Core material for plate chokes

[vinyvamos]

I am currently sourcing laminations for some trafo spinning and when thinking about plate chokes I am somewhat lacking in knowledge of what I can acheive. I have digested CJ's awesome thread on the V72 chokes, and have searched for other info here too.

I will be buying a bunch of EI48 M6 lams, so I assume that I can wind plate chokes on those with the I's all at one end, set a gap, and then maybe lacquer the whole thing to keep it still? Terrible plan? The other option is for me to get some ferrite powder 2-piece cores, but I really would like to know how this will change my plate choke spec. Will I need a physically larger powder core compared to M6 lams for a given L and DC mA? I assume I can set the gap easier and then just put epoxy on the join. I will need to experiment with multichamber winding and such. Steep learning curve to come!!

One example I have of this here is the Carnhill VTB2379 500H / 5mA choke. It is a hefty beast to be putting on a PCB, which is how it is designed and how I have used it already. It is a powder EE(?) core. For example, if I was to wind that choke on an M6 lam core, will it end up smaller? I need this, as my PCBs are already very tight for space.

Any info you can give is much appreciated! Cheers!

 

[Tubetec]

Regular silicon steel wont get you the Henries you need for a V series plate choke , the other thing to remember about these special plate chokes is the bobbin is divided into three side by side sections and series connected , you'll find more about the winding details in CJ documents.

I hadnt seen that Carnhill inductor before , that is up in the range you would need for a plate choke ,but theres no details about the construction ,by sectionalising the winding some of the self capacitance of the coils cancel and that allows your choke to provide a high load impedence at high frequencies and that translates into good high frequency performance from your amp , with one large single coil choke  incidental capacitance will end up having a marked effect within the audio band .  I also notice they have a new range of mains transformers dedicated to tube amps .

 

[vinyvamos]

Hmm ok so it would need to be a part-nickel core then yes? Or how do the powder cores compare? Maybe metglas or other powder core with high perm.??

 

[abbey road d enfer]

Hmm ok so it would need to be a part-nickel core then yes? Or how do the powder cores compare? Maybe metglas or other powder core with high perm.??

You need Permalloy lams. Ferrite cores have too low permeability; they have other advantages, particularly at RF.

 

[CJ]

you can wind V72 chokes on a steel M6 core. 

remember, there is DC on the choke, so it has a gap,

the original one piece snap in lams had a built in gap for stability,

you can use a paper gap for the EI 48 lams,

when you gap a core, the permeability drops to 5 to 10 percent of the original perm,

say you had some M6 rated at an effective perm of 10,000, the gap will drop this to 500 to 1000 depending on the thickness of the gap,

so it is a waste to use nickel lams anyway since the gap makes it like regular steel.

the way Telefunken gets those big Henries is by using many turns, around 20,0000 in this case.
inductance is a function of turns squared, so with 20,000, you have a factor of 400 million, compared to say a power supply choke of 2000  turns = 4 million, so by adding ten times the turns, you get 100 times the inductance.

now you do not really need 300 Henries for the V72,  this was kind of overkill, you would probably be hard pressed to notice the difference between 300 H and 100 H,  but it is nice to have that ultra flat response.

many turns of wire leads to capacitance which will short your hi frequency signal to the filter caps, you want that choked to block all frequencies,  so that the  signal is forced through the output cap into the primary of the OPT.
so you need 2 or 3 chambers to get the capacitance down.

C is reduced by 1/# of chambers^2, with 2 chambers, C is reduced by 1/2^2 = 1/4

3 chambers, 1/3^2 = 1/9 th of capacitance.

what size gap to use? start with about 0.005" and do some measurements.
note that is is hard to get an accurate reading with the choke out of circuit,
why? because there is no DC during the inductance test.
DC will steal some of the perm and tilt the B-H curve, so that AC inductance will be lowered a bit.
the 20,000 turns gives you great inductacnce, but it is not good in that it generates many ampere-turns,

at 1 ma plate current, you have 0.001 x 20,000 = 20 ampere turns, this H field is going to eat part of the core, what is left will be used to achieve maximum transfer of AC to the output transformer. the gap will help keep the DC from eating the whole core, that is it's purpose.  most of the magnetic energy is focused in the air between the E and I lams. Kind of like two resistors in series, one small  resistor is the core, the air gap is the large resistor so it will have to deal with most of the energy field generated by the DC.  This keeps the core from saturating from too much energy.  AC flux does not care about the gap, so Henries for AC is strictly a function of core cross section, turns, and perm, not by magnetic path length. (core path length plus air gap length)

Ferrite cores have better frequency response than M6, but this is not really important as the response in this case is dominated by coil capacitance. Ferrite is also brittle, hard to gap, so steel is the best material for chokes.  There might have been some V modules that used ferrite chokes, but they were not as popular as the 72 and 76.

 

[gyraf]

CJ, you're amazing.. Thanks!!

 

[vinyvamos]

Awesome CJ, all stellar info there and I think I grasped 99% of it on the first read  ;D.

Just about to finalise my order with DuarteNeves in Portugal and I need to decide on bobbins. I will get mostly plastic single chamber bobbins for making OPTs, but is it worth me getting some of their plastic dual chamber bobbins for these choke attempts? Bearing in mind that they are the ones normally designed for mains trafos so that there is plastic between primary and secondary. Is this plastic too thick for the dual chamber, lower capacitance approach??

Yes I had always thought that the henries of these chokes was a tad ridiculous, as if their winding dept. was being paid per turn of copper and the project managers were always bumping it up, or something . Same thing can be seen in the germanium TAB modules; ridiculous amounts of winding going on....

 

[Tubetec]

That is good news that the M6 lams can get you in the right ball park with regards Henries .
I have a very old twin coil choke , sections can be wired series or parrallel ,  its more a smoothing choke really , but its clear when you measure the capacitance ,say in the case of series , the reading from the centre tap to either end of the windings has a good chunk of capacitance aproximately equal ,measured end to  end the capacitance is in the pF range proving its canceling out to a large degree . 

Ive experimented with  adding spacers to full width bobbins and its very hard to match the mechanical stability of the split  moulded in . Its probably likely knowing German engineering that twin partitions are  very sufficient for all normal purposes  , in any case three coils will be impossible with a 50/50 split bobbin , so two or four are possible  options , two half width bobbins with the 50/50 partitions might be a possiblity.

A question I'd like to ask is if were winding a twin coil anode load choke are we better off winding both sections in the same run , so the traverse mechanism keeps both windings in step , or could we get more beneficial cancelation of capacitance by say winding the first coil left to right and the second right to left handed ,or for that matter  clockwise vs anticlock wound coils , theres a lot of variables can creep in .

 

[emrr]

Can’t recall if CJ took down the UTC A or O series plate choke.  Curious....

 

[CJ]

i do not think i have had the opportunity to take apart a UTC choke, so i will check evilbay for fresh meat!


you can build your own three chamber bobbins, place the bobbin down on a piece of old album cover cardboard and trace out the inside and outside, cut the outer section maybe a bit short so you allow for the pen tracing further out than you want, you do not want the core lams hanging up on the cardboard when you lace the core,

cut the inside by using a straight cut like you were cutting the paper in half, only stop cutting when you reach the outside of the inner pen trace, then cut the inside allowing about  3/32" extra diameter  since the inner trace does not include the width of the bobbin material,

when you finish cutting the flange, go to where you made the cut in the middle and trim both ends so that you have about a 3/16" gap, this is for two things, it allows the wire to cross into the next chamber, and it makes it easier to slip the cardboard flange onto the bobbin without bending it to the point of causing a crease.

mark the bobbin with a sharpie where you want the flange to sit, then tape it down.

the wire will tend to dam up against the cardboard and push it over, so use light tension and do not wind all the way to the wall. the wire will fill that area by itself as you add layers. you can place wood spacers in the unused chambers while winding the first chamber, but as soon as you remove them, the wire will push the cardboard over, plus, the spacers are a pain to make.

you should use a layer yellow mylar tape over the turns as you wind, maybe after about every 1000 to 1500 turns. this is what Telefunken does. this does two things. it cuts down a bit on capacitance, and it keeps the wires from vibrating. you do not want to dip the choke in varnish as the varnish will add too much capacitance. so you run the choke dry in the circuit. 

now a brain teaser. which side of the choke should you connect to the plate? the Start or the Finish?

this is a dual chamber bobbin for the Reddi DI transformers that we wind. we made a few tools,
we chopped a bobbin down to make tracing easier, and we made a metal plate for marking the line where the flange is to be placed. you want to keep the chambers even, to balance capacitance, and you do not want the flange to be out of alignment as if it wobbles during winding, it will snag the  thin wire very easily and you might not notice til many turns have been added, making unwind the mess to fix the problem a real pain which means you start over.  ???  make sure there are no rough edges on the flange that will catch wire. and this is another reason not to wind too close to the flange,  as mentioned before, this will prevent snags and the thin wire fills in nicely.

to tape the flange down, cut a piece of tape about bobbin wide, place it on top of the flange edge, then use needle nose pliers to pull the tape down both sides of the flange and all the way flush to the bobbin with a tight fit, this keeps the tape from causing a ridge to form when you wind,

Cosmo does make a three chamber bobbin for the V series chokes, but it is sold only to Tab/Funkenwerk.
You can ask Cosmo to make you a three chamber bobbin but it will cost you a $2000 setup fee.

i learned the trick of adding yellow tape as a background visual aid  to the winding table from a pro winder at the place i use to work at.  the red wire contrasts nicely against the yellow, allowing you to keep an eagle eye on your wire while it is spinning,

 

[vinyvamos]

All great info and tips again CJ, cheers! I am concerned about your LP collection though if you are chopping it all into bobbin flanges....  : ;D ;D

 

[Tubetec]

Thanks again CJ,
Ive tried sectioning the bobbin and yes , all the pit falls you mention happened . I was only able to wind a single coil at a time so the thing spread ,I also ended with wires from one coil crossing the the divider to the other section . There was no way I was going to try it out with HT volts applied as it could well have been couple of layers of enamel insulation between copper more that 100 volts dc apart.

My Cnc winder does have the ability for up to four wires to the wind head so a dual ,tri or quad bobbin  anode choke should be doable on it , Of course a custom wire guide would need to be made or maybe nozzles are the way to go  for the super fine wires needed .

I was looking at sewing machine thread tensioners and thinking maybe with added extra felt pads these might make neat adjustable wire tensioners.  I think I remember reading one of your posts on the V series chokes where you mentioned they were wound under considerable tension ,maybe for stablility as no varnish is used but also to squash more turns on  to maximise Henries. The other thing I had thought of was to possibly use a mini weighting scales to make a  tension gauge , costs only a couple of dollars so might be worth try.

I had a think about which end of the choke would be best connected to the anode and my only guess was the start of the winding ie innermost end on the bobbin might help screen things better possibly , from measuring a dual coil choke with the LCR  I can't see any benefit start or finish first in terms of capacitance cancelation .

 

[CJ]

you could wind three sections at the same time if you made a precision wire guide with three equally spaced holes,

the problem is buying three spools of small wire and keeping them all going good at the same time,

i would pick the start also as the lead that goes to the plate, this is the place with the most AC, (superimposed on DC of course).  the lead that goes to the filter cap will be kept pretty constant by the capacitor.  so the place which has the most vibration wire wise is the  the plate winds, since this wind is  under many wraps, it is held more secure than  the outer turns and so won't move around as much.

 

[vinyvamos]

Interesting that you mention the multi-head idea Tubetec. On the coil winder that I recently built I should now consider multi-feeds for this choke challenge!  ;D And as for wire tension measurement, my winder is controlled by an Arduino and one thing I did purchase was a tiny weight cell and interface module. My plan there was to weigh the wire spool before and after winding to measure how much wire I am using. Overkill really. But that weight cell could also be used to measure wire tension.

My winding head/nozzle is on a dot matrix printer carriage with a 0.1mm resolution. You type in all coil details into the arduino via serial terminal and it calculates step values etc. and turns per layer so that it can automatically change direction at the end of each layer.

To create back-tension the wire spool gets loaded onto a big old printer servo motor spindle  (my whole winder is made from printer parts mostly) and that motor has a MOSFET feeding it from 20V supply. PWM signal from the arduino can control how much back-tension gets put on that spool. I also have a reverse function on the pedal, in case I mess up. Then the back-tension becomes a rewind, pulling the wire nicely back onto the spool. This is more for power trafo winding, so for smaller winding I have fitted a small motor and drive board to the carriage itself and the small wire spool fits directly to that spindle. Again, back tension will be created by PWM control of that motor, firstly by braking the motor, and if more is needed I can feed power in. I have not tested that part yet though...

One way that I had hoped to roughly measure back-tension was to measure the current being drawn by both the wire spool motor and the bobbin motor. I have resistor shunts already fitted and reading from them into the Arduino, but have not played with it yet.

We will see how it goes!