Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on March 15, 2022, 10:48:06 pm

Title: Bias supply resistor question.
Post by: joesatch on March 15, 2022, 10:48:06 pm

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?

Title: Re: Bias supply resistor question.
Post by: sluckey on March 16, 2022, 02:54:15 am

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.