Lattice-wound RF choke replacement, 50 years later

I'm fixing up some older ham transmitters. 811 or 6146 finals, etc. in
the 50-200 watt class.

Invariably these have sat on the shelf for so long because the RF choke
for the final died a horrible death (sometimes charred beyond
recognition) in their final use. I myself have fried/burnt several RF
chokes and other final components in Heathkits etc. but that was many
years ago when I could order exact spare parts. Sometimes a capacitor
or two is burnt up too, but these I already have in the junkbox or can
buy off-the-shelf.

The chokes that burned up seem to be between 1mH and 2.5mH or so, and
were lattice-wound on what seems to be a ceramic form. Maybe it's a
ferrite, but I don't think so.

6146/811/sweep tube handbook construction articles from the 50's show
similar RF chokes in their circuits and part numbers like "National
R-100" and "Ohmite Z-214".

I look in modern catalogs and I see molded (almost certainly ferrite
core) RF chokes in the right inductance (low mH) and current (100mA to
a few hundred mA) range but they look nothing like the old
lattice-wound choke. Are these suitable substitutes for the originals?
Something in the back of my head says that ferrite core losses with
all that RF across the choke will make the thing burn up at transmitter
RF power levels even if I'm below the DC current limit for these parts.

AES lists some pi-wound 1mH and 2.5mH RF chokes (ferrite core) that
look more like the original. Better idea for these?

If I wanted to learn to rewind the original chokes (very often they
double as ceramic supports for the anti-parasitic RL network and plate
caps) are there any web resources that would tell me how to wind my own
lattice-wound RF chokes? I've wound my share of simple solenoidal
coils in repair/homebrew endeavors, but the slices in these RF chokes
seem very ornately wound. I think there's a Lindsay or similar book
about coil winding, is that on my "gotta own" list?

Oh well, back to replacing those smoking electrolytics in the power
supply, I know how to do that!

Tim.

 

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:

http://www.qsl.net/n7ws/RF_Choke.jpg

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

 

The choke must present a much higher impedance than the plate

===========================
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! www.micrometals.com

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. :-(

 

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

 

On 14 Aug 2005 01:25:38 -0700, "Andrew VK3BFA"

[snip]

Ceramic.

 

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

73 de VK3BFA Andrew

 

how was it done, how as the former made

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