Modern vs traditional heater wiring. How does it work?

[wcrankshaw]

I might be over thinking this, but I'm having trouble understanding how the "new" way of wiring tube heaters works. The old Fender way makes sense to me (I think). A transformer tap supplies 6.3V to one side of each tube heater, and that voltage is grounded out the other side of the heater. The other tap of that transformer winding is grounded. Right?

The new way has a center tap 0V wire with 2 more taps at ~3V on either side, right? So, to me this looks like one side of each tube heater is getting 3V, and the "ground" side is also getting 3V. Is that correct? If so, how does this equate to the same total of 6V going through the heater like in the older method? Feel free to school me if I'm completely missing the point.

[Dave H]

If you assume the heater circuit isn’t grounded for the moment you can see that the heater winding produces 6.3V between its ends and the ends are connected to each side of the filament so the filament has 6.3V across it. Now add the ground connection, this could be to either side of the winding or the centre tap (or to any other place in the circuit) and it won’t make any difference. The voltage across the filament will still be 6.3V.

[wcrankshaw]

Thanks for clarification. I guess what was throwing me off is how if you meter from either side of the heater to ground you get ~3V, not 6.

[Dave H]

I guess what was throwing me off is how if you meter from either side of the heater to ground you get ~3V, not 6.

That was quick! When the centre tap is grounded you do get 3V on each side wrt ground but the sides are out of phase so there is 3V+3V across the heater.

[wcrankshaw]

Ha yeah I've got nothing to do but sit in front of the computer right now. I had a feeling it had something to do with it being an AC supply, but I wasn't sure. Much appreciated. Ultimately, I was trying to figure out what voltage of high power LED's could be daisy chained onto the heater winding, and came across the question of getting a bulb close to 3V or close to 6V.

[Wilder Amplification]

It's what they call a "floating power supply". The CT is referenced to ground so you get 3.15VAC relative to ground, but the heaters "float" across the entire winding, which has 6.3VAC across it. They call it "floating" because both ends of the heaters are basically referenced 3.15VAC above ground due to the grounded center tap.