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Re: You're thinking of Class B [nt]

Posted by Tre' (P) on March 6, 2007 at 20:46:25

In Reply to: You're thinking of Class B [nt] posted by Ray Moth on March 6, 2007 at 19:00:23:

No I'm not. I'm thinking of Class A/B. A Class A/B amplifier operates in Class A until the input signal drives one tube to cutoff. Then it operates Class B.

In a Class A/B amplifier the current in one output tube will go down as the current in the other tube goes up. At some point the first tube will shut off and the current in the other tube will continue to increase.

In our example starting at 50ma. per tube; at 0 degrees into an input waveform, that will drive the amplifier to full power, there will be 100ma. of current flowing from the power supply. At 10 degrees into the waveform there will still be 100ma. flowing from the power supply (assuming linear tubes). Less than 50ma. in one tube but more than 50ma. in the other tube. At some point, before 90 degrees into the wave form, the tube with the grid that is being driven more negative will cutoff (the amplifier has entered into Class B operation)
and the other tube will be left on it's own to increase to 140ma.

There will never be less than 100ma. and never more than 140ma. of current being drawn from the power supply at any given moment.

In a Class A amplifier with the tubes biased to 50ma each there will be 100ma of current drawn from the power supply at all times. Never more, never less. (again, assuming linear tubes)

Tre'
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