Ceriatone Forum

American Style => 5F6-A Bassman => Topic started by: mutron80 on January 14, 2009, 04:25:22 AM

Title: BAssman Bias
Post by: mutron80 on January 14, 2009, 04:25:22 AM
I need de cathode current to adjust the bias.

Thanks and sorry for  my english.

Title: Re: BAssman Bias
Post by: wyatt on January 14, 2009, 06:39:23 AM
If you haven't already done so, you measure the Plate Voltage coming off pin 8 of the rectifier.  Careful doing so, it's high voltage. 

Then you measure the voltage from Pin 8 of each power tube socket to Ground (red probe on pin 8 of the socket, black probe on a chassis ground), this will be in milliVolts.   1mV=1mA of cathode current thanks to the 1-ohm resistor running from pin 8 of the power tube sockets to ground.

At the top of this page, select your power tube style and enter your plate voltage. 
http://www.webervst.com/tubes1/calcbias.htm (http://www.webervst.com/tubes1/calcbias.htm)

It will tell you what the cathode current should be for ~70% dissipation.  Adjust the bias pot and keep checking cathode current until you get the tubes in the ballpark.