Mesa/Boogie Vacuum Tube Audio owner manual Cathode resistor, Class a Operation

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 self-bias system it’s the cathode which becomes increasingly positive relative to the grid. Either way, the effect is identical.

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 auto-biased, it is because this system is unable to deliver as much power from the same tubes and transformers as can the fixed bias method. The key to this short fall is apparent by looking at the voltage charts above.

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 de-mystify Class A. In the section above we’ve discussed two separate methods of biasing tubes to draw their correct idle current as determined by the circuit designer. When we speak of classes of tube or transistor operation, we’re considering the effect of applying signal voltages of various magnitudes on top of the steady-state idle conditions.

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 steady-state bias by driving the grid further negative and reducing the amount of current flow through the tube. When the audio signal swings positive, it counteracts the negative bias by adding a positive voltage to a negative voltage such that the sum is not as negative as it first was and thus current flow increases.

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 non-linear region of a tube’s amplifying characteristic which occurs at and near cut-off which is when current ceases to flow. Any single-ended design must, by necessity, operate in Class A, otherwise tremendous distortion would occur as the amplifier literally cuts itself off by having a signal voltage that at times became so negative that all current ceased to flow.

But in a push-pull design, the signal is divided into two halves which are alternately amplified by each side of the circuit. To be more precise, a class AB amplifier would simultaneously use both halves of the amplifier at lower volumes that’s the Class A part, but at some level of signal magnitude, the negative half of the signal would drive one side of the push-pull circuit into cut-off while all the

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