GRETSCH: Expandafuzz

The Gretsch Expandafuzz is a bit of an odd-ball guitar effect from the late sixties / early seventies.  Not much out there on these in terms of information and video demos.

First oddity:  Apart from having a switch and a knob for almost everything, the one switch it doesn't have is for bypassing the entire effect - it's always on...  there's only a choice of the Expanda section (Bass, Mid, Treble) or Fuzz & the Expanda section on together.

Second oddity:  There's no master volume.  That's right - not only is it always on, but you also have to juggle levels using multiple pots and switches.  

Third oddity:  There's no choice between straight fuzz or boost.  It's either boost or boost with fuzz mixed in, so there's always clean signal present. 

Fourth oddity:  The schematic shows a 10k resistor from the output of every op-amp in the expanda section going to TP1 - TP4.  But there's no other reference to TP1 - TP4 anywhere else on the drawing.

Well, that's cause they don't actually connect to anything.  I believe these are just test points, so I've left them off the layout.  They do absolutely nothing in LTspice, and I could not find a connection anywhere after staring at photos of the circuit board for way too long.   Photos are here by the way  

GRETSCH EXPANDAFUZZ - VERO LAYOUT

The input stage switch could be removed and just hardwire the fuzz on all the time and have a normal true-bypass switch - that's what I'll be doing.   I'll probably get rid of the blend with the boost and just go for straight-out fuzz...  big surprise right.  You may also want to add a master volume control.

I didn't bother trying to keep the layout small, as it will need a big box with so many switches and knobs.  You could also just leave some switches off if you want to save some space, and hardwire bass, middle and treble on all the time, and just use the pots to control the mix.

There's also a power inverter with filtering included, which is not present on the original as it would have run off 9V batteries.  This is a dual rail supply running +/- 9v.

I also left off the small 22pf compensation caps that were required for the vintage opamp.  Chances are a modern op amp will sound a little different, but not significantly so.   

ORIGINAL SCHEMATIC

LT SPICE ANALYSIS

Note that the switches on the expanda stages are not there.  Switches are not actually a component in LTspice, so the schematic looks a bit different in terms of switching (or lack of).

Frequency response of the Fuzz & Boost section

Frequency response of the Expanda and output stage (all pots on 100%)

Traces of the boost and fuzz stage, along with the output of this section - all pots at 100%

Check out the size of the square wave off the output of the fuzz opamp - that's a healthy +/- 9v  (or 18v) square wave.  This is before it's tamed by hard clipping diodes and resistors in series; otherwise, it would destroy everything that follows it.  Although you can bypass or reduce the 220k resistor after the diodes if you want more fuzz.

The output of the pedal - all pots 100%