[MrFixr55]

With one of the 6F6s and the 6J5 accidentally switched, the radio may work but would not work well. The heater (Pins 2 and 7), cathode (Pin 8), control grid (Pin 5) and plate (Pin 3) connections for a 6J5 and 6F6 are the same. If a 6F6 were in the phase invertor position, the tube would amplify (poorly)and correctly invert phase for one of the 2 output tubes like a phase inverter should. Th 6J5 in a 6F6 position would amplify poorly.

The best way to verify correct location s to verify the diagram per the official Philco service document. Per this document, the 6J5 Phase Invertor is the furthest tube on the rear of the middle chassis from the transformer. The 2 6F6s will have pin 8 of each 6F6 socket at chassis ground and pin 3 of each 6F6 socket to the output transformer. The 6J5 socket will have two 0.03 uF caps (capacitors), one at pin 3, the other at pin 8. The other end of each of these caps will connect to pin 5 of each 6F6 socket. From this information, you should be able locate the correct sockets for the correct tubes.

The 6F6 is a fairly common tube. The 6J5 a little less so, but these should be available from many sources for a reasonable price. Performance will definitely improve with the correct tubes in the correct sockets. A pair of 6V6s can be substituted for the 6F6s without wiring changes, but output transformer impedances and bias may be different, somewhat affecting performance. Best ti use the 2 6F6s and the 6J5, all in the correct sockets.

Please note that the 5X4 rectifier is not common, and not directly replaceable by such rectifiers as a 5U4 as the wiring is very different. Per an RCA document "Tube Substitution Document for Emergency Servicing of Civilian Receivers" published during WWII, a 5U4 can replace a 5X4, but the socket must be rewired. The RCA Receiving Tube Manual, Version RC14 published 1940 states that the maximum operating characteristics for a 5X4 are the same as a 5U4. However, filament connections are pins 7 and 8 for the 5X4 but pins 8 and 2 for the 5U4. Plate connections are pins 3 and 5 for the 5X4 but are pins 4 and 6 for the 5U4. It is possible to connect pins 8 and 2, pins 3 and 4 and pins 5 and 6 to allow plug and play use of either tube. However, I think that would reduce the clearance between plate pins and with approximately 700V between plates, this may allow for arc-over.