Ok this is going out on a limb, but tube sockets have been failing* while looking otherwise intact on recent repairs
try the 6CG7 socket, it may have a conductive dust path underneath. Trace those hold coil wires too. I don't have Sams 550 but the resistance table for all the 6CG7 pins needs to be checked.
*Both CTC16 color demodulator 6GU7 sockets and a 12BY7 in the second set could not be helped with deoxit or new tubes.