3-CT-P2-P4 (CT=Center Tap), secondary S1-S2-S3.
That's about my current pet project... now for the more general part:
For putzing around, you can wind with your hands. This is okay for a few hundred turns, or the occasional 1,000 or 2,000 turn choke, but at that point you'll want to get serious and at least mount the bobbin on a cordless drill (hint: spade bit..). But for a big project like this you'll need a winding jig. This can be as simple as the junker I threw together here, or a fully equipped industrial winding lathe...whatever it is, all that matters is that it works.
This particular device is my old spool holder (the spool axle slides) with two diagonal bits of plywood strapped on to hold a shaft, a bit of steel tube, which was flared at the end for something else, works great to hold it at that end.. bobbin mandrel and crank are held on with pins through the tube, removable so I can get the thing off when I'm done.
If you have a lot to do, you could turn it with a belt or gearing, and preferably a variable speed reversible motor.. but I'm too lazy to do that and only have two big things to do so I'll be happy enough turning the crank a good 2,500 times.
After the winding jig, later on when you've assembled and tested the transformer, you'll want to put some wax or varnish deep inside the windup to keep the wires from vibrating themselves to pieces (Lorentz' force I believe). The easiest way to do this is just plunk the deal into a can of thinned varnish or hot wax and let it sit for a few hours while you read the news (rec.audio.tubes that is :). Next step up would be bona fide transformer varnish. The best solution is to vacuum-impregnate it, and despite the sucking and penetration implicit in that term, it's not a sex act, it's a lot more useful (electronically that is) than that! :^) What's done is the transformer is thrown in a pot of impregnant as above, then the pot is placed in a vacuum chamber. As pressure drops, the air in the winding expands, then when the chamber is opened maybe 10 minutes later, atmospheric pressure forces the stuff deep into the transformer. Nice, but sounds too industrial, right? Nope, rumor has it a pressure cooker pot and an air conditioner will do it! Reverse the seal on the pot (sealing pressure out), rip the compressor out of the A/C and hook the inlet up to the pressure cooker. Given a good bit of time, it'll suck down to a good 27" Hg!
Now, I haven't gathered these parts yet, so I don't have any pics for you, sorry.. However I do have a good description of the process by Patrick Turner of Turner Audio, who is a regular poster and well of knowledge on rec.audio.tubes. Varnishing a Transformer
That's it for now. Tomorrow I guess I'll start winding this beast, if I don't have too much homework to do...
Here's a more accurate diagram. Started winding today...