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Small signal audio transformer design for a vacum tube microphone
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SK_Caterpilar_SK:
well seems like my calculations are rubbish.. I wound 222 turns of wire in two layers (110 and 100on top of it insulated with capton tape) and the meter shows me only 1H. Gapping the cores seems to raise the inductance that the meter reads (MIC-4070D LCR METER). I dont know really.. I measured the permeability with 26 turns of 0,51mm wire and the relative premeabillity was spot on 2700 and even a little overachieving. Now when I wound the real deal 600 ohm winding assuming my 222 turns same parameters the relative permeability calculates to be 800: rather low. I toook out the battery and powered it with my bench power supply because I thought it was acting up on me but apparently it isnt. I dont have another tester like this available

I dont know what to do next. Oscillator test? so I can determine based on the resonance the inductance? And even if and the numbers end up being the same, what then?. The 600 ohm winding isnt all that long(9,5meters long at worst).

Its the same core same everything as I said in my previous post. (RM core AL=8700nH/relative permability 2700, not gapped, code: B65815E N30, link to datasheet: https://www.tme.eu/Document/4a01e423b117af0ac949e52794f2a2d2/rm_12.pdf )

Or my meter is acting up I cant really tell.
T3sl4co1l:
At what, 1kHz and 1V or less?

I like how it insists it is "some of the most advanced", yet doesn't actually say what it's doing on the highest inductance ranges, or even have selectable frequency or amplitude, or Kelvin leads I guess; all of which are features I would expect from something claiming to be. :-DD
https://webbuilder.asiannet.com/ftp/810/MIC-4070D%20User%20Manual.pdf

Anyway, at relatively high frequencies, you'll be limited by winding capacitance, and at low amplitudes, you'll be in the initial permeability regime.  Test with a signal generator and you should find it's in spec.

Make sure also that the core faces are clean, planar and well clamped.

Tim
The Electrician:
SK_Caterpillar_SK, you might find this useful: https://www.jensen-transformers.com/wp-content/uploads/2014/08/Audio-Transformers-Chapter.pdf
SK_Caterpilar_SK:
Yeah I just lost all my trust in the meter. The 200mH range said it's too much the 2H range says it's 400mH and the 20H range says it's anywhere between 2 up to 7H (absolute bs) and the 200H range shows it's 20H Soo...I guess pick your own.

I am already hunting for a old TESLA BM591 or BM595 or even a BM591 or one of those older vacuum tube LC meters.. Will see where this goes. I am going to make a oscillator out of it by just putting a cap in parallel (3uF foil brown dip) and look at the frequency on the scope and roughly calculated the inductange. I estimate it should come out roughly @40 some odd Hz.
SK_Caterpilar_SK:
alright well..it might have been correct about the 400 some odd mH. I am completely burnt out. I used a 3,3uF cap (measured 3,271uF) and with the coil in paralel on the scope I measure 125Hz (spot on).

That woul mean its only 495mH. Definitely far far far away from what was expected.. It should have been 5H.

I have also done this with a 1uF wima 630V capacitor and the frequency comes to be 227.3Hz and that means the inductor is 490mH.

I have definitelx done goofed up somewhere. I am starting to suspect they may have given me wrong cores or something.. One is labeled N30 all over and the other one isnt. When you order you order a set not piece by piece. And I also have to say I forgot to buy the clips but I held the transformer very firm and pressed both half cores together where the clips originally are.

Same results arround 490mH

1uF cap gets 227.3Hz which is 490mH XL=700ohms
220nF cap gets me 500Hz which is 460mH XL=1.447k ohms
10uF cap gets me 71.9Hz which is 489mH XL=221 ohms
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