[Amps] High temp magnet wire?

[Jim Thomson]

Date: Tue, 04 Dec 2018 20:52:14 +0000
From: Manfred Mornhinweg <manfred at ludens.cl>
To: amps at contesting.com
Subject: Re: [Amps] High temp magnet wire?


Jim,

> ##  I tested  22 gauge  plane jane magnet wire, with my current limited
> lab supply, and  ran  5 Amps  CCS through it for  1 hr, stone cold. 
> 22  gauge was used on a buddys  3x6 amp, all bands.

AWG#22 is good for roughly one ampere in tight windings inside large 
transformers, and at least 3A in bundles. 5A is a bit high, but in free 
air it's OK. If you wind that wire close-spaced into a solenoidal plate 
choke, with air flow restricted by the former and by the windings being 
close together, 5A would be too much, specially when considering 
elevated ambient temperature. But the 1A or so of ham amps is of course 
fine.

The resistance of #22 copper wire is 0.054 ohm per meter, at room 
temperature, rising when it warms up. A plate choke wound with #22 wire 
might end up having 0.5 to 1 ohm or so of total resistance. At 5A that 
would be up to 25W dissipation, which is a bit on the high side. But at 
1A the dissipation is at most 1W, negligible for a choke this size.

> The RF current through the plate choke is miniscule at best.

This is a point I would like to challenge. How much inductance would you 
use? 100 microhenry? If you have 2kV rms RF on the plate, on 160 meters 
that would cause a plate choke current of 1.77 amperes! And due to skin 
effect, the resistance of #22 wire rises to 0.18 ohm per meter at 
1.8MHz. That's about 6 to 12W additional dissipation. And that's at room 
temperature. Inside the hot amplifier, and considering the heat rise due 
to the choke's own dissipation, the resistance is much higher, so the 
dissipation rises too. On the other hand there is a moderating effect in 
our favor: The skin depht increases when the bulk resistance of copper 
increases, so the RF resistance of a wire does not rise as fast with 
temperature as its DC resistance does.

> On the lower bands, like  160-80-40-30m, the plate choke
> ends up being parallel  resonated with the tune cap, so the chokes Z  skyrockets.

Ops, not so!!! The choke's Z isn't changed by the tuning cap or any 
other external part! What's right is that the RF current in the choke 
(lagging by 90 degrees) is canceled by a portion of the RF current in 
the capacitances from plate to ground (leading by 90 degrees, thus 180 
degrees out of phase with the choke's RF current), but this doesn't 
reduce the RF current in the choke!

> ##  I tested  24 gauge, but cant find my  results from testing. Suffice to say, 24 gauge  will
> handle a lot more than  27 gauge.

Every 3 gauge numbers the DC resistance changes by a 1:2 ratio, and the 
surface of the wire changes by a 1:1.41 ratio. So #24 wire in free air 
can take somewhat less than twice the DC current that #27 wire can handle.

Inside a compact transformer winding, #24 can take exactly twice as much 
DC (or line frequency) current as #27 can, assuming that in both cases 
the transformer is the same size.

The RF resistance of wires changes less than their DC resistance, being 
roughly proportional to the wire's circumference (and thus its diameter) 
rather than to its cross sectional area. For this reason #24 wire can 
handle only about 41% more RF current than #27 can.

I have to add that in calculating the RF resistance of wires I neglected 
the fact that when they are close-wound into a coil, there are 
current-crowding effects caused by the adjacent turns, which drive the 
RF resistance further up.

What we need to keep in mind is that in a typical tube amp plate choke, 
far more heating is caused by RF current on the lowest band, than by the 
DC. If the wire is dimensioned just with DC in mind, it will likely burn 
out.

Manfred

##  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. 
And the chokes Z  will be a lot higher than its  XL.   JI  depicts a chart for the infamous
225 uh  ameritron choke, depicting choke Z  vs  freq.   And no tune cap depicted either,
or tube C... just the plate choke.   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.     JI depicts some sky high  Z  numbers, etc,
when he tested that choke  with his VNA.... on all  9 x bands.   Some bands  were better than others. But imo,
without the extra  C that is normally used, the  VNA  results are invalid.   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. 

##  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. 

##  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.  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. 
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. 

## New choke  setup is a 45 uh choke at right angles to a 135 uh choke,  with a bypass cap at  base of
135 uh choke..at the chassis.  Another cap is  wired at the junction of the 2 x chokes.... with the cold end
of this 2nd cap.... going to a SPST relay, then  to chassis.   Also,  4 x  1 meg resistors, 3 watt mof types
are wired in series... and wired  between cold end of junction / 2nd cap..and chassis.   With relay open,
cold end of junction cap is  DC grounded.  With relay closed,  cold end of junction cap is DC grounded,
and RF grounded..... and the 135 uh choke is now effectively out of the circuit.    45 uh used on 80-15m,
and both chokes used on 160m only.    45 uh choke since replaced with a 70 uh choke..since amp only
goes up to 15m.     Same deal though,  both chokes used on 160m, but the  70 uh choke  used on
80-15m.   Both chokes wound with  22 gauge magnet wire. 

##  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.

##  On a sorta similar note, say with  CM  chokes used at the feedpoint of a balanced yagi , or  dipole, 
RMS  RF voltage /  CM chokes  XL  is never used to calculate   CM current on coax braid.  Its
always the RMS voltage  /  CM chokes Z =    CM current.   And  CM current is easily measured 
with a clamp on RF ammeter...which jives with rms voltage /  Z.  

## 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   VE7RF.