07-22-2013, 08:33 PM
They are not interchangable. In some circuits, I think the 6K8 will sort of work, but get yourself a 6A8. This all presuming someone hasn't modified the circuit to use a 6K8. You can tell whether this might be an off model by looking at the tube socket from the top. That's something Zenith was always good at, printing the tube number on the socket.