Amplifier 12: Triode Push Pull Amplifier
Inside amplifier 12 the screen grids are connected directly to their anodes. They behave as one and therefore this is called triode behavior. This is not totally true, because between the anode and screen grid we find grid G3 (pin-1) which is still connected to the cathode.
Amplifier 13: Super Pentode Push Pull Amplifier
Inside amplifier 13 we find a strange combination of negative and positive feedback. The cathodes are connected to the secondary winding. This delivers negative feedback. The amplification factor lowers, the distortion is suppressed and the damping factor gets larger. However, the screen grids are connected inverse to their UL-taps on the primary, which is positive feedback. This combination of negative and positive feedback gives very interesting results, they will be discussed following.
Amplifier 15: Super Triode © push pull amplifier
In amplifier 15 the screen grids are connected to the ultra linear taps P-2 and P-4 of the output transformer. This means that almost the full output power of the power tubes is available. The cathodes again are connected to the secondary winding of the OPT. This creates extra local feedback. Click 
here to download the measurement results.
Amplifier 14: Pentode Push Pull Amplifier with Cathode Feedback
Amplifier 14 is a real pentode amplifier with lots of power. The screen grids are directly connected to the high voltage power supply. They seem to deliver no feedback (*). The cathodes again are connected to the secondary winding of the OPT. This creates strong local feedback. Click here to download the measurement results.
Lees meer: Amplifier 14: Pentode Push Pull Amplifier with Cathode Feedback
Amplifier 16: Triode push pull amplifier with cathode feedback
In amplifier 16 the screen grids are connected to their anodes. This means that half of the output power of the power tubes is available. The cathodes again are connected to the secondary winding of the OPT. This creates extra local feedback. Click here to download the measurement results.
Lees meer: Amplifier 16: Triode push pull amplifier with cathode feedback