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previous: Charles I don't think its a matter of being... -- 6/3/1998 9:07 PM View Thread

Re: Cathode bias...

6/4/1998 12:32 PM
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Re: Cathode bias...
When I referred to "high powered amps", it's not the power level per se that gives you the problem. It's the tube type. Low power tubes, lets say 6V6, 6AQ5, 6BQ5, 7189, 6973, etc., usually require a low grid bias voltage to keep them happy. And the difference in signal current from idle to max isn't that great comparing these tubes in a pp class A circuit with a class AB1 circuit. There is only one tube type that I can think of right now that has a fairly high power output capability but doesn't require a large grid bias voltage, and that's the 7591, 7868, 6GM5 design. These aren't very common in most of what we build these days, so I generalized when I said "high powered amp".  
 
If you're objective is maximum power, then you go with fixed bias. That's how to get the maximum power out of any tube. If you can get the power you need from some of the smaller bottles, then maybe you can design for simplicity along with some of the sonic characteristics provided by cathode bias. A 6L6 amp originally designed with fixed bias & high supply voltages is not the best place to start for cathode bias mods. An early, low voltage, 6L6 amp can be run in class A, without a large negative bias, and can still work well with cathode biasing.  
 
If you see the top of the composite waveform flattening, that's the result of the input signal exceeding the bias voltage. Crossover distortion is seen on the waveform at the zero signal axis, where the waveform is "crossing over" the line, where one tube stops conducting before the other tube starts conducting. Instead of a seamless handoff, theres a glitch like a scratch on a record or joint in a railroad track, creating a discontinuity in the sound.  
 
Flat tops can't be used to tell you what operating class you're in. If you want to see how each tube is conducting in a push pull pair, short the signal to ground going to one grid. You'll now be able to see the conduction waveform for a single tube. With a sine wave input, if you see only a half-wave on one side of the line, without any output on the other side, it's out of range of class AB and into B. If there is partial wave conduction on the other side, that's "greater than 50% conduction" which is class AB. If there's equal half waves either side of the line (conducting 100% of the time), that's class A.

 
Replies:
J Fletcher If you short the signal to ground f... -- 6/4/1998 12:44 PM
Charles We're on the same wavelength (pun i... -- 6/4/1998 4:34 PM