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Estimating inductance per turn

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#21
SwedishWings said:
@45, thanks! I tried this before and for could for some reason never make sense of the figures, maybe because i misunderstood some of the factors or mixed up the units. I wound 500 turns and measured inductance under ~50% load and measured 7H. Plugging that in gives mu = 4690 which would be close to expected results. Does this look right to you:

View attachment 1141626
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The estimate is correct. Only remember that you will get a mu of 4500-5000 only if you have minimal air gap and for signal that generates about 500 Gauss or more induction. At very low level mu will be about 1/4 to 1/3 (a few volts applied however corresponds to the power level in the mW region and so negligible distortion, in the worst case from the tubes). Also the reference frequency for measurements should be 50 Hz. At 100Hz 0.35 mm lamination will certainly lose a bit more than thinner lamination (again no problem for the actual use as the higher the frequency the less important will be the function of the core). 0.35 mm is perfectly standard lamination.
 
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#23
45 said:
The estimate is correct. Only remember that you will get a mu of 4500-5000 only if you have minimal air gap and for signal that generates about 500 Gauss or more induction. At very low level mu will be about 1/4 to 1/3 (a few volts applied however corresponds to the power level in the mW region and so negligible distortion, in the worst case from the tubes). Also the reference frequency for measurements should be 50 Hz. At 100Hz 0.35 mm lamination will certainly lose a bit more than thinner lamination (again no problem for the actual use as the higher the frequency the less important will be the function of the core). 0.35 mm is perfectly standard lamination.
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Many thanks for setting me on the right track! This is one of the few things I never fully got my head around. One of the reasons, perhaps, is that reference literature is scarce and hard to digest, at least what I've found so far. I've mostly used RDH4 and Robert G. Wolperts paper from 1989 for reference. How did you learn the subject?
 
#24
SwedishWings said:
This is a PP amp (2 x EF80), so no need for gap. I cut off the center divider and will wind 3:2 interleave this time as the increased permeability allows much less turns. I used 4:3 interleave on the last transformer for that reason, pictured below. Primary impedance was 20K, but the next will be 25K to get a better fitted load line given 250V that i have available.

View attachment 1141630
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You should wind 1/2 turns on the outer primaries respect to the inner ones. This is because the internal primary is coupled to 2 secondaries, the external ones only "see" one secondary. Doing 1/2 turns for the outer ones will make sure that the magnetomotive force is heavenly distributed across the core, hence lower losses and especially higher leakage inductance.
In the picture above you have 1211x4=4844 turns
You could do 1614 for P1 and P2 and 807 for P3 and P3.

Same idea if you change the number of primaries. However if you are aiming at 20K impedance with almost 5000 turns I would actually go for more interleaving. Otherwise your leakage inductance will be rather high and will get significant drop well before 20KHz.
 
#27
I know this post is a bit out of date…. But I’ve been looking for a source of Z11 lams now for a couple of weeks.

I’ve been comparing my Dynaco ST-35 OT’s with 29M6 to a z09 (9mil lams) I picked up on eBay and the inductance is much greater. Z11 might be perfect .

Z11 does not seem to be a thing in the US. I recall the DIYaudio forum was a bit more international. Thought I’d check here.

Any suggestions?
 
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