Bypassing BMP tone control...

[gutsofgold]

AMZ shows that you can simply connect the .01 cap with the 22k resistor to bypass the tone.
http://www.muzique.com/lab/tbypass.htm

On the GGG schematic of the 80s muff that actually had a tone bypass switch, it seems that it switches out the ENTIRE tone circuit and puts a single fixed resistor in place of it (150k resistor in place of a 100k pot).
http://www.generalguitargadgets.com/pdf/ggg_bmp_tbp_sc.pdf

Are there any major differences between the two methods?
I have this idea of setting the tone exactly the way I want it and keeping it there with no knob. And in place of a tone knob I'll have a blend knob to 'alter' the sound. Which way would you recommend?

[gutsofgold]

It also just occured to me that I could use a dual pot for the sustain or the tone. For example, I could use a dual pot with the sustain so that as the sustain is turned down more clean signal is brought back in.

Or the tone control could be a dual that adds more bass as it is turned closer to the bass side.

Too many ways to accomplish a simple task 

[Mark Hammer]

I learned something from a nifty little article Steve Morrison (moderator/webmaster of the Music Electronics Forum, formerly AMPAGE) had in POLYPHONY some 25 years ago.  A great many passive tone controls work by essentially selectively bleeding frequency content to ground.  If you make the path to ground much higher impedance (such as, oh I dunno, infinite), that selective bleed is effectively cancelled.  Steve implemented this principle in the Fender tonestack by simply lifting the contact between the midrange pot (normally connected to ground) and ground.  I tried it out on my old blackface Tremolux, and sonofagun if it didn't a) all but eliminate any effect of the tone controls, and b) increase the preamp output level to get a little more grind out of the amp.

In the BMP, the tone control provides for two parallel paths: one that provides for a highpass filtered signal, and the other that provides for a lowpass filtered signal.  These two parallel paths meet at the wiper of the tone pot, and the relative amount of resistance on each leg of the tone pot determines how much of each path is contributed to the output at the tone pot wiper.  The highpass section provides its action via a resistor to ground, and the lowpass section provides its action via a cap to ground.  As the resistance of those two paths to ground increases, several things happen.  In the case of the lowpass section, adding resistance to ground in series with the cap will reduce the  effect of the lowpass filtering.  In the case of the highpass section, drastically increasing the resistance to ground will lower the rolloff point (i.e., the frequency where bass content starts to be cut out) to the point where ALL frequencies pass relatively unimpeded (i.e., pretty much no effect).

So, connect the ground side of that resistor and capacitor to a 1M resistor, and connect the 1M resistor to ground instead.  Now, use a SPST toggle to simply shunt that 1M resistor when you want the tone circuit to be in effect, or unshunt the 1M resistor when you don't want any tone circuit in effect.  Do note that with the 1M resistance to ground, the signal at the wiper of the tone control will be hotter, so if you want to maintain the same volume with the tone-bypass, you will need to turn your volume pot down.

[gutsofgold]

Well I am trying to keep it still 3 knobs. So essentially I wanted to find a happy medium with the tone control and keep it permanently set there. Then use the once tone knob as a blend knob for a blend circuit I'm adding.

Could I just run a jumper where the tone pot used to be and keep the 1M resister connected between the pot/cap and ground?

One last thing,  , do you know how this method of bypass SOUNDS different than the other two ?

[bumblebee]

you could disconnect the tone pot and replace it with 2 resistors.but first you would want to set the tone control to where you like it, de-solder it and measure the resistance from lug 3 to 2 and 2 to 1 then get resistors of those values and solder them to the PCB in the place the pot was. this will give you a set amount of tone, predetermined by you. 
(this is how the old little big muffs set sustain and tone)

then you could use the pot thats still in the tone position for the blend you was talking about.
this method is about as simple as you can get and it also results in not having to add any holes to you BMP as the extra blend pot is already there.

[Solidhex]

Hey Mark

  I was a little confused about the method of tone bypass involving the 1m resistor. You had mentioned using a SPST switch to toggle between the regular tone operation and bypass. I understand Orman's method of interrupting the tone stacks path to ground with a SPST but wouldn't you need a SPDT to toggle between the 1M resistor to ground and the regular straight path to ground? Is there any benefit to this method as opposed to just cutting the tonestacks path to ground?

--Brad

[Mark Hammer]

The recommendation for the resistor+switch method is simply to avoid the audible popping that will undoubtedly occur if the bass side's cap has its ground connection lifted.  I suppose it is possible that the cap can drain off via the path through the treble side resistor if you leave those two connected to each other but not ground, but I figured the constant drain path to ground would provide anti-pop insurance.  The toggle simply transforms that ground path from "straight wire" to some higher resistance.  There is an excellent chance that resistances significantly lower than 1M are sufficient to cancel the impact of the tone control.  Some users may find that something in the vicinity of 47k-100k is sufficient to eliminate the EQ curve of the tone control to their satisfaction.  The adventurous may even wish to stick a 500k-1M trim-pot in there and monkey around until they find a value that suits their tastes.