Adding more tone controls

[SpareRibs]

Hello,
OK, I have restored this PA back to stock, with the exception of removing the 330K resistor from the wiper of the phono jack.
By going through the mike, the .5 meg potentiometer would offer less resistance than the 1meg phono input. It also goes directly into the 12AX7, utilizing all of the amp.
I also gave up the idea of modding the Negative feedback loop, so it is back to the original configuration. I figured I never play loud enough to drive the amp to distortion anyway.
Now to my question. Would it be possible to incorporate a tone stack working off of the treble control ? I know the amp is primitive, however at this point it has lots of treble or very little. Most of the tone stacks I have seen start at the treble and work from there.
Any input will be greatly appreciated.

[Malcolm Irving]

... at this point it has lots of treble or very little...

I think the way the circuit is, it should give you a smooth roll-off on the treble. I wonder if the treble pot is not tracking properly? Maybe a new pot might give you more control?

The existing treble control is a shunt low pass filter, so you would need to change that to slot in the usual tone stack (rather than just add to what's there). The tone stack would be in an unusual place in the circuit (after the 'top' stage of a paraphase inverter) but I think it should still work in principle. You would loose quite a lot of gain by adding a tone stack.

Notice that the input stage is grid-leak biased (which is regarded as old-fashioned, but can be fun).

A more conventional place to introduce a tone stack might be after the first stage and before the volume control - then just remove the existing treble control circuit.