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previous: MJ Harnish I'm currently considering to build ... -- 11/22/1998 8:55 AM View Thread

Re: Class A, cathode biased woes

11/22/1998 9:43 AM
Charles
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Re: Class A, cathode biased woes
92ma per tube seems high, even for class A operation with EL34's.  
 
One effect of cathode bias is the bias shift you get as the current thru the cathode increases. Here's an example... You might normally bias 6L6's at 35ma or 37ma and if you use a cathode resistor to get that value, you'll find that the voltage drop across that resistor is about 45ish volts. BUT, when the amp is cranked up, the tube is trying to conduct a whole lot more, and the voltage drop across the resistor increases. The increased negative relationship of the grid to cathode tends to make each tube decrease the amount of simultanious waveform conduction, and the amp slides closer to the Class B threshold. In cases such as that, we have to idle the tubes much higher so that at full bore we don't get crossover distortion. For Class A operation the bias voltage should be approximately halfway point between the voltage that causes shutoff and the voltage that causes saturation.  
 
On a practical note, you could use a cathode resistor that doesn't give Class A operation, then add some resistance in parallel with it while monitoring the output waveform of one tube (or output half). You can select the highest value that still give symmetrical waves at the highest expected input signal level.  
 
BTW, your math is right. I don't think there is a way to get more than a ballpark value of the cathode resistor based upon the expected operation of a generic tube of that type. The actual value will still need to be determined by the tubes you use.

 
Replies:
MJ Harnish Thanks for the info.So the sche... -- 11/22/1998 11:59 AM