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Fixed bias

142.119.53.183
Posted on January 22, 2021 at 14:17:42
vetmedrobert@gmail.com
Audiophile

Posts: 453
Location: Ontario
Joined: September 20, 2007
 Does one adjust to desired grid voltage or to desired current? Sorry if this is a stupid question but it seems that I could not come up with the right search terms.

 

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RE: Fixed bias, posted on January 22, 2021 at 15:49:28
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 Current. It's all about the amount of heat dissipated in the plate at idle.

Voltage (plate to cathode) times the current equals the number of watts dissipated by the plate (this would include the screen grid it there is one) of the tube.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Fixed bias, posted on January 22, 2021 at 19:34:59
vetmedrobert@gmail.com
Audiophile

Posts: 453
Location: Ontario
Joined: September 20, 2007
 This is what I thought, thanks for confirming it.

 

RE: Fixed bias, posted on January 24, 2021 at 21:10:37
Tube Cadet
Audiophile

Posts: 105
Location: NJ
Joined: November 22, 2002
 A fixed bias voltage is applied to the grid to obtain the desired target tube (bias) current. A bias circuit can be implemented via a simple voltage divider utilizing a variable resistor to allow one to set the bias voltage needed. As the tube ages the bias voltage may need to be adjusted. To measure the bias current you take a voltage reading at the cathode resistor and apply ohms law ( I = E/R) to determine the current through the tube. To simplify this a 10 ohm cathode resistor is sometimes used, for example on my amp I adjust the bias variable resistor to read 270 mV on the bias test point (cathode resistor) which translate to 27ma of tube current. Bottom line is you are adjusting the bias voltage to obtain the desired current.

 

RE: Fixed bias, posted on January 25, 2021 at 14:52:47
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 Just FYI - you might want to check out my FAQ post. Item #4 deals with your question - and there are some other useful items in their as well.

 

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