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Posted: **Wed 04/02/08 8:14 pm**

Phil_S wrote:Your typical 2*EL84 + 3*12AX7+EZ81 has a filament requirement of 3.42A. This is too close to the rated limit for 24AWG wire. Use at least 20AWG, but 18AWG is the recommended size. Remember 3.42A * 6.3V = ~22W and you really want big enough wire to handle this.

IMO, the small wire is a fire hazard (or at least at risk of melting the insulation) and should be replaced.

Because it is a low voltage circuit, you can use cheap wire. You don't need 600v rated (the expensive stuff). I like to use solid core. The solid core is easier to deal with when you've got to thread two leads into one pin on a 9-pin socket.

The engineering data does not support your opinion. The copper in the xmfr is likely 22AWG. How could it be a fire hazard? (That is a very strong statement)

Posted: **Thu 04/03/08 7:39 am**

For my heater circuit I use 20AWG wire I get from dead PC power supplies. I find a lot of nice colors

Posted: **Thu 04/03/08 9:48 am**

MichaelJ1965 wrote:
Phil_S wrote: IMO, the small wire is a fire hazard (or at least at risk of melting the insulation) and should be replaced.
The engineering data does not support your opinion. The copper in the xmfr is likely 22AWG. How could it be a fire hazard? (That is a very strong statement)

Hmmm....perhaps I've overstated this a bit and I don't mean to be alarmist.

It seems to me that wire is rated on the basis of how much heat the insulation can take before it melts. If the wire rating is inadequate for the application, then what are the consequences? My conclusion is that melted insulation > short to chassis > either a hot chassis or something starts to smoke (like the PT). Where there's smoke, there's fire.

I'm open to another suggestion. Is there something I'm missing here? I'm not an engineer, so please just explain this so I can understand what's really going on. Thanks.

Posted: **Thu 04/03/08 3:09 pm**

It's also rated on the resistance of the wire over a certain length. The power dissipated is directly related to the resistance, and that power dissipation in turn heats up the wire. With shorter lengths the resistance is lower.

Posted: **Fri 04/04/08 12:23 am**

Phil s,
The HEATER windings of the PT are likely 18 gauge,so you had better take another look.I have not seen many PT's with 22 gauge heater windings!

Posted: **Fri 04/04/08 2:02 am**

Common heater (6,3V) winding is 1,2-1,3 mm or 0,047"-0,051" ( double lacquired).
So,yes Phil S is tehnically right.
Not only that we have safety with 18AWG but we also relieve PT from additional heat,relieving from heat with just a few degrees in C° or F prolongs transformers life.
Ask yourself question why is heaters winding allways last layer on transformer.

Posted: **Fri 04/04/08 7:53 am**

You can use whatever wire size you want.

The comment I made regards the 3.42 Amps is the nominal current draw, the chart says that you can use 24 AWG wire. The chart also goes on to say that these numbers are conservative.

I just felt that the line of reasoning was incorrect saying that 24 AWG was recommended by the chart --- therefore use 18AWG or the amp will catch on fire. That's wrong. I built my first amp using 18 AWG, twisted, etc.. and it was an unnecessary pain in the arse.

There is enough Urban Legend regarding amp design that we don't need more.

Posted: **Fri 04/04/08 7:54 am**

leslie wrote: Ask yourself question why is heaters winding allways last layer on transformer.

(Imagining some guy at the factory....)

"Oh, crap. I knew there was something else I was supposed to put on there! "

Posted: **Fri 04/04/08 10:38 am**

it's really not that hard to twist and install 18 gauge wire into sockets. (at least, i've never had an issue with it)

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Heater wire?