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RE: Schottky diodes

The C3D04060 has a pretty high voltage drop, which is a tradeoff to get the higher operating voltage. This is not a particularly good selection for a low voltage heater supply.

300B's don't need a ton of current, 1N5820 ought to do fine.

Diode selection is a balance between forward voltage drop, device dissipation (load current * forward voltage at that current), and supply voltage vs. load voltage.

If you use the C3D04060 to heat your 300B's, you'll see a little over 1V of drop across each diode at 1.25A, so we'll say 1.375W of dissipation. The datasheet doesn't specify the thermal resistance to ambient, but it may be somewhere around 60-70C/W, so you may see a 90 degree temperature rise in the diode, which will reduce its lifetime and burn the board that it's attached to (unless you heatsink it).

If you go to a diode that drops less than 0.5V, you end up with much less heat dissipated, as well as more available voltage.

Turn-on will rarely damage these parts, as they are almost always cool at turn-on, and the stress of initially heating a filament isn't going to raise the device temperature quickly enough to matter.

If you are modifying an amplifier that is a commercial design, I would suggest that you are doing a lot more harm than good.

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