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This circuit is balanced but not differential.

Posted by Ralph on February 7, 2024 at 09:56:31:
So it needs an input transformer, otherwise you'll have no Common Mode Rejection Ratio (CMRR). Jensen Transformers makes good transformers for this sort of thing.

I'd use a 49.9K plate load at a minimum! The 100K is out to lunch.

If you set up a B- supply, you can run a proper differential circuit with good CMRR values without an input transformer.

Alternatively you could use solid state Constant Current Sources (CCS) replacing the resistor/capacitor values in the cathode circuits right now. But they would work better if you rectified and filtered the 12V supply. You could tie one side to ground and use the minus output for a B- for the CCS circuits. This would increase the effectiveness the CCS circuits to improve the differential effect.

Do not use a single-stage CCS- they hardly work at all. A 2 stage device or one that uses an opamp works much better.

No need at all for the rectifier tube. You can use some HEXFREDs and get the same low noise.