Help with choke deisgn

[JDeV]

I am in the process of building a tube pre-amp and require some chokes for the HT PSU. Here in South Africa, that is not easy to find, so I approached a local transformer manufacturer to make some for me. He never did this before but is willing to try if I give him the basic info needed.
The specs I got:
Choke #1: 10H, 100mA, 400V
Choke #2: 20H, 50mA, 400V

(I know currents is overrated for tube pre-amp, but want it like that.)

He mentioned that he would need at least the airgap.

Can anybody tell me what the airgap in both cases should be and also give other relevant info to be able to make these items. He will use EI cores and laminates that is used in small power transformers (normal 230:12V , 50VA trafo's), I don't know what material.

Thanx for any help.

[Sch3mat1c]

Depends on flux, which depends on turns and current. Him being the winder, he knows how many turns to use, and you gave the current spec., so he shouldn't have to ask you this. Maybe he's BS'ing you.

In general, between zero (no spacer material; by the nature of the assembly however, there is still an airgap) and .02" (layer of cardboard or several sheets of paper) for larger chokes.
For your units, I'd guess 5 or 10mil, i.e. .005 to .01" thickness, which is .12 to .25mm to you.

Tim

[fdegrove]

Hi,

No offense but I'm surprised to learn that in a country as big as SA nobody has experience with winding choke or OPTs for that matter.

If that's the case is importing really that expensive?

I'm just asking, my questions may be totally naieve but I used to export to your country and no one ever complained about import regulations.

Cheers,

[JDeV]

Originally posted by fdegrove
Hi,

No offense but I'm surprised to learn that in a country as big as SA nobody has experience with winding choke or OPTs for that matter.

If that's the case is importing really that expensive?

The problem is that in SA the DIY audio market is very small, and people with experience in valve equipment are rare to find. Because of the small market, the odd client usually order from abroad any parts needed. Local manufacturers are willing to try and make a OPT or choke, but they don't have experience in such components, so the end result is usually not close to the well known imported brands. Even with something "as simple as" a choke, the local trafo winders will try to but can not guarantee a good product, anyway the ones I approached so far (that is in Cape Town).
For importing, the cost for an article is not that high (getting better every day lately) but the weight factor sometimes double the price when shipping is added. Then again, a 10H, 50mA choke don't weigh a ton, so will most propably end up importing after all - if my local guy don't succeed in winding 1.

[richwalters]

The internet has opened the world up for us DIYers. For example, the UK has choke/ tranny manufacturers dotted about. There is one near my home town of Ipswich United (UK). I personally wouldn't trust anyone without serious tranny experience to wind my o/p transformer for me. I'm sure it would be wound like a grass bale.
A choke is just as important as a shorted turn is a disaster.
Occasionally I will rewind a mains transformer (if I can split the laminations), but I will not attempt an o/p tranny. We all have the physics and math, but your problem in SA is finding someone who has real competence in doing the practical aspects of winding.
Sorry I couldn't be more helpful
Living abroard as I do, one has to swallow the freight costs but I perservere.
r):

[Jonathan Bright]

I don't want to start a new thread. I have a question about winding an iron core inductor for a crossover. I will cut up an old transformer with a hacksaw or "anglegrinder". I am not going to attempt to do the theory to achieve a specific value but experiment until I get the inductance I need.

But the question I have is this: in cutting up the laminates I suspect I will magnitise then a little due to the stress of the mechanical process. Is that correct and do I need some remedial cure such as running DC through them later.....
Sorry, I'm not even sure if I'm asking the right question...
Thanks for your understanding, Jonathan

[godfrey]

Hi JDeV

Have you tried Eloff Transformers in Milnerton? When I spoke to them a few years ago, they were happy to take on a custom audio transformer job.

I don't think they had any actual experience with audio stuff, but they seemed confident. (/?)

Cheers - Godfrey

Edit: Oops, just noticed this thread is 6 years old ...

[godfrey]

@ Jonathan:
I'm not sure about magnetization but I'd be worried about creating shorts between the steel plates. Can't you get it apart without cutting?