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RE: Thomas stereo organ amplifier 6973 to 6cz5 conversion

Looking at the grid and working "backwards"...

There should be a resistor about 1KΩ connected directly to the grid pin. This is the grid stopper. I always use carbon comp for this, which is non-inductive. The purpose of this resistor is to reduce the Q of grid circuit inductance, some of which is inside the tube. The other end of the grid stopper is connected to A) the coupling cap from the previous stage, and B) the grid resistor (the one I mentioned above). I see below that Tom has found a 100KΩ spec for this, so that's what you should use. The other end of the grid resistor connects to the output of the negative bias supply. This replaces the resistor that is connected to ground in a self-bias amp.





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