Schematic has been changed with C14 added as the screen bypass capacitor.
Usually a screen bypass capacitor is built into the socket. There are sockets
for Audio Amplifier without the capacitor. When I built that AMP with the
4CX1000A I used one of those sockets. I knew about the need of an external
capacitor. But I simply forgot it in the schematic.
I am sorry for any inconvenience that someone had in copying this project.