Amplifier 17: Push pull triode cathode follower amplifier
In amplifier 17 again the screen grids are connected to their anodes. This means that half of the output power of the power tubes is available.
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Amplifier 18: Parallel Push Pull Pentode Amplifier
The power tubes in amplifier 18 are in pentode mode by means of a very smart circuit; now they can deliver their full output power. Two floating high voltage supplies Vht1 and Vht2 deliver the energy to this interesting amplifier. Each power tube is part of its own closed current circle including the primary winding between taps P-2 and P-4. The driver stage around B1 has not enough headroom to get the full output power, but an example is given later how to solve this. Click 
here to download the specifications of this special amplifier.
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Amplifier 19: Push pull pentode with twice the output power
The power tubes B2 and B3 can deliver twice the output power by raising the supply voltage from 360 V to 740 V and using a primary impedance Zaa = 8 kOhm. By doing so, the alternating current through the tubes stays as it was, while the alternating voltage doubles.
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Amplifier 20: High power push pull pentode with local cathode feedback
Amplifier 20 does not differ so much from amplifier 19. The voltages at the screen grids and the center tap of the OPT are the same. The major difference is the local feedback at the cathodes of the power tubes. However, this small difference has major impact. See the schematics for more details and form-20 (click here) for the measurement results.
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Amplifiers 1-2-3: Single Ended with saturated OPT-core
I know that the expectations are high. Finally its time for Single Ended amplifiers. How disappointed will you be, hearing from me that amplifiers 1 to 3 are NOT good, the output transformer is in heavy saturation. In spite of this, these amplifiers teach us several important things. We can look inside the core of the OPT and we can hear the special silky sound of single ended amplifiers. See for more details the schematics below and the specifications in form-1-2-3 (click here).
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