Gio wrote:
Hi Ed. Note that the heater current draw at 6.3V is 300 mA. At 12.6V it is 150 mA. Do you have a schematic drawn up.
Cheers
I have one drawn up, but it's too large to upload. I might be able to redo it in a smaller file format but I can't right now, as I'm traveling.
The 12A_7 tubes have a filament that's center-tapped. You can choose 6.3v operation at 300 mA or 12.6v operation at 150mA. Each side of the filament wants 150 mA at 6v and you get to choose how you give that out. With tubes like 6DJ8, 6N2P, etc, you don't because the two are hard-wired in parallel from pins 4-5.
So, I could run each MOSFET at 12v/150 mA, with the remaining 12v dropped by a current source and half of the tube filament. The current source transistor maintains a constant 6v on the filament, and therefore a constant 150 mA through the IRF510. Everybody is happy. The only problems this causes is a 2-watt dissipation in the output device that forces me to be aggressive on the heat sinking. Not a huge deal. The other thing is that it costs a whole 300 mA from the power supply, not counting the small amount that I'm going to spend on the tube plates (which might be 0.5-1 mA each plate) and other things like LED indicators and whatnot.
A more efficient route would be to run the tube filament separately, as a 12.6v, 150 mA string, and then bias each MOSFET to 50 mA. Total current draw would be 250 mA for the power and filament stage.
My thinking is this - if I were too worried about efficiency, I'd be designing an amplifier that wasn't class-A. Then again, for a headphone amplifier, how crazy do we really want to get?
Ed AE7TE