ferrite rod material for GG filament choke

[W4AMV]

I am looking for the permeability for the "classic" ferrite material used in the balun choke providing the filament feed for the 811A...etc.. In lieu of the rod, I was considering a toroid structure to reduce the size of the final unit and I have large toroids on hand; ui in the 800 range. The classic BW FC-xx like FC-25a or the Lafayette MS-333 antenna rod don't provide a tag to the actual ui of the ferrite. I believe the L on these fellows is about 40 uH or so and of course I need to handle the current 4-10 A. Thanks!

Alan

[W0BTU]

1/2" rods of 61 material are typically used for filament chokes. The permeability is about 125. Amidon has rods and even a kit which includes the wire.

https://www.amidoncorp.com/items/10 Rods
https://www.amidoncorp.com/items/61 Kit
http://www.fair-rite.com/newfair/materials61.htm Specs

The reason you don't see toroids used for this purpose is because the filament current can saturate a toriodal core quicker than a rod, reducing its effectiveness at RF.

[WA1GFZ]

bifilar choke does not saturate due to current. Fields cancel with both leads on the rod. Type 61 will have the lowest loss above 40 meters.

[W4AMV]

Interesting comment on the saturation. So, an AIR GAP is required and it begs the question, if the torus is cut and an air gap introduced would that work? The choke need not be a balun and evidence is 8877 amplifier designs seen show independent rods for each filamnet feed. So... the eye opener is the comment on torus vs. rod and lack of saturation in the rod format. Thanks again!

[W4AMV]

A quick research look and indeed the air - gap will slice the B-H curve and reduce the ability for the core to saturate. Thanks on the heads up on rod- vs toroid!

""Introducing an air gap in the magnetic
flux path sheers the hysteresis loop so that it requires more magnetizing force to saturate the
core. The more air gap introduced into the flux path the lower the permeability (ratio of B/H).
Note that the saturation flux density is unchanged even though a gapped core requires more
magnetizing force before reaching saturation... ""

Reference: Specify Saturation Properties of Ferrite Cores to Prevent Field Failure
by
George G. Orenchak
General Manager
TSC Ferrite International
Abstract

[WA1GFZ]

Again it will not saturate due to heater current because the fields cancel
You only need to worry and calculate for DC cathode current.

[k7yoo]

RF parts has the chokes already wound at a good price.

[W0BTU]

Again it will not saturate due to heater current because the fields cancel
You only need to worry and calculate for DC cathode current.

I think you're right. I stand corrected. :-)

http://lists.contesting.com/_amps/2006-12/msg00130.html

So, is the use of rods for filament chokes just design tradition?

[W4AMV]

ok, well this is getting confusing. Lets focus on the 811A. No cathode. In lieu of the ROD, can we use a toroid. If I use 2- toroids of suitable size, each wound with #12 or 14 G wire and I obtain L~ 40 uH for each unit, will that work? A single 811A is just 4 A filament current. I think the goal in this thread is to filter out the folklore. Must the 811A filament feed be via a ROD?

[WA1GFZ]

the heater is the cathode in an 811. The center tap of the heater transformer secondary is the DC cathode connection to the tube. Since the choke is bifilar wound one lead is seeing reverse current of the other bifilar winding so fields cancel.
The only current that will effect core performance is the DC offset due to tube DC current and the AC peak drive voltage.
The 1/2 inch rod donated by Lou McCoy was used in my first dual 811 linear back around '75 now sits in my 4CX3000A linear.

[W4AMV]

So, therefore, a torus bifilar wound works?

[KM1H]

A filament choke is there to provide a high impedance between the RF driven filament and the filament transformer at the other end. Loss is what you want and 61 mix is no good unless its for CB or 6M.

For a 4 holer use the 7 1/2"  x 1/2" 33 mix or the 4" for 2 tubes.

The FC-25A and others are 33 mix.

I have both sizes here and plenty of enamel wire up to #10 (for big tubes), PM for pricing, etc.


Carl

[K1JJ]

I haven't seen it mentioned yet, but Bill/ HG suggested a great idea years ago for a large current bifilar choke.  

Take the two straight fil leads and hold them together. Slip about 12" of large ferrite beads over both wires and secure. No turns required. That's it.

For larger directly heated tubes, (3CX-3000A7, etc) the fil currents can get big and using heavy wire on feriite rods is not easy. By slipping beads over, it's easy and the fil path is short.  I once measured the inductance of one of these chokes on 160M and it was more than enuff inductance.  I don't recall the permabilty/type material of the beads I used, but use something suitable for the frequencies involved. The beads were the large 1" diameter X 1.5" long types.

All of my homebrew linears employ this method these days.

This will work just as well for smaller amps, but for a smaller linear, the standard rod-wound or commercial "FC-XX" series chokes are fine. Just thought I'd mention this in case someone was looking into a bigger fil requirement.


T

[KM1H]

I used to sell the 12" x 3/4" 33 rod for the serious QRO years ago but Fair-Rite quit making it.

You can also use one choke for each tube when dealing with a pair of 4X1's or similar.

Luckily a lot of big tubes (YC-156, 3CX5000, and up) these days are cathode type which minimizes the problem.

[W0BTU]

61 mix is no good unless its for CB or 6M.

Coming from no less an amplifier guru than KM1H, this took the wind out of me.

I've always used 61 material for directly-heated filament chokes in the few linear amplifiers I've built or modified, from 160 through 10 meters. 61 Mix is also the choice of Amidon Associates.

But coming from Carl, what can I say? :-)

[Steve - K4HX]

So, therefore, a torus bifilar wound works?

Is anyone gonna answer this question. He's asked it like three time now.

I'd say yes, since I've seen it done. Anyone else?

[K1JJ]

heheh - looks like we blew that one.

Yes, I've seen several manufacturers using toroidal bilfilar chokes. I think my Henry amp 3-500Z's uses one. There's no reason why most any configuration like this will not work as long it is of enough inductance.

The only caution I would suggest it to keep it a single layer with no turns overlapped back on itself to prevent cancellation or stray inter-winding capacitance effects. Just use the normal toroid winding procedures and you shud be FB.

Also, wind BOTH wires on the same toroid(s). I have seen hum produced by using separate leads and ferrite.  Frank/GFZ, I am referring to your problem with the 4X3 amp years ago. Is this correct?

Hope this answers what you axed about.

T

[W0BTU]

So, therefore, a torus bifilar wound works?

Is anyone gonna answer this question. He's asked it like three time now. I'd say yes, since I've seen it done.

My answer to the question is, forget about making any type of toroidal filament choke.

It may very well be that you can do this, but of all the filament chokes I've ever seen throughout the years in numerous publications, 99% of them have been wound on a rod.

I just came down from my library after poring over many different directly-heated cathode filament amplifier designs, and without exception, ALL of them used rods, and not toroids.

If you want to experiment and build a toroidal filament choke, go ahead. But you are in uncharted (read: undocumented) territory.

[Steve - K4HX]

Henry used toroidal fil chokes in a few thousand amps. It's pretty well documented.

[W0BTU]

Yes, but Henry Radio had issues with them.

http://www.google.com/#hl=en&xhr=t&q=henry+toroidal+filament+choke

[K1JJ]

Yes, but Henry Radio had issues with them.

http://www.google.com/#hl=en&xhr=t&q=henry+toroidal+filament+choke

Interesting.  I didn't read all the links, but this sounds familiar....

"This is KX5JT.  You guys may remember a year ago after acquiring a
Henry 2K4 amplifier that I pressed into A.M. service, I had to replace
the improperly designed toroidal filament choke.  It was NOT bifilar
wound.  A bifilar wound replacement took care of the AC hum on the
carrier."

I will have to look at my own Henry next time it's apart to see if it is bifilar wound. It has no hum on the carrier, but worth a check anyway.  As I said in a previous post, Frank/GFZ solved his hum problem too once he wound both fil leads closely coupled on the same toroidal core, in the same direction.

I can't see why a toroid as a ferrite medium would not work correctly once wound this way - but open to opinions...

T

[W4AMV]

Well, I appreciate all the feedback. Items like this are particularly interesting and the question is mainly prompted by what I have on hand. If I could find a good reason not to fabricate the choke in torus fashion then I would. However, I could not. That said, I am happy to take the risk and see "what lesson is learned".

Thanks!

[Steve - K4HX]

That's documentation. 

Yes, but Henry Radio had issues with them.

http://www.google.com/#hl=en&xhr=t&q=henry+toroidal+filament+choke

[WA1GFZ]

Yup, I had two short rods side by side and the amp could hum a nice tune.
When I swapped over to bifilar the problem went away.
Tom, I think you used type 43 beads on that big choke you built..

[WD8BIL]

Type 43 is good down to 1 MHz. Type 31 covers down to 500kHz. Either one should work for this.
I'd be interested in trying a fil. choke wound on a tooide in typical common mode fashion ie... opposing directions.