In this section, main features of that special biasing will be discussed. However, since there is little interest in make pentodes or beam tetrodes operating in class A2, we will deal only with triodes. We have previously seen that class A1 is limited by apparition of grid current; if we relax this constraint (assuming that the driver stage has a low enough internal impedance so that dynamic grid resistance is high with respect to it), useful domain of operation is extended and, like in the case of pentodes, load resistance can be choosen to be :
with an associated efficiency :
This seems prodigiously interesting since most powerful triodes are designed to run continuously with grid current and promise high power even in single-ended mode. However, stress put on the driver is stringent, so that few designs will reach the top quality.
Main trouble with class A2 is that for a grid current and a driver internal resistance , the product must remain well below the maximum grid excursion . A rule of the thumb is to compute dynamic impedance of grid at maximum positive modulation. Since at positive biasing the grid-cathode system behaves essentially like a conducting diode, we have approximately with G the perveance of that equivalent diode. Dynamic impedance at can be computed as :
We see that it is lower than the expected value . A driver for a tube running in class A2 must have an internal impedance ten times lower than this value. Let us take the example of the 833A, a directly heated triode with 300W plate dissipation and an amplification factor of 35. Since is high, the useful operating rang in class A1 will be very narrow unless high voltage plate are applied (2500V to 3000V will be required). Class A2 biasing is much more appealing : with 1250V and , optimal load resistance is 5 kOmega and output power . Maximum positive grid voltage will be , with an associated grid current as indicated on the datasheet. The dynamic resistance computes as 1.6 k . The driver tube must have an internal impedance in the 100 range or lower. Very few triodes can have such a low value; some tubes used in serie regulation will match this requirement (for example the well known 6C33C). Other options are possible, like using a step down interstage transformer so that grid impedance appears higher. In the previous example a 2:1 transformer will give 4.4 k resistance and a driver near 400 . An no-compromise approach will use a higher ratio, like 4:1 or 6:1 in order to allow directly heated triodes as drivers (300B for the first ratio and 845 for the second). It worth note that interstage transformers are more difficult to build when ratio is above 10:1.
In many cases, feedback can be used in order to reduce both distorsion and output impedance : those quantities are roughly reduced by the feedback ratio. A 6C33C based driver will be able to reach 20 , yielding a nearly perfect grid control. When an interstage transformer is present, it worth including it in the feedback loop (it's an inner feedback loop in case of global feedback) since this results in minimizing phase shift effect from that component, thus increasing phase margin of the outer feedback loop (most class A2 designs must use global feedback). Note that feedback can be obtained by wiring the driver in cathode follower too !
Here are the key points for class A2 :