Variable coupling caps

[Zozobra]

I know that one tried and tested way of selecting coupling caps in an amp is to use something like the orange/matamp FAC rotary system. Would something like this work using just a common garden pot? Excuse the rough sketch!



If it would then I can see it being much more tonally flexible than the FAC method and I can avoid taking up a lot of space with a rotary pot with lots of caps hanging off the back of it! Thoughts and opinions greatly appreciated as always.

[Steve Conner]

It'll work almost like you expect, but not quite.

Even when the pot is all the way to the "1n" end, you still have 47n across it with 1M in series. If the input impedance of the next stage is also 1M, then you'll only get 6dB of rolloff before it levels out. The bass cut will be much less than with the FAC rotary switch set to select a 1n cap.

[Zozobra]

Ah okay, so some experimentation with values (as always) is required! Thanks. I should have this damn prototyping rig of mine built in the coming weeks so I can actually try these things first before asking dumb questions!

[Steve Conner]

Yes, and you'll find it hard to get a worthwhile effect without a 10:1 ratio between the pot and the next stage grid leak resistor.

[Zozobra]

ah, so if I was to have a gain/volume pot in between the stages it would interact with the bass boost pot or have I missed a trick here? I found that the 2210 does something similar but its part of a tone stack so its not really applicable in my case I guess.
As it happens the next stage is quite likely to be an EF86 in the design I've been dreaming up.

[Steve Conner]

Yes, they would interact. You're basically reinventing the various one-knob tone stacks, with the R and C transposed so it controls bass instead of treble.

[echuta13]

I installed the same variable coupling circuit in an amp build and it just didn't have enough of an effect on the sound.

[diagrammatiks]

you basically just have a tone pot.

[Gaz]

I installed the same variable coupling circuit in an amp build and it just didn't have enough of an effect on the sound.

Yeah, Orange style works better, especially if its earlier in the circuit. Plus, rotary knobs are cool!