How to wire two pots in series?

[theundeadelvis]

Hello everybody! (crowd: Hello Dr. Nick!)

Anyway, I am wanting to add a trim pot (switchable) in series with the level pot of my Blackfire distortion. The reason is because I have a tone stack that can be switched off but it results in a massive volume boost. So, I want this trimmer to switched in series to bring the level down, but I can't for the life of me picture how I need to wire this? Thanks!

[Mark Hammer]

I've got a couple of versions of the Blackfire, so I'm not sure which one you are using, or what point you are inserting the tonestack.  In any event, one version (2005) has a 1M resistor in series with the input to the 100k output pot.  Another earlier version (2003) has a 2x470k pair just ahead of the volume pot, set up as a voltage divider.

Note that in the first example, the 1M resistor and 100k pot behave like a pot turned down 10/11 of the way.  Think of it like a 1100k pot where the wiper simply can't budge past the 100k point.  Naturally, that drops the output level substantially.

Okay, put that on hold.

A second stream of thought.  If you lift the ground connection from just about any Marshall/Fender-style tonestack, you effectively cancel the effect of the tone-control settings, simultaneously getting a noticeable increae in signal level because there is no more passive bleed.  Over 25 years ago, Steve "T-Boy" Morrison (webmaster of the former Ampage, now Music Electronics Forum) published an article in Polyphony outlining a solo-boost mod for Fender amps that did exactly that.  I installed it on my old blackface Tremolux head and it did exactly what was promised, effectively doubling the output level and removing all tonal adjustments at once.

So, here's an idea.  If you lift the ground connection on your tonestack AND change the value of the resistance in series with the volume pot, you can achieve tone/no-tone switchover with constant volume.  You would do this, of course, by having a higher value resistance when the tonestack is disabled, for more output attenuation, and a lower resistance when the tonestack is engaged (for less output attenuation).  If you're using the more recent version of the Blackfire, consider tacking a 2nd 1M resistor in parallel with the existing 1M resistor when the tonestack is engaged, then lifting the parallel resistor when the ground connection on the tonestack is lifted.  That can be achieved with a DPDT toggle, which also means it can be achieved with a 3PDT stompswitch with LED indication.

Of course, you can also use a trimpot to more accurately set the desired parallel resistance.  I'd suggest a 330k fixed resistor and 1M trimpot in series to be on the safe side.

[CGDARK]

You can put the trimmer before the volume pot as a rheostat (as a simple variable resistor) or as a normal pot (a voltage divider). As a voltage divider just connect the trimmer as you used to wire the volume pot (terminal #3 to circuit and terminal #1 to ground or Vref) and connect the wiper of the trimmer (terminal #2) to the volume pot terminal #3 then the terminal #2 of the pot to the output jack or switch and terminal #1 to ground or Vref. That will do what you want.

CG

[theundeadelvis]

Thank you very much! I knew it would be fairly simple, but it kept making my head hurt thinking about it! 

[theundeadelvis]

Mark, I used this layout. Which resistor would I add another in parallel with the tonestack engaged? VR2 is the level pot.



Thanks!