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
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