Mid Boost/Cut for this Tonestack

[JoeK]

I'm trying to add a switch to cut and boost the mids on this existing tonestack. It's a hi-gain channel with a cathode follower driving the tone stack. I'm thinking I need to add/subtract the mids at the bottom of the stack above and below the bass? Was hoping to implement the switching with a DPDT ON-OFF-ON. If anybody has some thoughts I'd be very grateful.

[Chuck H]

How about this... Alter variables to taste. The mid boost acts on the upper mids (470p) or upper and lower mids (1000p) and doesn't alter the bass level, BUT, it is dependent on the treble control setting. The mid cut can be adjusted via the resistor value from full cut (no resistor) to just a little cut (4.7k). Hope this helps.

[JoeK]

Thanks for that Chuck. So to confirm all original parts stay as they are and I'm paralleling the PF cap with the original 50pf when switch is thrown. Throw switch the other way and I'm paralleling the 10K tail? resistor with another value resistor. I'll try it out and report back. Cheers Joe.

[Chuck H]

Yupper. The mid boost is the old Rivera circuit as seen on some Fender SF amps (one of the few good circuits to come out of the SF era) and the mid cut basically just turns down the mid pot (if you had one per se')

The 10M resistor keeps the parallel cap virtually out of the circuit but the cap remaind charged so there is no POP when the switch is thrown. You could do it without the 10M resistor if the POP isn't a problem and you want to minimize circuitry. And who keeps 10M resistors on hand? Just wire the circuit as if you clipped the 10M resistor out.

[Chuck H]

Ahh. After mocking it up on TSC I see that the unusual impedance due to the pot values (as compared to typical "Fender" values) changes the knee frequencies more dramatically than I anticipated. Also... There is a flaw in your drawing. The way it's drawn (which may be how it actually is wired I guess) Doesn't allow for much low end through the stack at all and doesn't match the schematic in your previous thread on this amp. I've amended my original mod value estimates and (corrected?) the drawing as it should be.

[JoeK]

Nice catch! I should have gone over my own drawing with a hi-lighter before I posted. I was also scratching my head about how your redraw looks just like my hand writing? Your a dab hand with the photo editing. Thanks again.

[JoeK]

I just wired this in and tested it seems to work fine. I had a 300pf handy and thought that was in the middle of the range so put that across the existing 50pf with the 10meg (had some 5megs leftover from same amp) and no pops. I'll fine tune this later with the new owner and see if the mids are scooped enough for his taste. The mids plus side has the 10K tail resistor being bypassed to ground and I was wondering if this will drop all the signal to ground if the bass and treble pots are both on zero. This turns out to be the case so to keep a boost and not lose all signal (just to keep a worried user happy) could I get away with having 1k or less still in series? I guess the TSC you referred to Chuck is Duncan's Tone stack calc? Is it easy to use?

[Chuck H]

TSC is especially easy to use. I even use it to SPICE non tonstack ideas by manipulating the values to extremes and it's always accurate. The mid boost is dependent on the treble control. That is, if the treble control is on 0 there will be no effect from the boost position. If the mid scoop is too subtle now a 1k will make it even more subtle. Though you probably wouldn't notice other than the stack will still pass some signal with all the controls @ 0. If the mid scoop is too subtle that's a bummer since direct to ground is as much mid scoop as this TS offers. Perhaps a speaker upgrade to one with less mids??? Then the overall tone of the amp would be less middy, the mid cut would get you the rest of the way and the mid boost would still be an option for more mids.