We want to use 6CZ5 power tubes in our Valco amp instead of the original 6973s but there was a problem.  The issue was with the valve's screen grid (G2).

The screen grid is a drain for electrons that bounce off the plate (P) and would otherwise interfere with the signal on the control grid (G1).

Here you see the terminal diagram (also called the basing or pin-out) of a 6973 vacuum tube.

The red line in the diagram emphasizes the fact that:

• Pin-1 connects to the screen grid.
• Pin-8 connects to the screen grid.

This is the original wiring of the Valco's left power tube socket.  Notice that:

• Pin-1 is unused.
• Pin-8 gets the screen grid voltage.

[Pin-8 gets the voltage via a red wire and relays it to the right socket via a brown wire.]

This is the original wiring of the right tube socket.  Again,

• Pin-1 is unused.
• Pin-8 gets the screen grid voltage.

[Pin-8 gets the voltage via the brown wire from the left socket.]

This is the terminal diagram of a 6CZ5.  The red line emphasizes the fact that:

• Pin-1 connects to the screen grid.
• Pin-8 is not to be used, even as a tie point.

Pin-8 is an Internal Connection, not the screen grid.  So the 6CZ5 tube won't work in our Valco amp that relies on pin-8 being the screen grid.

So, on the left tube socket, we unsoldered the two wires that were connected to pin-8 and soldered them to pin-1 instead so that:

• Pin-1 gets the screen grid voltage.
• Pin-8 is unused.

On the right tube socket, we unsoldered the one wire on pin-8 and soldered it to pin-1 instead so that:

• Pin-1 gets the screen grid voltage
• Pin-8 is unused

Now, the 6CZ5 tubes are compatible with our Montgomery Ward amp.

The original, 6973 tubes are compatible too since they don't care whether pin-1 or pin-8 gets the screen grid voltage.

Next, we'll tackle the amplifier's broken tremolo control.