WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
6p3s is basically equivalent to 6L6GB or 5881.
I have used for years 6p3s-e in push-pull circuits with 450 V anode and screen voltage without any problem.
The dissipation power must be kept at max. 20 W.