SE transformer question
 

I've almost finished winding 2 output transformers for a guy making a SE 2A3 stereo amp, he supplied blank bobbins only and I've wound them to his specifications - he intends to complete the transformers, connecting up the individual windings. This is a project I was contemplating myself sometime in the future so I was curious about the primary inductance and how much it changed with a DC bias, I just happened to have a stack of laminations that fitted his bobbins so I assembled the first TX with 2 sheets of photo copy paper (around 0.15mm) as a core gap, my homebuilt inductance bridge came up with a primary inductance of 67Hy, with 70mA of DC bias the inductance fell to 28Hy - this seems like a huge drop that I wasn't expecting, is anyone able to comment is this what one should expect ?

Re: SE transformer question
 

Can't comment exactly as there's not enough data - but it doesn't sound unreasonable. What is the core cross-section area, and how many turns are on the primary?

A single-ended output transformer has to not saturate with the standing DC current, and have sufficient margin from saturation to cope with the AC flux swing when the output stage is driven with maximum signal at the lowest frequency of interest. Assuming normal transformer iron (approaching saturation at 1.6T) good practice would have it at a quiescent level of about 0.9T so that an upwards and downwards swing of 0.7T can be catered for without hitting saturation, which would cause distortion.

Re: SE transformer question
 

These sorts of things are always fun to experiment with; lots of turns on the primary passing DC will increase the likelihood of core saturation [amps x turns] - if you're happy to sacrifice low-frequency response then use the fewest possible turns on the primary to minimise the core-magnetisation-effect.

[my experience is of AM transmitters, where we deliberately engineered a cutoff of audio frequencies below 300Hz because they contributed nothing to communications-efficiency; as a result the modulator-transformers looked rather lightweight compared with the iron seemingly popular these days with the audiophile types].

Re: SE transformer question
 

Yes. But having increased the turns, you then increase the air gap to nullify the effect of the increased turns, bringing the core flux back to where it was.

And then, because inductance is proportional to turns SQUARED, you find you've got yourself more henries!

Re: SE transformer question
 

The core is a good size lump of iron with 38mm x 44mm central core x-section - primary turns total 3600 - the winding layout is not at all exotic see the pic - I had previously loaned him a pair of 2A3's and was invited to come and listen to the SE amp he had built using Hammond OT's, played through speakers of his own design - 2 way, JBL 15" with Celestion Horns. It was very impressive with bags of volume and was hard to believe it was only around 3 watts max.

Re: SE transformer question
 

OK - so the core's cross-section area is 1,672mm² (generously sized!) and allowing for a lamination stacking factor of 95%, we get 1,588mm² of iron.

It's fairly easy to derive a useful formula relating turns, inductance, current, area, and flux density:

N = L x I / (B x A) where N= turns, L = inductance, I = current, B = flux density, and A = area , all in standard SI units of course.

Morphing this around, we get L = B x A x N / I,

and assuming 0.9T for flux density (thus leaving margin for flux swing); 1588mm² core area; 3600 turns; and 50mA current for the 2A3, we get:

L = 103H.

So you need to gap the core to bring down inductance to this figure. Too small a gap, the inductance will be higher, and from the above formula, you'll be closer to saturation (about 1.6T for most grades of iron) at 50mA.

But - you've got 67H so that's more than enough gap! Which means that the inductance should be holding up to way more than 50mA! Something needs reconciling!

Without access to your set-up, I'm going to postulate that it's down to the rather soggy nature of iron at small excitation levels, the incremental permeability is not the same as the large-scale permeability. I would be inclined to test the transformer on 50Hz AC and monitor the current waveform as the voltage is wound up. Downside is that it'll need quite a high voltage, but if it 'looks' nice and sinusoidal up to your 70mA peak, there's nothing to worry about.

It might be easier to energise the transformer on the secondary, scaling voltages down by the turns ratio, and currents up by the turns ratio, just to keep voltages 'sensible'.

Re: SE transformer question
 

Thanks kalee20 for the detailed post, I'll copy and paste it into a txt file for the time I tackle my own project - I'm not sure what iron (grain orientation, lam thickness etc) will be used in the final transformer, I just happened to have a stack of lams from a transformer I dismantled and intended to rewind as part of the stepper motor power supply for my manual to CNC mill conversion, I have a vague recollection that it came from an old solid state PA amp and was used to drive the 100V audio line, these days 100v lines are outmoded and the usual thing is to use powered speakers, I ended up rewinding a huge toroid instead.

The amp builder wanted the winding leadout wires fed through spaghetti tubing before anchoring and then winding, on one of the transformers I did not not notice that when starting the first secondary winding, the wire had slipped some time prior to anchoring, and I went ahead and finished all of the windings, it was then that I saw the wire wasn't poking out of the spaghetti tube - I cut a short section of the tube off hoping to see the wire, I kept cutting and looking until I had cut off all of the spaghetti tube and still no wire emerged. No option but to unwind it right back and rewind with new wire, I didn't want to chance using the same wire.
For now I'll just hand them over to him and let him worry about the inductance and core gap, if I get invited to a listening test I'll report on how it went.

Re: SE transformer question
 

You havent said what thickness the iron you have is ??. As it was origionally driven by solid state,
( Strong like bull, smart like bulldozer ) it might be "very ordinary " iron ". I used to use John Sankey iron and it was a known and very predictable quantity. Exact inductances with SE transformers is pretty easy to do on paper, and VERY hard to wind on iron.
I would not be worrying so much about inductance, as waiting till its running and do the listen test.
I would suggest you forget "testing " with fancy test equipment, and listen to the music. If its what the amp builder wanted, its right.

Joe

Re: SE transformer question
 

Lamination thickness shouldn't matter much, really - standard thicknesses are optimised for 50/60Hz (mainly what's easy to stamp) and 400Hz (thickness reduced for eddy current reduction). And audio lowest frequencies are not far from 50Hz so if it's fine at 50Hz it'll be even better at (say) 20Hz.

The high audio frequencies are no issue because the flux swing is vastly reduced, so eddy current losses don't kick in (unlike a 400Hz transformer which isn't designed to handle low frequencies, so operates at a much higher flux swing to be able to make it small and light).

Agree with joebog1 - it's not easy to end up with a particular inductance, the more so because the figure you get depends how you measure it! The trick is to have the inductance high enough that it doesn't really matter what it is - oh, and be sure the iron doesn't saturate (as already discussed!) - and everything will be fine!

Re: SE transformer question
 

The lams I used for the inductance test are .5mm thick (.020") I don't think they are anything special just ordinary 50Hz power Tx stuff, PA amps especially the older ones are usually optimised for speech so nothing better is required. They still had very visible remnants of varnish on them so the stacking would have been quite imperfect.

I tend to agree - it is the final listening test that counts I'm sure the guy won't mind if I attend a listening test.

Re: SE transformer question
 

.5mm or 20 thou is pretty chunky iron !!. "Most " transformer iron is .35 mm, or 14 thou. Special iron
( mainly, for dare I mention it, chokes and HIGH quality SE service is 7 thou )
can handle somewhat more flux density, and has a much better stack factor, providing it is stamped from a sharp die. As mentioned, John Sankey ( Newcastle Australia ) were specialists in lamination production, and made some of the very best ever available. If you can find some old or burned 2nd hand iron, and are still on your self education, I would recommend another try if you are not overly successful first time around. Sectioning is another thing to look at, as is winding with bifilar or tri filar winding tecniques. This reduces gaps or voids and distsributes the current over a wider winding area.

I have, on several occasions wound SE stuff in verticle sections, rather than horizontal sections.
They measured better, but sounded the same. They also run cooler.
Of greatest importance is wire tension when winding. Exactly the same wire tension has always created a better transfomer, as has precise insulation between layers. I still prefer paper insulation, rather than mylar tape for the best examples.
Known as onion paper, it used be available down to 1.5 thou thickness. Was supplied by "Mica and Insulating Supplies Sydney " now long deceased of course. Mostly useless information now, but just me musing on SE transformers, and chokes. I posted it hoping you will derive some useful information.

My best regards

Joe