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Re: How do you tell if an amp is cathode-biased?


 
5/23/2005 9:42 PM
Enzo
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Re: How do you tell if an amp is cathode-biased?
That is micro-micro, but I'm nitpicking.  
 
People throw the term bias around too freely.  
 
SOme people use it to mean that there must be a bias voltage in the amp - always true. Other people use it to mean the voltage must be adjusted. " I had to bias my amp." When both meanings get into the same discussion, confusion results.  
 
All amps have to be biased in the sense that there must be a bias voltage difference between the grid and cathode. But all amps do NOT have to be biased in the sense of having the bias voltage addjusted.  
 
Cathode bias is easy to spot, the output tube cathodes are not grounded, they run through a resistor to ground. That resistor will typically be 50 to 500 ohms depending on tube type and the particular circuit. If it has 1 ohm resistors, that is not cathode bias, those are sensing resistors as mentioned above.  
 
Fixed bias amps are the alternative to cathode biasing. Amps with adjustable bias are still called fixed bias circuits, the fixed voltage is adjustable. Once set, it stays the same no matter what the tube does. In cathode bias the current throught the tube determines the voltage across the resistor - that is what Ohm's law calculates - so it is not fixed.  
 
In the case of fixed bias amps, the cathodes will be grounded, or at least they will have only that 1 ohm resistor to ground. The other factor is that in those cases there must be a separate negative voltage supply applied to the grids.  
 
So:  
Big cathode resistor means cathode bias. Negative grid voltage power supply means fixed bias. SImple as that.
 
5/26/2005 3:40 PM
GlennW
Thanks everybody. You've all been a big help.  
 
I haven't unsoldered anything, but I will when the replacement parts arrive. I'll order  
 
http://www.mouser.com/index.cfm?&handler=data.listcategory&D=*71RH5250*&terms=71-RH5-250&Ntt=*71RH5250*&Dk=1&Ns=SField&N=0&crc=true  
 
for the cathode resistor, and  
 
http://www.mouser.com/index.cfm?&handler=data.listcategory&D=*75TE1207.5*&terms=75-TE1207.5&Ntt=*75TE1207.5*&Dk=1&Ns=SField&N=0&crc=true  
 
for the coupling cap. I guess they'll work ok. I'll try to learn how to take some voltage measurements and let you know what I find. The schematic has what I think are voltage measurements in various places. This will be a good learning experience.  
 
Thanks again.
 
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