MOSRITE: Fuzzrite, Germanium Version

There are more than a few versions of the germanium Mosrite Fuzzrite out there - seems like they went through a few different construction technics and capacitor combinations along the way.  The details mostly seem to be lost to time (and rust).

Here's a bit of info that I've pulled together from various sources while I was trying to work out what was going on with all the variations.  

STANDARD MOSRITE FUZZRITE SCHEMATIC - GERMANIUM VERSION

The basic structure of any Fuzzrite is a transistor (Q1) smashing another transistor (Q2) with a lot of level.  The signal from each collector, which is out of phase with the other, is mixed together at the depth pot.  Q2 passes by a high-pass filter before the mixing occurs.  

There's phase cancellation happening, volume loss around the middle of the depth control (phase and additional resistance), one transistor is more distorted than the other, and of course, Q2 is really bright due to the high pass filter.  This is the signature sound of the Fuzzrite, and why it doesn't quite sound like anything else.

FREQUENCY RESPONSE

There's a pretty clear difference between the two - the most obvious being the huge dip in the response with the 47n caps.  It's like a notch filter is at work.  The 100n caps have a bit more low-end, and less of a notch.  On both, the notch disappears pretty quickly as soon as the pot started to turn towards Q2.

The value of the Depth pot does make a difference - a 500k pot reduces the notch.  Changes to the volume pot don't seem to make a huge difference, although I would not recommend attempting to recreate a 33k pot, which I think is probably an incorrect value.

CAP VALUES

100n caps were used on early versions, later changing to 50n ceramics.  Some good references and photos on the Fuzz Boxes website.   

2n caps seem to be the most common, with the one coming off Q2 paired with a 22k resistor to form a high-pass filter.

There does seem to be some evidence of other cap values being used for the depth control - I recently saw a post on Instagram with 7.5n tropical fish caps in this position.  They seemed to be original.  I've also seen photos of a 1n cap on Q1 and a 2n on Q2.  Maybe this is the classic vintage construction technique, of use what we have available today or was cheap at the time it was needed.  Close enough right?

I was surprised to see the 7.5n caps were not radically different - noticeably so I'm sure.

POT VALUES

The most likely scenarios for pots are either 350k or 500k pots.  Despite 350k being an odd value, it is what was used in a lot of Mosrite guitars.   I have seen the code BA8-11-3901 on a volume pot, which is a Mosrite part number for a 500k pot.

On Guitar FX Layouts, Travis pulled his vintage GE Fuzzrite apart and listed 470k and C350k as the values for the pots.  Probably a safe bet that these are correct values.

A 1.2meg resistor across pins 1 & 3 of a 500k pot with give you a 352k pot, so close enough, especially considering the 20% variance on most pots.

  

TRANSISTORS - FROM THE RCA HANDBOOK

It's generally agreed to run something a little hotter for Q1 (2N2613), and something lower hfe for Q2 (2N408).  

But like most things, don't take the internet as gospel, you might prefer the sound of something different.  Apart from that "correct" transistors for the Fuzzrite are crazy expensive on eBay.

FUZZRITE VERO LAYOUT

Here's a simple layout intended for battery operation, with stock values.  Change caps to the nearest modern value, as it really makes no difference (well very little anyway).