[Diabolical Artificer]

After messing about trying to get a valve tone stage to work on one of my amp builds, I've come to the conclusion it's only an exercise for masochists. I've had great success incorporating Rod Elliots opamp based tone stage into a valve preamp and another amp, but the valve tone controls, sheesh.

The problem is you have high voltage DC going through pots or you try and block said HV with caps, but the caps add extra phase shift, which can make things unstable and implementing a bypass SW isn't trivial, as you've found, though I managed it with mine but differently to how you did it. Where you put the volume pot in the circuit effects things too, that's what I found. I used the circuit back in post one,altered a bit.

So in a nutshell I wouldn't use valves again in a tone control. A one two three four...so it's no nay never oh never no more, will I use valves in a tonestack, oh never no more ( all this sung at full whack to the tune of The Wild Rover) the experience aged me immensely : )

Andy.