5 - 4 AUTO BIAS: ( Continued )
Note that the voltage relationship between the three elements is similar and that the difference between cathode and the grid voltages is the same in both examples: The grid is 50 Volts more negative or less positive in both. Thus as far as the electrons are concerned, the static operating conditions between the two systems are quite similar.
Cathode | ...........................................+ 50 (voltage generated across |
| cathode resistor) |
Plate | + 450 |
Grid | 0 |
However, there are two important differences between the systems. First, if the tube in the cathode biased system tries to draw too much plate current, the voltage drop across the cathode resistor increases, raising the positive cathode voltage. This has the same effect as would increasing the fixed negative bias voltage, only it happens automatically. Idle current through the tube decreases whenever the grid becomes more negative relative to the cathode. Only in the
In mechanical terms, cathode bias is like having a governor on an engine’s throttle: if the engine tries to speed up, the throttle is automatically reduced. If it tries to slow down, the throttle is increased.
If you wonder why every tube amplifier isn’t
The maximum power a tube can transfer is dependent on the difference between cathode and plate voltages. In order to generate the cathode bias, the difference between plate and cathodes is 400 Volts versus 450 Volts in the fixed bias example above.
5 - 5 CLASS A OPERATION:
Often considered the Holy Grail of tube operation, Class A is just as widely misunderstood. While there is no way an Owner’s Manual can successfully convey all of its intricacies, we will try to give you that little bit of knowledge which is not dangerous but may help to
Remember, the idle current flowing through the tubes under zero signal conditions has been established by biasing via either method so that the grid is charged somewhat negative in relation to the cathode. If it becomes more negative, less current flows. When it is less negative, more current flows. Now, if we apply an audio signal to that grid and look at it instant by instant, the effect is exactly the same as changing the bias voltage. As the audio signal swings negative, it augments the
What Class A operation means is simply that current will continue to flow AT ALL TIMES through all the tubes during all normal signal voltages. That’s it! The only reason this is important is because it avoids the
But in a
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