Lattice-wound RF choke replacement, 50 years later

Tim,

The idea behind the multiple section windings on these chokes is the prevention of unwanted resonances.

The choke must present a much higher impedance than the plate impedance over a wide frequency range. A single solenoid that has enough inductance at the low frequency end is going to have a lot of self-capacitance and will wind up (no pun intended) with unwanted resonances at some other frequency(ies) in the operating range. Above any parallel resonance the reactance will go capacitive with disastrous results.

In an attempt to control this, the chokes are effectively broken up into several smaller and dissimilar chokes connected in series.

Here is an example of a solenoid wound choke that uses different winding lengths and diameters to place resonances outside the range of operation:

I see no theoretical reason why ferrite loaded chokes cannot be used as long as all of the design caveats are observed.

=========================== Suitability of any choke for this application can be readily checked (in circuit )with a grid dipper ,which should NOT indicate resonance within any of the used bands.

Frank GM0CSZ / KN6WH

From: Wes Stewart on Fri 12 Aug 2005 10:09

Absolutely. A self-parallel-resonant inductor above its resonant frequency will appear as a CAPACITOR.

Excellent photo quality there, Wes. Also, it amused me to see so MANY sections on that RFC assembly! :-) I've seen quite a few and "dipped the plate, peaked the grid" on many an HF transmitter of olden times using such RF chokes.

I can only add that the Micrometals *free* toroid calculator incorporates approximate self-resonant frequencies in their program's calculations. Excellent program for toroidal inductor design...can't beat the price! :-)

Problem is, if this is a restoration kind of project similar to old automobile restoration, toroidal forms won't do...just weren't many in the old transmitters of the 1950s and before. Anything of the same construction type is going to be an expensive special-order thing now. :-(

snipped-for-privacy@ieee.org

Micrometals manufactures powdered iron, not ferrite, cores. The two materials are quite different. Among the larger manufacturers of ferrites are Fair-Rite, Magnetics, Indiana General, Siemens, and Ferroxcube.

When making an RFC using a core of any magnetic material, you need to insure that the core won't saturate from the flux generated by the DC current. This is done by including flux density in the calculations, and comparing it with the flux density which the material will support before saturating. Powdered iron cores will tolerate a much greater flux density than ferrites without saturating -- but the permeability is much lower, also, which necessitates more turns and therefore higher flux density for a given inductance and DC current. The presence of an air gap in the magnetic path reduces the flux density for a given inductance, so saturation isn't nearly as much of a problem with a solenoidal inductor than with a toroid. However, once again, there's a trade -- the air gap reduces the effective core permeability.

If you don't want to calculate the flux density, you can evaluate an inductor by measuring the inductance or RF impedance while the normal amount of DC current is flowing through it. The current has to come from a source with considerably higher impedance than the inductor's, say from a high voltage source with series resistor, or a transistor connected as a current source. The measurement device is connected through capacitors. Some saturation isn't a problem as long as the impedance remains high enough for the purpose at hand -- as the saturation level increases the impedance will drop.

Low frequency ferrite -- with an air gap -- can be a good choice for an RFC because at HF the impedance is resistive or, at worst, looks like an extremely low Q inductance. This results in an impedance that stays relatively constant for a very wide bandwidth (typically decades), and is just about completely free of resonance effects. But again, flux density has to be considered whenever it has to pass DC current.

Roy Lewallen, W7EL

In fact, this isn't quite a "restoration to original build" type thing, it's more of a "fix some old stuff that's been sitting on the shelf for years and put it to use" type thing. Most of these units are in pretty good shape, and I'm guessing that after the RF chokesdied they were swapped around at hamfests for a few years with each owner realizing they didn't have that choke and passing it on to the next guy... eventually ending up with me.

In fact, Pi-w ound chokes are still listed in the Hammond catalog and a couple of them are stocked by AES. I ordered a 2.5mH one which ought to be able to handle the 100mA or so that a single 6146 will need and I'll grid-dip it and try it out.

I would still like to find a good reference on lattice-wound and Pi-wound coil winding in a do-it-yourselfer's workshop. I think I remember seeing a book that shows how to build a little coil winding machine that'll do some of the ornate pi winding, and now I kick myself for not buying it at the time.

Even though the choke fires took out some stuff, the ceramic core seems to be intact in most cases and I think I could rewind something useful and authentic-looking on them. (OK, I'd use enameled wire and not that funky real skinny cloth kind!). Again the intention is to put these rigs back on the air after fixing whatever is necessary. Having powered them up, most of them are fine right up to the final stage (OK, they generally need new electrolytics in the PS) where everything stops because of plate current. I probably will replace the tubes in mostcases and I'm guessing that after I get plate current flowing again and replacing the miscellaneous capacitors blown up in the final compartment they'll need to be reneutralized.

Tim.

hi Tim, yes, a reference to how to do pie-wound chokes would be good - jumble wound would be fine, but its probably like cable lacing - you just want to know how to do it for the sheer satisfaction of it hi hi !

the picture link that Wes posted looks pretty good aesthetically (and evey toher way as well) - how was it done, how as the former made up - looks like plastic, hopefully a follow-up post from The Man? - these things also provide a useful way of getting B+ to the anodes of the tubes......

I too have a "accidental" collection of old valve type amateur HF radios - (too many, in fact) that need to be got on air as a first step

- will worry about the "authenticity" of the process later.

Andrew VK3BFA

Simple way. Wind a single layer 175uH choke on a ceramic form with 24-26 wire. Good for 600ma and 160-10M. I use these on all my HB from 30W to 1KW.

================================= I'm interested. What is the approx diameter and length of the form and approx number of turns?

---------------------- No turns depends on dia of form but the size of the form don't matter. I use forms from fried chokes, 3/8 inch to 1 inch. With 26 wire and 3/8 dia, expect 3 inches of winding or so then use analyzer to get about 170uH and check for zeros around 23MHz and a second around 33MHz. Tweak the 170uH to get the zero at 23MHz in situ but with the PINET unhooked... GL 73 W7ZFB

Thanks for info.

===========================

number

matter.

Ceramic.

Hi Wes, well, not going to be able to homebrew THAT former then !

What's wrong with, say, G-10 glass epoxy rod for winding RF chokes around?

Is it flammability/gasses made in a choke fire that would make G-10 be not preferred?

Tim.

Hi Tim, got no idea really - over the years I have sort of got used to things bursting into flames occassionally(sp) hi hi , so dont worry about it but just give it a go and see what happens. I also dont leave things running un-attended, so am not the complete lunatic I would seem.

To me, the main criteria is whats on hand - there are not REALLY huge voltages and currents there, so a bit of latitude in the materials selection process seems to me, at any rate, quite reasonable.

No doubt one of the space shuttle designers on this group will promptly debunk my theories.....

And yes, I sometimes DO represent the lunatic fringe of radio - guess I never really grew up. But I have fun.....

Andrew VK3BFA