Heaters/Filaments, Centre Taps and Current Draw

[frequencycentral]

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[paulster]

It's easier to convert into watts for each filament, add them all up, and then work out a current consumption from your composite power consumption figure.

In this case you'll end up with 12.6V/300mA.

If you look at the 12AU7 example you'll find that you can have 6.3V x 0.3A = 1.89W or 12.6V x 0.15A = 1.89W. The power consumption is the same regardless of which way you wire it.

In your case each tube is 6.3V x 0.3A = 1.89W

Total power consumption for 2 tubes is 2 x 1.89W = 3.78W

Divide that by 12.6V (nominal supply voltage) and you get 0.3A.

[frequencycentral]

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[paulster]

I'm impressed. It's not often Maplin sells something that even meets, let alone exceeds, the published specifications!

[gearhead]

Alright, I am now confused.

Being a bit parochial, what would the current draw be for a Liverpool and an Express?

3 X 12AX7
4 X EL-84 (or 2 X EL-34)

Tell me if I got this right:

For Liverpool,
12AX7: that's 1.89W per heater times 6 heaters = 11.34W
EL-84: that's .76 A x 6.3V = 4.78W, times 4 heaters = 19.1W

Total is 31.44W. With a nominal power of 12.6v, current is 2.5A

According to this table: http://www.powerstream.com/Wire_Size.htm

I'd need to use 16ga to be safe. However, I know lots of folks here that use 20ga?

[paulster]

You only need to calculate 3 12AX7 heaters since the current consumption figures have already considered whether the filaments are in series or parallel for 6.3V or 12.6V operation.

Since you're running everything at 6.3V in an Express or Liverpool you can simply add up the current figures.

Liverpool: (3 x 0.3) + (4 x 0.76) = 3.94A
Express: (3x 0.3) + (2 x 1.5) = 3.9A

I'm not going to calculate wire gauges because we don't really use them in the UK, just current ratings on our wire. Just bear in mind that the preamp tubes can be wired with smaller wire than the power tubes because their cabling only needs to carry their filament current, assuming that the power tubes are wired to the PT and then the preamp tubes follow these.

[ChrisM]

Hey Rick (kurtlives from DIYStomp)

I am pretty sure the 12AU7 draws 300mA per current when run at 12.6V DC using pins 4 and 5? Or are you talking about 150mA per triode?

[Structo]

150ma @ 12.6v and 300ma @ 6.3v.

[Voodoo_Man]

I'm not going to calculate wire gauges because we don't really use them in the UK, just current ratings on our wire. Just bear in mind that the preamp tubes can be wired with smaller wire than the power tubes because their cabling only needs to carry their filament current, assuming that the power tubes are wired to the PT and then the preamp tubes follow these.

My real 1966 Super uses AWG 20 (.032 in.) solid wire for all the heaters. Cloth of course. It actually measured .033 in. Close enough.

God Bless,
John

[Voodoo_Man]

Alright, I am now confused.

I'd need to use 16ga to be safe. However, I know lots of folks here that use 20ga?

Only if you wire a cigarette lighter & air horns in the circuit too. 16 is way over kill. As I stated in another post, my super reverb uses 20 for all the heaters. That's 1 x 12AT7, 4 x 12AX7, 2 x 6L6, and the GZ34

God Bless,
John

[Jana]

Only if you wire a cigarette lighter & air horns in the circuit

Well it never hurts to plan ahead for future expansion!

[Voodoo_Man]

Only if you wire a cigarette lighter & air horns in the circuit

Well it never hurts to plan ahead for future expansion!

Well, it'd be a blow dryer & curling iron in your case. Aren't those current hogs? Or maybe some AWG 8 for an arc welder.

God Bless,

john

[frequencycentral]

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[frequencycentral]

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[paulster]

Use V=IR (R=V/I) to figure the resistance of each heater at 6.3V using the tube data for the heater current.

Then you'll be able to calculate the resistor to put in parallel with the higher value resistance to make both the same, at which point they'll get half the supply voltage each.

This will result in power being dissipated in the resistor as heat though, so it doesn't really help to minimise your overall current draw.