SOLA SOUND: Hybrid MKI Tone Bender

I've had a couple of questions about these recently, and out of curiosity I decided to take a closer look.  In 2024, Sola Sound released a few versions of a hybrid MKI, built by Dave Main of D*A*M.

So what is it?  It’s essentially a standard MKI circuit, but with the first two germanium transistors replaced by silicon transistors.  Because silicon devices don’t leak like germanium, a few extra resistors are needed to get things working properly.

As this is an incomplete trace and there are circuit variants - use the info below as a guide only.

That being said, I have breadboarded this with success - transistors matter....  It sounded a lot like the video demo below, and it was easy to tune playing with input and coupling capacitor values.

SOLA SOUND HYBRID MKI LAYOUT

There aren’t many photos available - I couldn't make out a few values, but I got most of it.  You’ll want to experiment on a breadboard and dial things in before committing to solder.

I ended up making a slightly different layout, as I didn't like some of the placment on this one.

The two values that I just could not see anywhere:  R3 & R7.  For R7, start large and scale down, it seems to tame the gain somewhat - it can also assist with noise if this is a problem.   For R3 I tried a couple of different values.  I have 4k7 on the breadboard at the moment.

The cap values that I could not really see are C1 & C2 on the input & C3.  C3 is just a suitable large electrolytic cap, normally a 22u.

I’m currently using 8k2 for R5 with a 50k pot.  Could probably use a 25k pot and skip R6.  

Groke

Note that R9 is actually 2 x 15k resistor in parallel, wich may as well be the stock value of 8k2.  

The Griffin

Some difference with R5 & R6 are visible. 

Brandy Snap

Looks like a 15n input cap with a 4n7 to ground.  1u coupling cap.  Maybe a 1m5 resistor on the output.