Pandora's Box

"A lost pedal rediscovered growing from trees on ZZ Top stages"



The Pandora's Box overdrive rack effect module compares to the little-known Bixonic Expandora.  Could easily be classified as a distortion, as at least half of its settings go beyond overdrive, well beyond it.  It is an unusual circuit for an unusual effect.  It goes from mild overdrive to crunch to distortion to a wildly-out-of-control squealing mess of noise that cannot be tamed once it is let out of the box.  Rumored to be one of Billy Gibbons' preferred overdrive/distortion effects.  The effect has a Bass/Guitar switch, so maybe Dusty tried one, too...



Perhaps no tone quest is filled with so many false leads, tall tales, and misinformation as any that attempts to figure out what secret weapons have been used by Billy Gibbons over the years.  (OK, well maybe EVH tone comes close...) The Bixonic Expandora was tossed into the mix some years ago, when it was rumored that he had been using one.  Then things got silly when a "tree" of six Expandoras, cabled in serial, made its appearance on the stage at some ZZ Top shows.

Anyone that has ever played through an Expandora knows that it is a quirky pedal, that goes far beyond out-of-control, much-too-high distortion.  At its maximum settings it is unusable for anything other than producing various types of noise.  Using six of them in a row to create some sort of usable guitar tone, even on low gain settings, is nonsense.  But you would also know that a single Expandora has an incredible range of overdrive and distortion available, is touch sensitive, and can provide some really great drive tones once you figure out how to tame it.  Maybe Billy did that.  Perhaps he even had more than one of them set for different tones, since it would be difficult to change settings during a show even if Elwood was doing it for him.  Using more than one at a time seems highly unlikely...

Pandora's Box compares to the Bixonic Expandora.  The original pedal had controls for Gain, Tone, and Level.  It also had two or three DIP switches, depending on the version.  One switch selected between Bass and Guitar.  The other two switches together had a total of 4 setting combinations.  These setting ranged from Overdrive to Crunch to Distortion to Forbidden, which is way, way , way too much distortion that squeals, shrieks, and is completely unusable with the Gain turned more than just barely on.

Part of the circuit is similar to a Proco Rat, and part is unique to this pedal.  The harder you hit the strings, the more distortion you get.  Back off the guitar volume and it cleans up.  If it wasn't for the wild, out-of-control 4th switch setting, perhaps the pedal would have caught on better.

Pandora's Box adds a few options to the original circuit, including the ability to tame the beast on the 4th switch setting.  We call it the "Chaos Control" and offer it as an option, along with a couple of popular Rat mods.  We don't offer options for changing the components.  The circuit is picky.  If you want something changed let us know and we can give it a try.  We'll use low noise metal resistors and low noise caps due to the extreme gain.  You'll also get the original circuit's LM308N op amp for the Rat-like portion of the circuit and a 4558 for other parts.  We'll use the unusual RN2206 transistor of the original, as long as we can continue to find them.  The original also used an unusual surface mount optocoupler.  We sometimes use the same one or a completely compatible optocoupler that is not surface mount, whichever we can keep in stock.



  • Pandora's Box - compares to Bixonic Expandora






  • Smooth Switch - This is a variation of the Reutz Rat mod.  It decreases the gain a bit and reduces the low cut (or treble boost), making the sound smoother.  This is switched so you can still select the stock sound or the modified sound.
  • Smooth Control - This is another variation of the Reutz Rat mod which allows you to adjust the cut frequency.  All "off" gives you the stock circuit sound.  Turning it up flattens the frequency response, reduces the gain, and results in a smoother sound.  This control is interactive with the Gain and Volume controls and the Chaos control if it is installed.
  • Custom Clipping - The default clipping configuration is (2) 1N4148 diodes.  You can pick another configuration if you choose.  However, there is another constraint you must consider.  There is also a Sensitivity diode that impacts the playing dynamics.  Clipping diodes and Sensitivity diodes must be compatible.  Without going into a long explanation here, we'll just say that we'll help you with this if you choose to change the clipping diodes.
  • Clipping Switch - This provides a switch to change between two clipping diode configs.  Again, we will help with Sensitivity diode selection to make sure the diodes all stay compatible.
  • Sensitivity Diode - You can also change the Sensitivity diode without changing the clipping diodes.  Again, we'll help with the details, but this may result in either increased or decreased playing dynamics, depending on which you prefer.  Not recommended unless you are familiar with the Expandora.
  • Chaos Trimmer - If you can use the Forbidden mode, it is extremely high gain, almost like a fuzz, and can sound great.  If you can use it.  It is a very finicky mode because of extremely high gain.  If you want to get some use from the Forbidden mode, this option or the Chaos Control is highly recommended.  The Chaos Trimmer option tames the max gain of the Forbidden mode.  It is an internal trimmer.  To set it, turn on Forbidden mode, crank the Gain all up, and turn the trimmer until you get a usable, controllable sound.  The Chaos Trimmer will also have a tiny effect on the gain of the other 3 modes.  Depending on the Chaos setting, you quite likely won't be able to notice the difference in the other 3 modes, or you can choose to change them more noticeably by cutting more gain with the Chaos Trimmer.  It will give you a little less gain on each mode, but since the modes overlap and provide a range of gain levels, you may just be moving sweetspots to different settings, possibly in a different mode.  At the highest gain setting of the Distortion mode, there will be a little less gain available, but then you'll have the Forbidden mode to work with.  So you don't really lose much, if anything, but you gain some lower gain options.  With the Chaos trimmer, the tone won't clean up quite as much with the guitar volume.  You may lose a bit of the sensitivity between playing lightly and digging in.  Not all settings of the Chaos trimmer are usable.  If you turn it down too much, you'll lose all your gain.  If you turn it all the way up, it may not help you tame the Forbidden mode.  There is a range somewhere in between that should give you best results.
  • Chaos Control - Same as Chaos Trimmer, but an external control instead of an internal trimmer.  This is probably the better option for those that use different guitars, change up the signal chains, or otherwise change the signal going into the module which could throw off the Chaos Trimmer setting.


Front Panel

  • Gain
  • Tone
  • Level
  • D1 switch
  • D2 switch
  • Guitar/Bass switch
  • (optional) Smooth switch
  • (optional) Smooth control
  • (optional) Clipping switch
  • (optional) Chaos control


Rear Panel

  • Audio In
  • Audio Out
  • On/Off Footswitch
  • On/Off Override
  • DC Power
  • DC Power LED


Module Width

  • 2" standard
  • left wing and right wing panels not presently available


Power Consumption (aprox)

13 - 17mA


Base Configurations

Part # Description List Price
MOD-PANDORA Pandora's Box module $229



Part # Description List Price
MOPT-PANDORA-SMTH Smooth Control $19
MOPT-PANDORA-CLIP Custom Clipping $0 - TBD
MOPT-PANDORA-SWCLIP Clipping Switch $19 - TBD
MOPT-PANDORA-SDIODE Sensitivity Diode $0
MOPT-PANDORA-CHAOS Chaos Control $19



Part # Description List Price
MPT-PANDORA-CA Replacement pot, switch, jack, LED cable assembly $19
MPT-PANDORA-KNB Replacement control knob $2
MPT-PANDORA-PB Replacement power board $29
MPT-PANDORA-SB Replacement switching board $29