[Amps] High temp magnet wire?

[Manfred Mornhinweg]

Jim,

> ##  I dont buy into the RF  RMS  vac /   XL  =  RF current flow 
> through the choke. Even  W8JI  doesnt   buy into it either.    Its 
> more like  RF  RMS  vac  /  Choke Z.

I fully agree with that! It's just that I was assuming that at least on
the lowest bands, which are more predictable, the Z will closely match
the XL.

On the higher bands anything can happen, due to parasitic effects (AKA
resonances).

I wanted to get a better feeling for those parasitics. So I dug out a
big old plate choke from my junk box, and measured its impedance.
The choke is tight-wound with 175 turns of #23 wire, on a 1.5 inch
porcelain tube. It has 1/4 inch wide metal clamps around the ends,
forming shorted turns touching the first and last wire turns. It is
supported by brackets and huge brass bolts that run deep into the
porcelain tube. Both the shorted turns and the brass bolts must be
reducing the inductance below the value it would be without them.
Calculation without considering the effects of these elements predicts
roughly 350 microhenry, while measurement done at very low frequency
gives only slightly more than half that much.

I'm not positively sure where this choke came from, but I think it's
from an antique AM broadcast transmitter of roughly 1kW, handmade in
Chile or Argentina, probably in the 1940's or early 1950's. It might
also have been from a marine transmitter, but that's less likely.

Impedance measurement gives the following:

1830kHz: 2100 ohm, 86 degrees
3600kHz: 4125 ohm, 89 degrees
7080kHz: 8000 ohm, 81 degrees
13110kHz: 1310 ohm, zero degrees (first series resonance)
14250kHz: 5700 ohm, -80 degrees (capacitive, above resonance)
28300kHz: 44000 ohm, 42 degrees
29990kHz: 45000 ohm, 23 degrees

My interpretation of this is: The choke behaves as a reasonably clean 
inductor on the lower bands up to 40 meters, with its impedance closely 
following its inductive reactance. The Q peaks around the 80 meter band. 
At 13110kHz it has its first series resonance. At that frequency it has 
a rather low impedance of only 1310 ohm, and what's more important, this 
is purely resistive! So the loss (and heating) at this frequency would 
be extreme, and the choke would burn up in a matter of seconds if 
operating at that frequency.

Above that frequency the impedance becomes capacitive and rises again. 
Slightly above 30MHz apparently there is a parallel resonance, but I 
can't check the exact frequency because I was using my HF radio as a 
signal generator, and it doesn't transmit above 30MHz.

It looks like this choke would be usable in a ham amplifier, but the 
operator must never try to use the amp near 13MHz, or Very Bad Things 
will happen. Bad luck for "freebanders".

> And the chokes Z  will be a lot higher than its  XL. 

Not on the low bands, according to my measurements. Z closely follows XL 
there. And going higher, first Z goes DOWN, way below XL!

> I tested that ameritron choke..which is wound with 27 gauge
>  wire, with turns butt tight,,, except  for the  2 x gaps.... and it 
> runs stone cold at 1.5 A   CCS..with my small lab supply.  Any more, 
> and it heats up up..and  quick like.

The quick heating has to do both with the losses rising with the square 
of the current, and with the temperature coefficient of the conductivity 
of copper.

But I would like to insists on the point that RF causes a lot more loss 
than DC, on the worst bands, in a typical amplifier application. tetsing 
with DC alone is pointless.

 > JI depicts some sky high  Z
> numbers, etc, when he tested that choke  with his VNA.... on all  9 x
>  bands.

Would like to see that. Is it piblished somewhere?

> Some bands  were better than others. But imo, without the 
> extra  C that is normally used, the  VNA  results are invalid.

Jim, any capacitance you connect EXTERNALLY to a choke does not change 
the choke's impedance, AT ALL, NOT A BIT. It DOES change the impedance 
of the complete circuit, of course, but not that of the individual 
choke. So the choke's current to voltage ratio (its impedance) on a 
given frequency will remain the same, no matter what you connect to kit.

What CAN change things is if you mount the choke so close to the chassis 
or other stuff that stray capacitance to the choke's windings becomes 
important, or if the choke couples to the tank coil and picks up a lot 
of its magnetic field.

VNA test results should be correct, as long as the VNA works right, 
isn't taken outside its calibrated range, and the user knows how to use 
it. When I'm in doubt with VNA results, I prefer to go to the bare 
bones, using a signal generator, a scope, suitable test probes, and my 
trusty old pocket calculator.

 > It
> would be interesting to place aprx 35.2 pf  in parallel with the 
> ameritron choke.. then re-do the VNA  test on 160M.   35.2 pf is what
>  would be required to cancel out the  225 uh chokes  XL on 160m.

Measuring the parallel resonant circuit that way will show a very high 
impedance for the complete circuit, but still the choke's impedance 
hasn't changed.

> ##  Put a clamp on RF ammeter  at the cold end of the choke, or 
> better yet, for safety, put the clamp on RF ammeter at the cold end 
> of the bypass cap at the base of the choke. IE:  between chassis and 
> cold end of bypass cap.  Then measure it.

That's basically what I did, just that instead of a tube amp I used a 70 
volt 50 ohm signal generator (a 100W HF transmitter loaded by a 50 ohm 
dummy load). I used a scope to measure the voltage across the choke and 
the current through the choke. The amplitudes and the phase ratio of them.

> ##  With 7 kv under load of  3A, on a buddys  3x6 amp, the  RF RMS 
> voltage is aprx 4.2 kv.    He tried as little as  just 45 uh, wound 
> with 22 gauge magnet wire,  turns butt tight, with zero issues..and 
> thats  on  160m  over 4 x nights.    I was expecting it to burst into
>  flames.  Nothing happens. 

He must have had that choke parallel-resonant on the band. How was that 
choke built? With lots of interwinding capacitance? Anyway it's hard to 
believe...

> The only thing that happens is an extra 
> 176 pf  of tune cap C is required..... to cancel out the 508.5 ohms 
> of  XL of the  45 uh plate choke..on 160m.

In that case it MUST have blown up. That would have been 8A of RF 
current in that choke. It would have burned that wire. Except perhaps if 
there was strong air flow. Was that the case?

 > We don?t see 8.26 A of RF
> current, that you would expect with  4.2 kv  + 508.5 ohms of  XL..on 
> 160m.

You MUST have had that much current, unless either the choke was 
internally parallel-resonant near the frequency, or its inductance for 
some reason was much higher than you think.

> ##  I got into this years ago with the late Rich measures.  He 
> contends  that choke  RF current is RF RMS voltage  /  XL. Meanwhile
> he also parallel  resonates  the oem  10 uh filament choke, used on a
> heath  SB-220, on his 160m  mod, with an extra 900 pf cap across the
> C2 cap used on his 160m  PI tuned input.   In the case of the 10 uh
> bifilar fil choke,  if a normal, say calculated  values, using a  Q
> of 2-3 used for a 160M  PI, tuned input is  used, input swr is sky
> high.   An additional 900 pf required is to parallel resonate the
> 10uh  bifilar  fil choke.    Ok, then  input swr drops to  1:1  on
> 160m. That explains why   C2 values end up being on the high side of
> the calculated values..on 160m, when using various values of uh for
> the bifilar choke on a GG  triode.    Bypass caps used at cold end of
> bifilar, which puts the entire bifilar in parallel  with the C2 cap
> of the PI  tuned input.
> 
> ## I cant see a plate choke being any different.   Plate choke is in 
> parallel with the C1  tune cap...and also the tubes  anode to 
> grounded grid C.

 > ## What am I missing here ?   The required extra C on a  Tune cap is
 > always the correct amount to cancel out the chokes XL.  If we are not
 >  parallel  resonating the choke, what are we doing then ? Ditto with
 > the bifilar and the  160m PI  tuned   input.

Jim, you fail to separate two things which are physically different: The 
current in the choke, and the TOTAL current. That's is the choke's 
current cancelled by the current through the resonating capacitance. 
ONLY if this capacitance is internal to tyhe choke, and distributed 
along it, will the actual choke current be reduced. Whenever the 
resonating capacitance is external, or lumped, the current in the 
choke's windings will remain high.

By adding the precise amount of external capacitance you can always make 
the total current zero (for perfect components) or very low (for real, 
lossy components), but this doesn't reduce the current flowing in the 
choke. It only adds an equal current of opposite phase, flowing through 
the capacitor.

It's perfectly right that you have to provide this additional tuning 
capacitance, both in the plate circuit and in the filament/drive 
circuit, to achieve proper circuit operation - but this doesn't reduce 
the current in the chokes!

I wish we could get together, have a cup of tea, coffee, a glass of beer 
or whatever, in the company of some sheets of paper and a pen. A few 
drawings are worth more than a long-worded posting like this. A 
schematic, some arrows indicating current flow in each component and 
each connection, would make the issue crystal clear. But there is just 
too much distance from my place to yours...

Manfred

========================
Visit my hobby homepage!
http://ludens.cl
========================