Replacing heater wiring in Fender SF Champ amp

[Bob M.]

Hi all,

Today I'm replacing the heater wiring in a Champ amp with a twisted pair and and ground reference resistors. I have a choice of two wires to use : a 20 gauge, 600 volt rated wire or a 22 gauge, 300 volt rated wire. Normally, I'd use the 20 ga, 600 V wire but as this Champ has only 1x 6V6 and 1x 12AX7a for tubes with not alot of heater current involved, I'm wonder if I'm safe using the 22 gauge, 300 volt wire in this particular amp.

Thanks,

Bob M.

[Justin Thomas]

Let's see, the current for the tubes is .45A + .3A = .75A. Round up to 1A. 22guage will do 8A @ 80°C, using Google. Even allowing for a substantial margin of error, I think either will be fine... Use whatever you're comfortable working with.

Justin

[Bob M.]

Thanks for your information. I had to get this done so I wimped out and used the 20 gauge wire as I knew there would be no problems with this. Sometimes in the past, I have used a heavier gauge wire just on the power tubes and pilot light and then a smaller gauge wire on the preamp tubes (as long as there weren't too many small tubes). It's tough to manipulate the larger gauge wire and I value a neat and clean cosmetic look for the twisted pair string as well as being electronically sound.

BTW, this particular Champ came out so quiet, I was amazed. It makes almost no noise at all, that is, until your plug in that guitar with the single coil pickups.

Thanks,

Bob M.

[loudthud]

Let's see, the current for the tubes is .45A + .3A = .75A. Round up to 1A. 22guage will do 8A @ 80°C, using Google.

I ran into this issue laying out PCBs. Often, the amount of copper required to conduct a certain current will produce too much Voltage drop. 80°C is way too hot. You don't want wires so hot they will burn you. According to the table in my old Amateur Radio Handbook, #22 is good for 0.918A and #20 is good for 1.46A. #22 is 16.46 Ohms per 1000 Ft, #20 is 10.35 Ohms per 1000 Ft.

[Justin Thomas]

Thanks for the explanation! I'll investigate some more and learn some.

Justin

[mhuss]

The old 100 watt Hiwatt heads used 22ga solid PVC insulated wire for all four EL34 heaters (@1.5A each) daisy-chained, and I've never seen a problem. Same wire used to connect all four ECC8? preamp tubes (in fact it was the same wire that was used for signal connections).

It's both current and length that matters.

[Steve A.]

I ran into this issue laying out PCBs. Often, the amount of copper required to conduct a certain current will produce too much Voltage drop. 80°C is way too hot. You don't want wires so hot they will burn you. According to the table in my old Amateur Radio Handbook, #22 is good for 0.918A and #20 is good for 1.46A. #22 is 16.46 Ohms per 1000 Ft, #20 is 10.35 Ohms per 1000 Ft.

Are those amp ratings for 120vac or 6.3vac? Isn't it the wattage that determines the heat generated? Also perhaps the 80°C figure is for ambient temperature...
Just wondering...

Steve

[loudthud]

Are those amp ratings for 120vac or 6.3vac? Isn't it the wattage that determines the heat generated? Also perhaps the 80°C figure is for ambient temperature...
Just wondering...

The text isn't too clear. The chart is really for winding transformers and the amp rating is based on 700 "circular mills" per amp. A circular mill is just the diameter of the wire (in thousandths of an inch) squared. The text says 700 is suitable for small transformers, but numbers between 500 and 1000 are typically used. Current ratings are based on temperature raise, so Voltage isn't much of an issue. The numbers I gave above are probably very conservative for chassis wiring.

[Enzo]

Steve, wattage is current squared time resistance. Doesn't care about voltage.

Not sure what you mean by ambient temperature, 80 degrees C is 176 degrees F. That is a little higher than room temperature. It will burn you if it gets that hot.

[Chuck H]

Just to follow up on LT's post (which is the most relevant AFAICT)...

According to an on line calculator (available to anyone who searches for it) using 22AWG wire will cause a .097V drop if you use three feet for 1A at 6.3V. That's roughly the amount of wire you might use with a tightly twisted pair on that amp. Round up to .1V and it's still negligible. That said... I typically use 18ga. or 20ga. for filaments 'just cause' it's what I have in my arsenal of wire for that purpose. And THAT said... Anything that makes filament wiring easier, like the opportunity to use a smaller gauge wile, is welcome on my bench. I hate wiring filaments. It's a pedantic task and modern tube sockets don't accommodate a twisted pair well. My soldering is sloppy enough without having to negotiate tiny holes with multiple strands of big wire paralleled.

JM2C
Chuck

[Steve A.]

Steve, wattage is current squared time resistance. Doesn't care about voltage.

I grew up learning that wattage was current times voltage. 2A @ 120vac is 240 watts. (One of the permutations of Ohm's Law.)

Not sure what you mean by ambient temperature, 80 degrees C is 176 degrees F. That is a little higher than room temperature. It will burn you if it gets that hot.

Ratings are often quoted in terms of ambient temperature. More often than ratings specifying a particular temperature created by a specific voltage at a specific current...

I was questioning the ratings mentioned in the post. The chart at the following link indicates that 22GA wire can handle 7A for chassis wiring (short lengths, not bundled)...

American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies and wire breaking strength

So is current or wattage the critical rating here? 7A at 120 vac is 840 watts, 7A at 6.3 vac is around 44 watts.

Steve Ahola

[martin manning]

Don't forget that the only leads carrying the full filament circuit current are the ones from the power transformer, and they are sized accordingly by the manufacturer. Beyond that, the highest current loading in the filament wiring is along the two or three inches between the first and second power tubes (unless you run the PT leads to the pilot lamp first like Fender did). I'm with Mark; 22AWG is fine for 100W amps.

[mhuss]

Are those amp ratings for 120vac or 6.3vac? Isn't it the wattage that determines the heat generated? Also perhaps the 80°C figure is for ambient temperature...
Just wondering...

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." The higher the voltage, and lower the current. The main reason mains power is distributed from power plants at >100kV is that higher voltage means lower current, and therefore lower losses in the wire. This also means our European friends have a slight advantage using 240v vs. 120v.

To the point at hand, you can easily calculate the losses in any given wire by applying the Ohm's law. The resistance per foot or meter for a given wire size can be looked up, and heater current times resistance of, say, 6 inches /15 cm of wire will tell you how much voltage will be lost in the wire.
I x I x R will tell you how many watts of heat will be lost (and heating up) said wire.

[Malcolm Irving]

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

Edit: OK the total heat inside the chassis is reduced, if the wire is shorter, but as far as the temperature of that wire is concerned - not much help.

[Benedikt]

Current is the critical rating here. Any wattage dissipated over the resistance of the wire will be minimal.

There are a few handy derived formulas for power from the standards (voltage X Current), The one Enzo mentions and Voltage Squared divided by resistance. Of course these only hold true for purely resistive circuits when AC is involved.

The temperature rating is generally indicating the temperature of the ambient environment where the wire will live for that current rating to remain valid. If it's in a warmer environment you must de-rate the current rating (usually from a chart). 80 degrees C seems pretty high for this. usually Its around 30 or 45 degrees C.

Not all 22 awg gauge is equal in rating. but for an environment such as a small amp chassis just about anything 22 gauge should be expected to handle at least 5 amps. The sub amp rating is for power transmission implies a much more significant length than is relevant here.