Princeton tonestack bypass wiring question

[jordan86]

Hey guys!

I still love my Princeton Reverb that I built back in 2020, which so many of you helped me on. I did add eventually a "RAW" control on the back of the chassis - in the "ground switch" hole.  It's just a 500K pot that sits in series between the fixed 6.8K mid resistor and ground. As I crank it up, things get more "tweedy". Rough schematic attached. I like the varying levels of midrange on the raw pot but I'd love to add a complete bypass of the tonestack. You know, for when you just need a little "extra"

Could that be achieved by replacing the 500K pot with a switched pot or push pull pot? Would prefer to not drill anymore holes in the chassis if possible, just not sure how to implement that with a pot. I don't have much experience with push pull pots.

I'd be fine with anything from 25K-250K on the raw control if I could implement the bypass. Basically something to just break the ground connection on the tonestack. Unless I am thinking wrongly.

[sluckey]

Remove the ground from the bottom of your raw pot. Now put a switch between the raw pot and ground.

[bmccowan]

He's not wanting another hole in his head - er, chassis. Would this pot work?
https://www.amplifiedparts.com/products/potentiometer-alpha-audio-spst-rotary-switch-500k

[sluckey]

Push/pull switched pot would be better.

[jordan86]

So I think I know how to do it. I would need a push pull that is closed in the stock position and switch open in the pulled position. Is that normal? Or are they always open in / closed pull?

[sluckey]

The pots with a SPST switch are usually closed in the pull position. But if you get a pot with a DPDT switch you can wire it either way.

     https://www.tubesandmore.com/products/potentiometer-bourns-audio-knurled-shaft-push-pull-mini

[nandrewjackson]

Isn't at least some resistance to ground necessary at that point?


Is that EQ after a cathode or anode follower?

[jewishjay]

Isn't at least some resistance to ground necessary at that point?

Only if you want the tone controls to work. By disconnecting the tone controls from ground, it takes the tone stack out of circuit, giving a huge volume gain boost. Hence "raw" or direct sound.

[nandrewjackson]

I see, there's a volume control there in the Princeton reverb.


I think I'd plan on switching in a 3Meg resistance to ground under the raw control lug to ground point.


Minimize a possible "pop" from the switch?

[jordan86]

I don’t plan to hot swap it or use it live on the fly. Just studio and home use.

[mresistor]

I like it better to install a mini pot in the #2 input and wire it to the bass pot as described as a variable tone stack bypass.  Some of the intermediate positions of the pot are useful to me. 
YMMV