Bobtavia w/Spinal Tap Mod reviewed

[petemoore]

  Ok it cuts volume, and only works on a very small portion of the gain knob.
  But the STM is one interesting place!!
  I got a wide variety of tunings out of it !
  By itself it is a lower volume [about unity I don't know], thin well focused octave sound, clkclkclkclk clicking on the other side of a very small sweet zone where oscillation between notes, oscillation, different octa-harmonics etc. dwells.
  Using a comp. before [or boost or Fuzz] and a Rocket after ModBob w/STM engaged, well defined Octave under [certain small portions of the neck] shifts to pitch shift when exiting the area of the neck where that happens, above or below the clearer Oct-down portion. Notes don't make any sense unless 're-indexed'. Higher up on the neck with pre-gain/post fuzz and STM in got double octave up, or sounded as though it was testing the limits of the speaker frequencies, attack octave can fade quickly to II Octave sustains, very interesting responses to dynamics.
  I was able to repeat the 'repeater tremolo' or short deep med/fast tremolo effect [w/oct./] after each attack, lasting 5 cycles then sustaining strongly...very interesting effect on the high E string between almost exactly high G / high B, above/below this area strong octave.
  So the gain goes about all the way up, then back to just higher than the oscillations start...right before and over that line is all the cool stuff that starts coming out better with the volume up, another note is that the Rocket's output volume...after Bob...had an odd, surprizing and profound effect on...Good Octave between say 9 oclock and 12, rising volume and cancellation of and real signal between 1 and 3:30, then 4 oclock..the wierd sequences of triggered tremolos double oct. and strong bass octave...
  All of the above isn't necessarily 'usable' in the strict musical sense, but provides a very freaqey experience when seriously tuned to do so..many of the things mentioned like two octave, bass octave etc. were available with careful tuning, and then only on a small portion of the neck, sometimes only a small band of frequencies will respond in 'like' fashion..beyond that it is very cool, very fun, gets super wierd and some sounds that are complex sequences of mixed effects, quite a spacey trip to play !
  Thanks to Jeff Musser for drawing the BOBTAVIA EZ Build schematic, Doug Hammond for providing mods, and Matt Tucker for the Spinal Tap Mod.
  This excellent circuit is in the PB with PS, and is very consistant, well behaved.
  This Bobtavia gets strong Octave up, Ghost notes, Ring Mod sounds, and does sound different from what I remember Bobtavia last build...with alot of 'balance' parts in it, that stands to reason. I seem to remember the last Bob as having...more diodes in the recifier section, and vague memory that they weren't necessary, perhaps..whatever the case Bobtavia w/STM is a great build.

[DaveBullard]

What's the spinal tap mod?

[petemoore]

  It's a 2k2 resistor AFAICT.
  http://www.generalguitargadgets.com/diagrams/bobtavia_spinal_ez.jpg
  GGG>Cross reference >Schematics, regular Bobtavia is in there too.
  I just can't figure out whether to make that 2k2 a little bigger or smaller to wring out a little bit more consistant behaviour in the STM mode.
I was messing with the STM, and can't get the tick/oscillation to stop even with gain up/nothing else on, it finally settled down again, I still haven't been able to replicate the tremolo thing I described either...it was pretty fun tweeking the knobs and pseudo-osci-theremin-ing, but getting there...well I wasn't able to last time, but getting there was a venture in knob tweeking, fun for a time, maybe..takes alot of setting up and I wouldn't use it except for kicks.
 

[DaveBullard]

How about replacing that 2k2 with a 5k pot? That's what I would try, but I have a strong affinity with knobs.
Could the finikeyness be temperature dependant? Moisture absorption? I'm not a super genious, but if it's that sensitive in the first place...
I breadboarded the stock version last month and I intend to box it up, I think that I'll try that mod using the pot I suggested

[petemoore]

I think that I'll try that mod using the pot I suggested
  Cool, let me know how that goes, 5k..yupp!