[Radioroslyn]

As for the 6A7 mixer Let me see if I can muddy the waters for you. It's probably more concise to use grid numbers rather than names.

Pin 5 is grid1, the control grid for the oscillator section of the mixer. When the oscillator is operating properly should see a negative voltage there with a hi impedance voltmeter.

Pin 4 is grid 2, some call it the screen grid but it acts as a PLATE for grid 1. The combination of pin 6 the cathode, pin 5, and pin 4 forms a triode which is the oscillator section of the mixer. It must have hv on pin 4 to operate.

Pin 3 grids 3 and 5 act as screen grids. Their job is to speed up the electrons headed for the plate. It to must have hv on it to operate properly. Usually a bit less than grid 2 and the plate.

The cap is grid 4 it is the control grid for the amplifier section of the mixer. This is where the incoming signal is connected to the mixer. In the case of the 640 the signal is coming from the 78 rf amplifier. Some sets don't have a rf amp stage so this grid is connected to the antenna coil.

Think of the pentagrid converter tube as two tubes one on top of the other. The bottom tube is a triode ( oscillator) and the top is a tetrode (amplifier). The input of the amp (cap) is tuned to incoming signal and the output (plate) is tuned to the IF frequency. The oscillator section is oscillating at 460kc above the incoming signal and the result is a signal at 460kc which is amplified in the IF amp.

The G1 voltages on the 42 tubes is good. Won't have any effect on the mixer but call me George I was just curious.

Clear as mud right?

Terry