Bias supply resistor question.

[joesatch]

re-purposing a PT from a Hot Rod Deluxe. The bias tap is like 42VAC. In the schematic it runs straight to the reversed diode with no resistor in series. On most AB763 i see a 470ohm 2 watt resistor in series. What is this resistor for and do i need it?

[sluckey]

Most AB763 PTs have a 50VAC bias tap. The 470Ω drops a little voltage. It also provides some protection to the PT in case the diode and/or bias cap short.

Since your bias tap is only 42V I suggest building the bias supply like the HRD. It has better filtering.