[Radio Wrangler]

There seems to be some sort of cultural imperative driving people towards using triodes everywhere, but as Robert has pointed out, triodes don't have the necessary gain to make the active Baxendall tone control circuit work. Towards max boost it will run out of loop gain.

It's a feedback circuit and all the usual equations apply.

You could try a 2-stage amplifier, but then that doesn't invert so the feedback is the wrong sense. Alternatively a cascode would boost the gain.

Even with a pentode amplifying its little heart out, the gain is somewhat limited by the impedance of the feedback network loading the anode resistor. It's still not exactly wonderful, but nevertheless, a lot better than a triode alone.

What the circuit really needs is an NE5534. Don't worry, it won't pollute your sound. It's likely that some of what you listen to has already been through a load of 'em in the studio.

David