Replacing heater wiring in Fender SF Champ amp

[J M Fahey]

I grew up learning that wattage was current times voltage.

yes ... and also V^2/R and I^2*R
None is more accurate than the other, all are the same.

So is current or wattage the critical rating here?

Since we are talking wiring losses and temperature, current is the main parameter (if wire diameter is already chosen )

This also means our European friends have a slight advantage using 240v vs. 120v.

"Slight?" ... would you call 4:1 ratio "slight"?

The wire being short doesn't really help. A shorter wire has less resistance, but the heat produced is distributed over a shorter length.

A half truth here.
Yes, "heat produced" will be distributed along a shorter length ... but it will be lower, so temperature will not change.

As in: half the wire length: half the power loss: half the heat produced:same temperature rise.

Or to see it from other angle: doubled wire: doubled loss ... but also double heatsinking ... one compensates the other so actual wire temperature does not increase.

That's why chassis wiring (or home/industrial/distribution wiring) specs only acceptable current per given diameter, the heat is dissipated into surroundings.

Now on transformer wiring, you have this compact block of copper, with minimal ventilation.

As a side note, in my large amps, fans are set up to cool the transformer ... now you can imagine why.

Safe huge power from small (and cheap) transformers
*Always* maximize copper section, to the point of having to press or hammer windings to fit laminations .

Found it the best investment, transformers end up costing (and weighting) less.

[Malcolm Irving]

Wires and cables are rated for current. But it is not really the current that we are actually worried about, it is temperature rise in the cable. Current is being used as a ‘surrogate’.
The reason why there is such variability in what can be regarded as an appropriate current rating is that temperature rise also depends on all the following factors:
Surrounding air temperature (assuming the wire is in air),
Movement of the surrounding air (forced or natural),
Temperature of any nearby objects,
Thermal conductivity of the wire insulation (if any).

[Steve A.]

Power is expressed in watts, and the power 'wasted' in a wire depends on both the resistance and the current - this is why the power companies call these "I squared R losses."

Thanks for the explanation! I imagine that "I squared R losses" are what is used in computing the AWG charts...

Steve Ahola

[Bob M.]

And all this explanation for the lowly Champ amp. I'm sure Leo is standing and applauding in heaven!

Bob M.

[Steve A.]

And all this explanation for the lowly Champ amp. I'm sure Leo is standing and applauding in heaven!

Bob M.

Well, the discussion did meander around to other topics and I learned something about ampacity ratings. One item not discussed was the quality of the insulation; cheap solid core wire will often "skin back" when soldering and if undetected can lead to short circuits...

Steve A.

[J M Fahey]

The "lowly Champ" is just an example, it applies to *any* amplifier

As of Leo, he certainly deserves Heaven

[pdf64]

Another point to bear in mind is that the temperature rise that can be tolerated in the conductor may differ according to the temperature stability / resilience of its insulation.
So eg ptfe insulated wire may be a smaller gauge / rated for high current, than the cheap solid wire wire mentioned by Steve.

[Chuck H]

I love Teflon and silicone insulated wire (because I can't mess it up so badly with my clumsy soldering), but it's too expensive for everyday use. My current fav is irradiated PVC. Much more affordable than PTFE or silicone and only a little more costly, but much more heat stable than plain PVC.