Heater wire?
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- lovedat800
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Heater wire?
I just realized while reading a post that heater wire is supposed to be quite big ! It said 18 gauge. I never realized that when i built mine and i think i used what looks like probably about 22-24. yet my amp is quiet. then again, could it be responsible for sonic issues? Should i redo it, and what should i use?
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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.
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.
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http://www.powerstream.com/Wire_Size.htm
To answer one of your questions, I really don't think the wire gauge you use on the heaters will cause any sonic issues,
To answer one of your questions, I really don't think the wire gauge you use on the heaters will cause any sonic issues,
Last edited by Alexo on Wed 04/02/08 11:20 am, edited 1 time in total.
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A quad of 6550's might need 18 gauge,but a pair of EL84's will be very happy with 22 gauge.I use 20 gauge in all my amps,even 50 watt marshall styles with no issues.
Measure the heater voltages.If the last tube in line has significantly less voltage,it is being starved.If the voltage is less,it is being starved for current.
Use 20 gauge to be safe,as it is easier to work with than 22 gauge.
Measure the heater voltages.If the last tube in line has significantly less voltage,it is being starved.If the voltage is less,it is being starved for current.
Use 20 gauge to be safe,as it is easier to work with than 22 gauge.
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I'd think that it would depend on the load and how it's distributed. For example, if you've got a quad of 1.5A octals, 18ga for them, but you could drop the preamp wiring to 20 or 22ga. Of course, if you've got seven 12a_7's instead of three, maybe 20 is best. If they're all noval sockets, then 20ga at most. It's hard to shove a lot of 18ga connections into those lugs.
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Based on how many metres length? If your heater wires aren't too long, then the resistance and hence ohmic heating effects won't be much.Phil_S wrote:This is too close to the rated limit for 24AWG wire.
That formula isn't appropriate here IMO. The correct formulae to use for the power dissipated in the heater wire are V*V/R or I*I*R, where R is the resistance of the given length of wire. From that you can work out the actual temperature rise of the wire, given its heat capacity - provided you have the patience.Phil_S wrote:Remember 3.42A * 6.3V = ~22W and you really want big enough wire to handle this.
Personally, I'm using twin zip cord these days, just like in the original "first born" 18Ws, and I haven't noticed the wire getting even slightly warm Because the wire isn't twisted, it also keeps the effective lengths shorter and hence the resistance lower. And the close parallel conductors still defeat radiated hum.
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+1 on that stuff. Sure is nice not to have to twist those dang leads anymore, no hum issues at all. I use 18ga. but that's a little aggravating on novals....zaphod_phil wrote:
Personally, I'm using twin zip cord these days, just like in the original "first born" 18Ws, and I haven't noticed the wire getting even slightly warm Because the wire isn't twisted, it also keeps the effective lengths shorter and hence the resistance lower. And the close parallel conductors still defeat radiated hum.
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You can get zip chord in the normal guages at Rat-Shack. I haven't done an amp with zip chord yet, but it is appealing.
I generally use 22 guage on < 50 watt amps with no problems. As far as heaters are concerned, I think lead dress around the socket is the most important way to keep noise down.
I've built amps with twisted (which isn't all that hard if you have a drill) and straight wire across the preamp tubes, with twisted for longer runs. I've had no problems.
And remember, when you jump to the next tube your load diminishes. So the most important run is from the PT to the first prower tube. When you jump to the next power tube, you can subtract the needs of the first power tube and so on. So 24 guage might be fine if you jump directly from the PT to the first EL84.
I generally use 22 guage on < 50 watt amps with no problems. As far as heaters are concerned, I think lead dress around the socket is the most important way to keep noise down.
I've built amps with twisted (which isn't all that hard if you have a drill) and straight wire across the preamp tubes, with twisted for longer runs. I've had no problems.
And remember, when you jump to the next tube your load diminishes. So the most important run is from the PT to the first prower tube. When you jump to the next power tube, you can subtract the needs of the first power tube and so on. So 24 guage might be fine if you jump directly from the PT to the first EL84.
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Ah, ZPhil, if there is a run of 2 feet of 18AWG with 6VAC @ 3.5A, the voltage drop is about 0.1V. I admit that I am not schooled in this stuff, so I took a short cut and used this on line calculator. http://www.powerstream.com/Wire_Size.htmzaphod_phil wrote:That formula isn't appropriate here IMO. The correct formulae to use for the power dissipated in the heater wire are V*V/R or I*I*R, where R is the resistance of the given length of wire. From that you can work out the actual temperature rise of the wire, given its heat capacity - provided you have the patience.Phil_S wrote:Remember 3.42A * 6.3V = ~22W and you really want big enough wire to handle this.
So, while I'll gladly defer to you on the appropriate formula, I think, perhaps, you are having a good laugh about this one.
Still, on a serious note, it seems to me that one shouldn't beg for trouble by using wire that is too thin. I've been using 20AWG solid because I find that I have trouble working with fatter wire on a noval socket. I was tempted to try zip wire on my last build, and I don't know why I didn't.
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- zaphod_phil
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I agree, those are the standard ratings, and wouldn't laugh at anyone over them. However, as the explanatory note on that web page says, these are very conservative ratings, and "For short lengths of wire, such as is used in battery packs you should trade off the resistance and load with size, weight, and flexibility", which is pretty much what we're doing here. A drop of 0.1V with a 3.5A load equates to 0.35W being dissipated along 2 feet of wire, which isn't a great deal IMHO.Phil_S wrote:... if there is a run of 2 feet of 18AWG with 6VAC @ 3.5A, the voltage drop is about 0.1V. I admit that I am not schooled in this stuff, so I took a short cut and used this on line calculator. http://www.powerstream.com/Wire_Size.htm
Now for a pair (or quad) of EL34s or KT88s, I would definitely use thicker wire between the PT and power tubes, and then use the thinner stuff after that for the PI and preamp.
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6.5v...
Why worry? You're feeding them enough fire to keep them happy... even Oliver Twist on the far end there.
Zip cord? Interesting. Might be easier to route away from signal paths. Might look neater too. I may be too much of a sentimental traditionalist to try it, even if it is much more easier than those dang twisted heaters
Why worry? You're feeding them enough fire to keep them happy... even Oliver Twist on the far end there.
Zip cord? Interesting. Might be easier to route away from signal paths. Might look neater too. I may be too much of a sentimental traditionalist to try it, even if it is much more easier than those dang twisted heaters
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