Transformers and non-magnetic fixings

[Greg]

I'm in conversation with Tony regarding his build of his WAD 300B PP clone and he's asked me why James and I went for stainless steel non-magnetic fixings for the mains and O/P Tx's to chassis. I seem to recall some discussion on the old WAD forum on this and I came away with an opinion from what others wrote that non-magnetic is best although the effects may not be audible. Please can anyone refresh my memory on this or offer fresh opinion on any benifits over normal steel fixings? Thany you.

[Neal]

Greg, I seem to recall that steel mounting creates a 'shorted turn' in the transformer....now what that means in reality I've no idea! LOL! Nick or Paul may be able to shed some light.....

[Tony Moore]

Hi Neal,

Having an electrical connection at the top and bottom of the bolt would (I think) make a circuit (through the case and top cap) that could pick up stray magnetic field and be effectively a single shorted turn, reducing the effectiveness of the transformer, creating mechanical vibration and heat.

Wouldn't that also be true though of any conductor, magnetic or non-magnetic?

Assuming that both the magnetic or non-magnetic bolts were insulated at at least one end would there be a difference? Are those holes in the core supposed to be filled with a magnetic material? Are they part of the magnetic circuit? Are the magnetic properties of the bolts similar to the properties of the laminations even? What happens if you put in nothing, or non-mag bolts? Does that mean that the core now has less magnetic material? Is there much magnetic flux at the position of the holes anyway?

Baffled of Chester.

ps. we're talking about E-I cores specifically here. And in my case mains tx and OPTx with corner mounting holes.

[Toppsy]

Wouldn't using potted TX's overcome this problem?

[Nick]

Well, to create a shorted turn i a EI core I believe it would need to pass through the gap, if you imagine the flux path in the core the mounting bolts are not creating a loop around that.

The problem of a shorted turn is most common with a torroid, where its easy to ceratea loop around the flux path if you join the mounting bolt with something conductive.

There is flux outside the core, but just what bad effects that can cause (other than inducing hum) I am not sure.

Some time ago on the WD forum Peter stated that "it was easy to show" that there were currents created in any magnetic material near the TX, ie, this is why they used ali instead of steel cases. I asked at the time what was the way in which these currents caused any problem, but never got an answer.

AN will sell you TX's with copper end caps to reducethis effect, but I am still looking why its a problem.

I guess that inducing a field in a nearby magnetic material will take some of the flux from where it should be, but I don't know exactly where the "bad thing" is.

I can see that creating current in a chassis would be bad if you were using the chassis as a earth path, but thats a bad thing to do anyway, and if anything, a magnetic case will shield the components inside from external fields.

Simple answer from me is I don't know, maybe Paul or James can shed some light.

The cynic in me notes that I don't think the TX's on the power poles have copper or ali cases, but I don't know. Maybe Ali has some information about this?

[Paul Barker]

It's just a nitty gritty thing, but magnetic iron might hold magnetism instead of magnetising and demagnetising which transformer body is meant to. i.e. you don't make transformers out of permanent magnets, which your screws might be in part.

The issue of bridging the lam's is avoided by using insulated thinggies at each end so the bolt never touches any lam's.

I don't expect you'd notice the slightest difference in apower transformer maybe not even an audio but once you know a little you feel obliged to apply that knowledge. Some things do affect sound, which things we don't usually successfully predict, heaven forbid we should leave a stone unturned.

[Tony Moore]

Thanks Paul/Nick,

That's answered my question. I will do the job properly and use stainless steel studding. As you say, when you get to the stage of not really being sure if one thing or another might influence the sound negatively and it's only a small cost to do the "right thing" then it makes no sense to skimp and then always be wondering...

Cheers,
Tony

Now...whether or not to use those Humbucker pots....

[pre65]

Hi-from what Steve S has said (on the "other" forum i think) it would seem 2 fixed resistors may be the way to go sound wise !

If a pot balances the hum on its central setting then fixed resistors would seem to make good sense.

If i keep PT15 i will try it on that at some time.