adding a blend/intensity control to late 70s early 80s small stone EH4800?

[bobster]

hi guys- [i tried the search function but got nothing on this ]
im liking very much my old small stone . i got to thinking that maybe with an intensity or blend pot mixing dry and wet signals i could get a lower balance of effect sound where i could leave it on all the time for a more subtle shimmer if you catch my drift........
is there an easy[ish] way to do this ?
i dont mind drilling a couple holes etc as the pedal is already quite tatty with no battery cover and will not be getting sold so collector value etc doesnt come in here. maybe a mini pot could be installed alongside other controls and soldered in to the appropriate place on the board?
many thanks for any suggestions - bob

[Mark Hammer]

The Dry and Wet signals are passively mixed together at the output via fixed resistors.  If you work your way back from the output wire on the stompswitch to the circuit board, you should first encounter a 470k terminating resistor to ground and a 100nf (0.1uf) cap leading up to it.  This schematic - http://www.montagar.com/~patj/smlstone.gif - shows them.  (The notes on Aron's list of schems indicates that there is an error in the drawing, but the error is not in this part so we can use the drawing for reference in this instance.)  The two resistors in question are the 27k resistor coming from the last chip, and the 30k resistor coming from the transistor input stage.  If you make the 27k resistance larger, this will reduce the level of the phase shift signal, rendering it subtler.  If you lift the connection of the 30k resistor (on either side, actually), you'll get vibrato.

Max notch depth and phasing effect occurs when the wet and dry levels are perfectly matched.  Since those pedals use 5% resistors, the levels are only nominally matched by use of those particular resistor values.  I.E., sometimes you geta perfect one, sometimes you get just a really good one, and sometimes you get one that is not quite there.  If you replace the 27k resistor with a 22k value and a 100k pot in series with that, wired as variable resistor, you will be able to dial in a mixing resistance for the phase shift side that varies from a little less than the clean side, to bang on (wherever that might be given the +/-5% range each resistor can cover), to much greater than the clean side, yielding a phase shift sound that is "shallower" and pushed further to the background.  I've done this and can confirm it works.

100k may not be sufficient for every taste, so if it isn't enough for you, there are several routes you can go.  One is to use the next value up - 250k - as is, or to use any higher value pot with a suitable parallel resistor to set its maximum value (e.g., 250k with a parallel 220k fixed resistor means it can never go higher than a resistance value of 117k).  Another is to simply decide upon a couple of presets you want and toggle them in.  For instance, if you replaced the 27k phase shift side resistor with a value of 220k, you could use a 3-position on-off-on SPDT toggle to engage either no parallel resistor, or one of two parallel resistors to get, say, full, medium, and weak phase shift sound.  For some people that may be enough, and it certainly takes up less room and generally costs less than a pot and knob.

[bobster]

hey mark , thanks so much for that detailed reply .............
i have a little toggle switch layin around and some resistors so i'll get cracking on that one
cheers - rob