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Re-Gapping OPT

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I have a pair of transformers that were gapped for 35mA of current. I am running them considerably lower than this (~18mA) and thought that a smaller gap might improve performance a tad. So, the questions is a) is this assumption correct, and if so b) how difficult is this to do, and is it even possible? Any tips/suggestions?
Inductance and gap...


An Increase in air-gap will Reduce Inductance and increase the current at which the core saturates, it will also increase the low freq response to a higher figure--Great if you want to make a guitar output Tx.....:bawling:

The reverse is true, but we are talking about gaps in the order of 0.1mm to 0.2mm Total gap--Meaning 0.05 to 0.1mm thickness 'paper' sheet as the spacer Mag field passes gap twice don't forget And it reduces the current at which the core saturates. If its a high grade core the core distortion appears rather suddenly as the point of saturation is reached ....

If your Tx is rated at say 50mA and your'e running at 35mA, Dont even think of messing with it--It is quite a precision item.--. Its just not worth the effort unless you have graded 'shims' below 0.05mm and some way of installing them to ensure all the laminations are 'butted' up correctly to the shims, and a Good Inductance measurement device to see what effect the alteration has made--Having made a few O/P Tx, I can say that 'setting the gap' is the very hardest part-- Especially when your'e needing a matched pair of Tx......... :bigeyes:

Check out Yvesm Tx design proggie, Interesting to experiment with the gap, the coil winding/interleave numbers and core sizes and materials in there,-- Be easier to strike a new Tx from the data you can get using this great little proggie--Used it many times for various experiments/Tx design and chokes etc.... :D
Setting gaps

Couldn't one come up with a jig that has a micrometer assembly that would slide the E I laminations apart or together using Teflon strips between the laminations?


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Experimenting with the gap of a single ended transformer is not a big deal so long as you don't mind the trial and error method. I've done it. McMaster Carr sells plastic shim assortments of 15 color coded pieces for $33. (stock #9513K42 at www.mcmaster.com)

Better yet, 8x12 inch kapton sheets of various thicknesses are available on eBay from "paper street plastics" for $2 each. (.001 to .005") Standard 2 inch wide masking tape is .005 inches thick. Duct tape about twice that.

An inexpensive Chinese made one inch micrometer is available from Enco. (www.use-enco.com) Everyone should have a one inch mike at their disposal, and even these cheap imports will serve you well for home use. And they're almost always on sale. Look in the "hot deals" flyer.


OK.....I'm confused here now, are we talking of the spacing between laminations of the E I assemblies or the butting up of the E I sections?
See A variable or the B variable (See attached)


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It presumes that you are stacking all the eez and all the I's seperately. Generally e's and i's are interleaved... and the transdesing for worst case minimum gaps...

In a gapped SE transformer the "E" laminations are stacked together, and the "I" laminations are stacked together. The "E" laminations are then gapped from the "I" laminations with a non metallic shim. Non gapped P-P transformers usually have the "E" and "I" core sections are interleaved as in the diagram from Richard Ellis. The laminations are often coated with a varnish and vacuum baked which may make them hard to separate. The better quality transformers often have more varnish, while cheaper ones may not have any at all.

I have successfully taken a P-P transformer apart and restacked the laminations as one big E and one big I. I then tested the transformer as an SE transformer with 3 different gaps made from one, two, or three thicknesses of masking tape as the shim. In my case two layers of tape worked best. I started with an "80 watt" P-P transformer that was made for guitar amps and wound up with a SE transformer that could handle 5 to 10 watts depending on how much bass I wanted. I would say that the result sounded better than the $19 Edcor or the Hammond 125CSE, but not as good as a "real" SE transformer.

I would not attempt these experiments with any expensive (or useful) transformer, but I bought a bunch of these transformers cheap, so I experimented on some.
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