For a long time, I have considered designing and building an RL12P35 tube-based amplifier. 

Why? The German army took my grandparents’ house in the last days and used it for radio communication. We were the poorest family in the village, but the house was the last in the street at the highest spot. Before the russian army arrived, they flew without the radio equipment and left it with some RL12P35 spare tubes. So the tube has some emotional value for me to do something with these parts.

Design goal: Tube level NFB, 2 such tubes.

Option1: Single ended push pull arrangement. I simply tok the AG9015, but changed the ECC83 to an ECC83 in SRPP pair. It works somehow, even G2 is around the 200V wish, but the simulation shows distortion above ~6V Vp output. Ltspice file here.

Option2: Parallel Push Pull (Circlotron), whose theoretical drive is similar to SEPP. Here I copied this circuit, but replaced the output pentode. Simulation is close to useless… , probably to do with simulation method. Ltspice file here. 

Did not check yet loadline…. Will come.

https://wtfamps.com/load-line-calculations/
https://robrobinette.com/Drawing_Tube_Load_Lines.htm