AMPEG: Scrambler

I haven't looked at an Ampeg Scrambler in a long time - in fact, my first attempt a few years ago went nowhere and ended up in the bin.  

It's a pretty wild octave fuzz from the late sixties; not a huge number were produced so the originals are expensive now.  

Scramblers were designed for bass guitar, which is why there's a clean blend.  Another thing to note is that they have a pretty low output level compared to a regular fuzz, sometimes below unity, so a boost is often added or used in conjunction with an electric guitar.

AMPEG SCRAMBLER - VERO LAYOUT

I didn't include the diodes marked as 'not on units' on the original schematic, as after playing with the circuit on LTspice they seemed to make no difference at all.

 

IMPORTANT: If you use 2N5306 for this, check your pinouts, as they can vary.

ORIGINAL SCRAMBLER SCHEMATIC

VIDEO DEMO

LTSPICE ANALYSIS

Schematic

I used DIY Darlington transistors in spice, as it doesn't actually come with Darlington transistors as a standard component (LTspice has some quirks), and I could not get the version I tried to download as a model to work.  Anyway - seems to have worked here...

As mentioned above, I did not include the redundant diodes, but I did compare them side by side and found almost no difference at all - if there was, it was extremely minor, not enough for me to spot on my laptop screen.  

Frequency Response

I included comparisons of different values for the emitter capacitor on the boost stage (Q3 on my schematic).  The original has a 120u capacitor, which will be hard to find now.  There's not a huge amount of difference between 100u and 120u.  You could probably move the 47k resistor to the left, and add another capacitor in parallel if you want to get closer to the original value, or just go a bit larger and use a 220u.

Traces

Below is the output of boost stage after the rectifier diodes, which, as you can see separates the positive and negative sides of the wave, and sends them to the differential amplifier.  Differential amplifiers as the name suggests, amplify the difference between the two inputs, which basically doubles the frequency, as you can see below.