PULTEC Inductors

[AGhostStory]

I am in the process of building a PULTEC EQP1 passive eq circuit with a LANG PEQ2 output amplifier. I have a matched pair of vintage multitaped inductors that I sourced however it is unclear if they were made for audio applications or the exact makeup of them. Also, the inductances are roughly 15 times the values listed in the Pultec schematic hence I would need to multiply resistances, etc accordingly. Do you know any detrimental effects of multiplying resistances etc. on the sound?

As of yet, the prices of the CINEMAG and SOWTER inductors are out of my budget. I was thinking of perhaps winding my own toroidal inductors however I am unsure of the exact specifications that are authentic to the original PULTEC (or LANG) unit e.g. the core diameter, wire gauge etc. Does anyone have any information on this? From what I have seen here, the core material was molypermalloy however beyond this, I am unsure.

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[ruffrecords]

The inductor in an EQP1 is used for the peaking treble boost control. The sharpness of this peak is proportional to the inductance, and the frequency depends on the values of the inductance and the capacitance. Whilst you can reduce the capacitance value to ensure the frequency remain unchanged, you will seriously alter the sharpness of the peak unless you alter the characteristic resistance of the circuit by the same factor. This is highly likely to require some very odd resistor values and it is doubtfull if the circuit could be made to work properly with inductances 15 times the design values.

You would be much better winding your own inductor. This has been covered here - check out the magnetics section.

Cheers

Ian

 

[AGhostStory]

Thanks. Over the past few days, I have looked through the magnetics section etc. The only cores I appear to be able to source are on this website Micrometals Alloy Powder Cores. From the VintageWindings website, the PULTEC core seems to be approximately an inch wide. My two options appear to be here and here. The latter has a higher AL value of 63 compared to the former of 41 however I am unsure if any of these options are suitable. IIRC, I saw a thread in the magnetics section favoring a higher AL however I could be wrong since I don't really know what I am doing.

Edit: My mistake. There are other cores on that site with an approximate diameter of 1 inch. However I'm unsure what AL or perm value to select.

 

[AGhostStory]

Would you recommend a ferrite core instead since they are easier to source? What permeability and AL values should I roughly be looking for? I am going to experiment but I'm really unsure where to start (I've looked at the various threads however I can't find any reference to these values). On this often-linked site, an AL of 1900 was used however I'm not sure he/she was wiring a toroidal inductor.

 

[AGhostStory]

From Pultec Inductors again..., it is stated that the AL of the original was 87.6 uH. Is there a preferable manufacturer/core that has been tried by others and deemed successful on here?

 

[ruffrecords]

I have not wound my own inductors so the details of them I don't know. However, CJ, our resident magnetics wizard, has done a teardown of the Carnhill inductors which should give you enough info to be able to wind your own.

Cheers

ian

 

[volker]

@AGhostStory: You might ask user Moby about the core, he is also winding them, or buy from him.

 

[gyraf]

Don't get too wound up in the precise specs on the material - just wind the required turns onto whatever material is accessible to you, and evaluate from there. It's simply not all that important, contrary to what the magic-dust industry wants you to believe - yes, there is some variation in behavior, but the essence that we like from the pultecs' high boost can literally be had by winding enough wire onto a nail..

/Jakob E.

 

[AGhostStory]

@gyraf Yes, I've decided to just go with these https://www.mag-inc.com/Media/Magnetics/Datasheets/C055350A2.pdf (diameter of 0.928") mainly for price and availability. My main concern is not being able to fit all the turns onto the core. From calculations, I'm going to need 1219 turns for an inductance of 156uH. Will I be able to fit this onto the core or is it preferable to go for a larger diameter one for ease of winding (since I am doing it by hand)? Do you know what AWG of wire I should start with? I've heard some people using 34AWG however I'm worried I won't be able to fit all the turns on if the wire is too thick.

 

[volker]

@gyraf Yes, I've decided to just go with these https://www.mag-inc.com/Media/Magnetics/Datasheets/C055350A2.pdf (diameter of 0.928") mainly for price and availability. My main concern is not being able to fit all the turns onto the core. From calculations, I'm going to need 1219 turns for an inductance of 156uH. Will I be able to fit this onto the core or is it preferable to go for a larger diameter one for ease of winding (since I am doing it by hand)? Do you know what AWG of wire I should start with? I've heard some people using 34AWG however I'm worried I won't be able to fit all the turns on if the wire is too thick.

https://www.coilwindingmachines.eu/engineers_corner/toroidal_calculations.html
This should help. From the length and diameter of wire you can also easily calculate the DCR to know you're on the right track.

Before winding the complete thing I would recommend to put 50 or 100 turns on the core, measure the inductance, calculate the actual AL value from that and adjust the winding numbers accordingly. Although the one you selected is rather tightly specced at 8%, with a stereo pair you can still get very different results from two cores.

 

[AGhostStory]

@volker Thanks! Is it worth going for a higher diameter? I've never wound an inductor and I imagine a larger diameter core would be less fiddly. What AWG would you recommend? I have no idea where to start regarding AWG.

 

[volker]

@volker Thanks! Is it worth going for a higher diameter? I've never wound an inductor and I imagine a larger diameter core would be less fiddly. What AWG would you recommend? I have no idea where to start regarding AWG.

I don't know, but it is pretty easy using the calculator. Just plug the data of the core in, the number of turns, and try out different wire diameters. What you need to observe is the Theoretical Finished Inner Diameter "ID", that is the hole in the middle that will be left. This needs to be big enough so that your "shuttle" (or whatever would be the right term in English, the piece of wood or plastic you put your wire on) can still pass through. Keep in mind it says theoretical, yours will be smaller due to imperfect winding by hand.

Then you can look at the total length in meters, and put that into another calculator to get the DCR.

Using your core with AL of 105, you need about 1200 turns for 150mH. With the dimensions in the calculator, 0.14mm wire would leave a hole of 12.6mm. and a DCR of 39Ω. I don't know what the value is supposed to be, but for example going one size up to 0.16mm wire would give a hole 12.2mm and a DCR of 30Ω. So just play around with it, look what wire you can get and see if it's in the ballpark.

Like I said in the previous post, I would determine the actual AL value of your core first, so you know exactly how many turns you are going to need.