Calculation for heater current

[trem]

I did a search and found some info, but I would still like to verify. Regarding a 12ax7, they are rated at 0.15 series and 0.3 parallel. I assume they are referring to how the tubes are wired in relation to the transformer and not how the tube is wired relative to itself. So it would be parallel, at 0.3 per triode in a 12ax7. The 6L6 draws 0.9 amps per tube. The 5ar4 soaks up 1.90 amps, according to the book I have.
So how do you figure what gage heater wire to use? I know 18 gage is kind of standard. But how does one get to that number, and not 20 gage for example. How do I figure it out?
5AR4 x 1
6L6 x 2
12AX7 x 3
Thank You

[defaced]

12a_7 is a dual triode tube, each triode having its own heater. When the heaters in the tube are wired in parallel, which is very common, they draw a combined current level of 300ma - this is the configuration the data sheet is refering to. Same scenario for series connection, its relative to the heaters in the tube.

As for wire diameter, there are guides out there for current carrying capacity for wire given different cabling methods and environmental conditions. Theres also the topic of voltage drop produced by the wire for a given current draw through it. Google "awg wire ampacity". A site from power stream will come up, thats what you want. Id copy the link for you but Firefox is puking on me and wont let me use apostrophes or copy stuff.

[stokes]

The heater current for the AX7 is .3amps per tube not for each triode.The 5AR4 is on a seperate 5 volt winding so you dont count it with the other tubes.What is the formula?I dont know,but I've seen 18,20 and even 22 gauge used in different amps heater circuits.If you are concerned then go with the 18 gauge just to ease your mind.

[tubeswell]

AFAICT you only really need the thicker wires (18-20g) on the output tube filaments. 22g is fine for the pre-amp tube filaments

[Enzo]

I wouldn;t think of bothering with heavier wire than 18 myself. You have about a foot of the stuff. The tiny fraction of an ohm the wire represents is not going to cost you any amount of voltage you could be concerned with. Yes, 16 has lower resistance than 22. But it is so low for either that it doesn't matter much.

Your 6v heater winding will see a 2.7A current draw. And that is only to the first tube wired - presumably a 6L6. The wires to the second 6L6 only carry 1.8A, and from there the wires to the three 12AX7s carry .9A, then .6, then .3.


Your 12AX7 is a dual triode. Each triode has a 6v heater. one heater is from pin 4 to pin 9, and the other heater is wired from pin 9 to pin 5. Series mode means you put 12v between pins 4 and 5. Parallel mode means you wire pins 4 and 5 together, then put 6v between pins 9 and 4/5.

[jrfrond]

This is an area where you don't really need to worry. Use 18AWG and be done with it. You need to worry more about the VA rating of the PT's heater winding.

Just remember to twist your heater string wires up nice and tight with an electric drill. Another hint is to use two different color wires to facilitate watching your heater string phasing. I use green and yellow and twist up a few feet at a time, cutting it to length as necessary. The tighter the wires are twisted, the more efficient the humbucking effect is.

[trem]

Thanks once again for the help. The reason I asked is that the 18 AWG that I have, has a real wicked memory. It kind of has a heavy, oil resistant insulation. So it is almost impossible to get the two colors to stay wound together. My power tubes are done, but it was a struggle. So I was wondering if I might be able to switch to 20 AWG for the preamp tubes.
Plus I was just wondering how to figure the current requirements of the heater wire.
Thank You

[Enzo]

You have any dead consumer electronic stuff around? Dead TV set? SOmething? Often find various lengths of colored wire in those.

If you have wire that is fighting you, do yourself a favor - go buy a little roll of wire that will cooperate.

[jrfrond]

Yeah, fork over $10 and get two new spools of wire in pretty colors. OR.....just use the 20AWG. The run is so short that heater current will not be an issue, and in reality, current requirements diminish with each successive tube in the heater string.