12v heater artificial center tap, what resistors to use??

[ampfab]

i'm building a trainwreck type amp using an old Baldwin organ power transformer. i want to use a 12v heater supply to heat the 12ax7 tubes.
the 12v supply is not center tapped, i want to add an artificial center tap‎, should i use the usual 100 ohm resistors? or should i double the resistor value because of the higher voltage?

[darryl_h]

The value used is not very critical, as long as the power dissipation is well within the resistors' rating.

If the heater voltage is 12.6VAC, and you do use 100Ω resistors, the total power dissipated by both resistors would be 12.6^2/200≈0.8 watts. A pair of 1 watt resistors would work fine.

[ampfab]

Thank you

[rdjones]

More important to the function is that they be fairly closely matched.


rd

[ampfab]

I'll be sure to use low tolerance resistors. Could I simply ground pin 9 of the phase inverter?

[rdjones]

I'll be sure to use low tolerance resistors. Could I simply ground pin 9 of the phase inverter?

Hmmmmm ?
Interesting question !
I'm having to think about this one.

reddog Steve

[JazzGuitarGimp]

Could I simply ground pin 9 of the phase inverter?

As long as you don't do something that would cause current to flow from pin 9 to GND. For instance, trying to derive 6.3V for the power tubes by running their heaters from one side of the 12.6V winding to GND - this would definitely be a bad thing. So, as long as both ends of the 12.6VAC tap are used only for the 12V tubes, grounding pin 9 of the PI tube will balance the AC heater current about GND and there will be no current flowing from pin 9 of the PI to GND.

Like so:

[darryl_h]

I'm having to think about this one.

So am I.

It shouldn't do any harm, but maybe for the minimal cost of two 1W resistors, it's better to stick with the conventional method.

[Structo]

Didn't a lot of Fender tweeds do that? Half the heater circuit went to ground.

[img:800:518]https://lh4.googleusercontent.com/-58bJ ... 5205C1.JPG[/img]

[ampfab]

I may experiment and try both methods.
Thanks for your thoughts.

[ampfab]

Didn't a lot of Fender tweeds do that? Half the heater circuit went to ground.

[img:800:518]https://lh4.googleusercontent.com/-58bJ ... 5205C1.JPG[/img]

I would be using a 12v source, hooked to pins 4 and 5, but reference the circuit to ground by attaching pin 9 to ground. No current should flow through the circuit connected to ground. Like jazz's schematic.