[Amps] High temp magnet wire?

[Manfred Mornhinweg]

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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