Typical Class AB KT88 Output Stage



For comparison I tried a typical KT88 output stage (triode connected) in the SE Amp Cad. The KT88 is driven at 425V, 30mA into a 2kOhm load (4k push-pull).  All results are for ONE of the output tubes.

Full output swing into Class AB1 (grid negative)

SE Amp CAD results for typical KT88 output stage

Note that the Ip avg is 55mA (not 30mA) and the high 3rd harmonic. The 2nd harmonic will be reduced by the push-pull action, but the 3rd harmonic will not be reduced at all. Loop feedback must be used, and with 20dB feedback the figures will be something like 0.1% 2nd and 0.2% 3rd. This looks decent, but the feedback will not be perfect and high order harmonics are added. It is my opinion that  it must be better to make the stage as linear as possible and loose some power.

Load line for typical KT88 output stage

Transfer function for typical KT88 output stage
Mirror transfer function for typical KT88 output stage

Note the bad symmetry, which makes it impossible to get a straight line in push-pull operation. The symmetry is quite good at 0.5W (per tube) and this is likely to be the explanation to why an amplifier like this sounds pretty good after all, but it can be made better.

Full output swing into Class AB2 (grid positive)

SE Amp CAD results at peak power

The maximum peak output power is around 40W for two triode connected KT88 in push-pull for this output stage. Note the VERY high 3rd harmonic. When driven this hard the driver stage will not be perfect at all, and this effects the feedback performance negatively.

Output for one tube at peak power

'Sinus' output from one tube at full peak power.

Loadline at peak power

This tube will run hot.

Transfer function at peak power

Just imagine what happens when you add the mirror of this transfer function in the push-pull output stage. Not even directly heated triodes will perform well when driven like this (Class AB2).


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