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Old 19th Apr 2024, 3:57 pm   #1
Chris55000
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Default FX2239 Mullard Pot Core Data needed!

Hi!

Can anyone tell me how to calculate the inductance factor AL of a Mullard Pot Core please?

I'm looking for a modern equivalent to the Mullard FX2239 Ferrite Core Set for the Practical Electronics "Gemini" Preamplifier – Mullard quote the permeability (1150 min!) but not the Inductance Factor – the Ferrite is quoted as 3H1!

. . .I can calculate the nominal inductance the Gemini chokes need to be if I know the inductance factor – it's wound with three intermediate tappings!

Chris Williams
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Old 19th Apr 2024, 4:59 pm   #2
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Default Re: FX2239 Mullard Pot Core Data needed!

PS!

I haven't got an FX2239 so the usual method of winding ten turns and dividing the measured inductance by 100 is no good!

Chris Williams
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Old 19th Apr 2024, 5:18 pm   #3
ronbryan
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Default Re: FX2239 Mullard Pot Core Data needed!

The FX2239 (A5) has an inductance factor (L nH for 1 turn) of 3433 to 6686 from 25C to 70C or 2866 to 6686 from -10C to 70C according to book 3 part 2 of the Mullard Components and Materials Technical Handbook January 1970.

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Old 20th Apr 2024, 8:05 am   #4
kalee20
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Default Re: FX2239 Mullard Pot Core Data needed!

Quote:
Originally Posted by Chris55000 View Post
Hi!
Can anyone tell me how to calculate the inductance factor AL of a Mullard Pot Core please?
Same as anyone else's core!

Inductance factor is ur x u0 x A / l

Where:

ur = relative permeability (the 1150 min that you quote)

u0 = permeability of free space ( 4 x pi x 10^-7)

A = cross-sectional area in square metres (so divide by 1,000,000 if it's quoted in square millimetres)

l = path length in metres.

The above calculation will be a little bit inaccurate if the core shape has a geometry where the area varies around the magnetic circuit, but it'll still give a reasonable figure.

In many cases, particularly for self-oscillating converters, the inductance isn't the important characteristic though - what determines the frequency of operation is when the core saturates. And for a given operating voltage, that's determined by the cross-section area, the number of turns, and the ferrite material saturation flux density, whereas inductance is determined by permeability and number of turns squared.

Last edited by kalee20; 20th Apr 2024 at 8:07 am. Reason: Had to change Greek letters, which displayed as ?'s.
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Old 20th Apr 2024, 8:31 am   #5
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Default Re: FX2239 Mullard Pot Core Data needed!

If you're lucky the core data sheet will give you the effective path length for that size and shape along with the effective cross-sectional area.These take into account variation in section and corner-cutting by the flux.

In this case, the core seems to be gapless unless you use an inert shim. I used to use mostly Siemens cores and they would grind down the centre post to gap them. They even printed the Al value on the core halves (oL meant gapless) which helped a lot.

In general, magnetics suppliers just tol you the dimensions of the thing and curves for the material and you had to figure out everything yourself from there. No wonder people shied away and tried to use capacitors for everything! Coilcraft and Wurth came along later flogging finished mag components with finished component like specs.

It's almost as if resistor makers told you all the dimensions and the resistivity of their materials and never specified resistance or power ratings!

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Old 20th Apr 2024, 8:45 am   #6
kalee20
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Default Re: FX2239 Mullard Pot Core Data needed!

Is there any chance of putting up the circuit diagram (or a snippet thereof) showing how the inductive component is used?

It would help in advising on a modern replacement. Many times I've seen marginal designs, where the vital characteristics are dependent on loosely-specified and highly-variable parameters of the core. Gapless cores certainly have their place, as David indicates, but where a specific inductance is required, an air gap (and more turns, unfortunately) gives a much better-defined and stable inductance.
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Old 20th Apr 2024, 10:17 am   #7
chriswood1900
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Default Re: FX2239 Mullard Pot Core Data needed!

Quote:
Originally Posted by kalee20 View Post
Is there any chance of putting up the circuit diagram (or a snippet thereof) showing how the inductive component is used?

It would help in advising on a modern replacement. Many times I've seen marginal designs, where the vital characteristics are dependent on loosely-specified and highly-variable parameters of the core. Gapless cores certainly have their place, as David indicates, but where a specific inductance is required, an air gap (and more turns, unfortunately) gives a much better-defined and stable inductance.
Out of curiosity I was looking at this and found this info on the Gemini amp.
https://www.angelfire.com/sd/paulkemble/sound5l.html

And the article can be found here, https://www.worldradiohistory.com/UK...cs-1971-01.pdf
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Old 20th Apr 2024, 10:44 am   #8
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Default Re: FX2239 Mullard Pot Core Data needed!

I don't see an SMPS in either of those amplifiers. Is there some confusion with a PE project for an electronic ignition unit by the same name, mentioned earlier?

A friend had built the Gemini amplifier and it worked nicely, equivalent to the Quad 303 indeed.

David
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Old 20th Apr 2024, 10:49 am   #9
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Default Re: FX2239 Mullard Pot Core Data needed!

OK. Thanks for the link!

So... it's a tapped inductor, in a low-pass filter, in a hi-fi preamp.

I'd have to criticise this from a design point of view!

Firstly, for something as loosely specified as an ungapped ferrite pot core, Al varies hugely between samples (figures of -20% / +30% are commonplace), repeatability of inductance between samples won't be good. True, five inductors wound at the same time may show a small spread of values - but wait a year, place another order for cores, and evaluate those - it could be significantly different (the cores by now will have come from a different manufacturing batch).

It's like making a precision timer or filter and using electrolytic capacitors.

Secondly, inductance is poorly-defined anyway, the figure you measure varies with frequency, with excitation level... and even keeping those constant, it'll vary with temperature and with time.

Using a non-linear component will lead to distortion in an amplifier, and this is supposed to be a hi-fi circuit!

Ungapped cores and toroids are used in applications where inductance doesn't matter as long as it's high, and the higher the better. That's not the case with filters.

By far a better solution would be to use a core with an air gap (I'd suggest an RM12 size, which is PCB-mountable), Al of 630nH or 400nH (small gap), and recalculate the turns.
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Old 20th Apr 2024, 11:52 am   #10
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Default Re: FX2239 Mullard Pot Core Data needed!

Complete agreement.

oL core is asking for easy distortion possibly saturation and hysteresis.

I cloned the Quad33 filter. I used Siemens RM10, N28, Al 400, no adjuster.

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Old 20th Apr 2024, 12:27 pm   #11
kalee20
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Default Re: FX2239 Mullard Pot Core Data needed!

Flippin' heck!

From the south-west:

Quote:
Originally Posted by kalee20 View Post
By far a better solution would be to use a core with an air gap (I'd suggest an RM12 size, which is PCB-mountable), Al of 630nH or 400nH (small gap), and recalculate the turns.
From way up north, it had already happened and been validated... there had been no collusion!

Quote:
Originally Posted by Radio Wrangler View Post
I cloned the Quad33 filter. I used Siemens RM10, N28, Al 400, no adjuster.
There's strength in numbers for you!

The smaller size RM10 will involve slightly higher winding resistance, which almost certainly won't be a limiting factor here (AF filters are usually low-Q depending on what response characteristic is wanted), and the smaller size is always beneficial otherwise.
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