No problem, just don't ask me to translate the whole book
CAPTION:
Output power Wo, distortion dtot, cathode current Ik and input AC voltage Vi in function of the load impedance for two EL34 tubes in triode mode in push-pull class A bias with a B+ of 425V. The left side of the figure shows the case of bypassed cathode resistor; in the (more important) right half of the figure an unbypassed shared cathode resistor of 235 ohms was used.
TEXT:
Very low distortion can be achieved in push-pull finals with class A triodes. 25W pentodes or tetrodes in triode operation can reach below 1% distortion @ 12...15W output power and 425V B+.
Power and distortion depend strongly on the load impedance, as seen in the figure for EL34 tubes. The tubes are run in triode mode and are biased just below the maximum dissipation of 25W.
Below 7kohms load impedance it is possibel to use a shared cathode resistor or two separate ones, and they should be bypassed. Above 7kohms the push-push stage works better with an unbypassed shared cathode resistor. The bias approaches class A as the load is increased; optimal conditions are reached around 10kohms, when output power is 14W at a THD significantly lower than 0.5%.
Finals such as these (e.g. Williamson) were recently very popular and many ~12W output power hifi amps were built to this recipe. Since the inherent distortion is so low, less feedback is necessary than with pentodes or tetrodes of the same power rating. Additionally, the risk of instability is much less in 3- or 4-stage amplifiers where most of the feedback is global and includes the output transformer.
The major drawback of this arrangement is the low efficiency, leading to a costly and heavy power supply and output transformer for a comparatively low output power.