Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on July 05, 2022, 08:56:38 am

Title: Bias circuit question.
Post by: joesatch on July 05, 2022, 08:56:38 am

typical plexi style bias circuit. What resistor would i change below if more negative bias voltage is needed?

Title: Re: Bias circuit question.
Post by: thetragichero on July 05, 2022, 09:27:06 am

decrease r54 or install a resistor between p1 and ground (this second one is a better idea to me because then you remove the possibility of turning the bias voltage all the way down to zero. maybe it's me but i try to make things as "idiot proof" as possible)

Title: Re: Bias circuit question.
Post by: sluckey on July 05, 2022, 10:09:05 am

A lot of people refer to R63 as the "bias range" resistor. Decrease size for more negative voltage.

Decreasing R54 will result in LESS negative voltage. There is no danger of reducing the negative voltage to zero with the circuit as is.

Title: Re: Bias circuit question.
Post by: thetragichero on July 05, 2022, 12:26:28 pm

ahhh picture is missing the top which would show bias voltage being taken from the junction of r54 and r59. my bad, see above correct answer

Title: Re: Bias circuit question.
Post by: nandrewjackson on July 05, 2022, 05:46:19 pm

+1 for decrease R63