Date: Tue, 04 Dec 2018 20:52:14 +0000
From: Manfred Mornhinweg <manfred@ludens.cl>
To: amps@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.
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