SONIC VI: Silicon MKI Tone Bender

The SonicVI silicon MKI Tone Bender is firmly based on the original, with a few additions and some minor changes.  

Being silicon based, the transistors do not self-bias, so additional resistors have been added from V- to the base of Q1 & Q3. 

The transistors used are early Mullard PNP silicon OC202 which are quite low-gain and still available at reasonable prices.  In terms of gain, it's down in the 40-120hfe range.  

I tested some random vintage silicon transistors that I had handy, and around 85hfe did the job.  Being silicon, this is easily flipped to NPN by reversing the electrolytic caps, as long as you have similar transistors.  I've tested with PN2222A and it was fine.  Out of the transistors tested, the lower gain vintage ones sounded a tad better to my ears - I suspect this has more to do with hfe than age. 

ASSUMPTIONS

Given I did the layout based on a photo from Reverb, there are some unknowns - not too many though, as most of the values are clearly visible.  

Trimmer:  A 25k to 50k trimmer works well, set on the edge of gating to get that MKI decay.   

Attack:  The Attack pot is paired with a 150k resistor - 50k is fine, quite possibly with a low value resistor hidden under the board.  Tune it so it just cuts off with the pot on zero.  I'd say test this with a couple of different size pots (50k or smaller), to see what suits you.

Level:  I've assumed the value of the Level pot to is 500k, which is the same as a MKI

SONIC VI SILICON MKI TONE BENDER - EYELET BOARD LAYOUT

I added the 100uf that's seen off-board in the photo to the layout, as it's just a bit easier to attach here, vs the DC jack.

photo care of Reverb listing

MULLARD OC202 DATASHEET

SONIC VI SILICON MKI vs PIGDOG MKI

STANDARD VERO VERSION - NPN

I guess not many people will be interested in the eyelet version, as cool as it may look - and this is NPN that will fit in a 1590B enclosure.

version 1a - error corrected on v1.

BREADBOARD TESTING - WORK IN PROGRESS

It’s sounding really nice on the breadboard.  Sharp attack, same frequency response as a stock MKI, and there’s even the spitty decay on the end of notes.

It did pickup radio on the breadboard and squelch a bit - a low value cap on the input sorts this out.  Not sure what I even have on there at the moment as I picked up a random small greenie off the bench - think it's 3n3.  Needs a cap on the power supply, again, helps with noise.    

Tested with;

2N4036 PNP Silicon (vintage ones, although they were in production until quite recently)

Q1   86hfe    Q2   81hfe      Q3 106hfe 

PN2222A NPN Silicon (new)

Q1 - Q3, approx 220hfe