Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on July 29, 2016, 07:17:26 pm

Title: need help tweaking a bias circuit
Post by: pbman1953 on July 29, 2016, 07:17:26 pm

Traynor YBA-3A- with KT -120's


The 120's work but I need help tweaking the bias circuit to bring down the cathode side.


Here's the layout-


http://www.0rigami.com/vb/YBA3__cool-schematics.gif (http://www.0rigami.com/vb/YBA3__cool-schematics.gif)




At this point here are the values in the amp now:


C24 & 25 are 10uf


R36 is 3K.


R37 is  47K


The bias pot is 20k




The highest tube is 81.6 (IK)  and the lowest is 73.8 (IK).


I've been decreasing R36 but it seems to creep back up. The pot is pinned and measuring "0" on my ohm meter.

Any thoughts?

Title: Re: need help tweaking a bias circuit
Post by: sluckey on July 29, 2016, 07:54:36 pm

There is a 130K/2W resistor shown just above the PT. Change that to 100K and see if the cathode current doesn't decrease.

Title: Re: need help tweaking a bias circuit
Post by: pbman1953 on July 30, 2016, 11:02:38 am

The 130K was really a 150K. I tried the 100k and it cut it too much and  now I'm at 135 with  22k resistor from 3K.


Thanks!!!