Vibra-Tone question

[SonicVI]

By Vibra-tone I'm referring to RG's fake Fender brownface vibrato.  Can somebody explain the purpose of the 2n3906 connected to the input jack ring?  thanks.

[bluesdevil]

I gotta look at the schematic to refresh my memory, but I believe it's part of a circuit to turn power on when plugging in. 

[R.G.]

If you research my posts here, you may be able to find my technical disagreement with using the input jack ground return for grounding the battery lead.

This semi-standard trick to turn batteries off when there is no plug in the input jack forces all of the pedal's battery power to run down the input jack wire. It is a setup for having input noise problems like clicks when the LED comes on and forces another 20ma through that wire suddenly.

A simple PNP transistor fixes that. The PNP switches the positive battery lead and the negative lead is always connected to circuit ground. The input ground wire only carries the base current for the PNP. The base current is not only much smaller than the whole pedal's current, it's also almost perfectly constant, so there is no variation for the input to amplify.  This cures the problem with the input jack battery switching trick.

This is another place where pedal effects are so easy that you can get away with sloppy circuitry. The ordinary connection of the battery negative to the input jack ring contact works ...almost... all the time, so people use it. It's only when you get bitten by noise and clicks that can't be found that you have a problem. Mostly even then people never figure out what's causing the problem, so the practice never gets changed.

[SonicVI]

Thanks RG, that's interesting,  I suspected it was something to do with switching the power on, but I'd never seen anything like that before and thought maybe it was a necessity for this circuit specifically.  Can I just eliminate this if I'm not going to be using a battery and jack switching? 

The reason I was curious is that I'm putting this circuit in a box together with another tremolo circuit and was wondering if it was cause any kind of conflict with the power or grounding of the other circuit.  I suspect there will not be.  thanks again.