3-band EQ for the McGrath Stereo amp

 

When I discovered that I couldn't use my variable-frequency midrange circuit with the Baxandall topology, this also threw into doubt my 3-band EQ for the McGrath Stereo amp.  I had planned to use a conventional Baxandall bass & treble EQ, and add my v-mid circuit in parallel with the other two, in the same feedback loop.  Now that I know that's not possible, I was left in a quandry.  Run a separate v-mid circuit in series with the Baxandall?  (I've seen this done in some other amps like Ampeg which have some degree of variable mid frequency.)  But I couldn't see a way to do this without adding at least one triode stage, so it would no longer fit in a single 12AX7.

But last night it occurred to me, I could just ditch Baxandall entirely, and implement the bass and treble with the same "cathodyne" circuit that I'm using for the v-mid: all three bands in parallel (just like I wanted to do originally with the Baxandall).  I have drawn this up, above (just topologically, no values yet).  This will of course have some interaction between the controls, and so wouldn't be suited to audiophile or hi-fi applications, but I think it may work well for guitar amps.  In particular, bands which are at the extreme min or max positions will have minimal interaction.  Also, it preserves the critical property that midway position is flat.  I suppose I'll want to wire up a prototype, to make sure it works as I think it will...