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bias question (just installed 1 ohm cathode resistors)


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7/7/1998 9:14 AM
jason
bias question (just installed 1 ohm cathode resistors)
I just added a bias pot and 1 ohm cathode resistors to my bassman reissue and was wondering what value of current I should be looking for on the cathode resistors. I read on RG's tube amp fact that the value read will be 5 to 10ma higher than what the plate current actually is. Any help would be appreciated.  
thanks,  
jason
 
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7/7/1998 11:13 AM
gus

The total current should = the plate current and the screen and control grid currents.  
You can measure the voltage drops across the 1.5k and 470ohm resistors and subtract that value from the cathode current for the true plate current.
 
7/7/1998 9:58 PM
Bruce

Just a quick question Gus,  
What does the 1k5 resistor have to do with the static bias bias current?  
I can see the screen measurement from the drop on the 470 ohm resistor.  
 
Bruce
 
7/8/1998 9:48 AM
gus

Bruce  
Very little effect my point was to show the current flow paths.
 
7/8/1998 10:07 PM
Bruce

Gotcha Gus.  
I rushed through that too fast the first time around.  
 
The error factor in that measurement is, of course, in the favor of longer tube life with the plate current a bit less then what you are actually measuring across the resistor.  
 
Cheers.  
Bruce
 

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