[sdiy] more sh-101 mods?

[Paul Higgins]

I'm aware that Mesa/Boogie uses tons of vactrols for switching purposes 
(I have an old modified Quad Preamp, and it has 32 of them!).  But 
their TriAxis, a MIDI-controlled preamp, uses Vactrols in its control 
circuitry as well.  The TriAxis is really an amazing piece of 
engineering, which unfortunately made it to market just as guitarists' 
fashion sense went back to the Stone Age (non-programmable pedalboards, 
single-channel amps, etc.).  Only a few pro players use them (John 
Petrucci of Dream Theater is a particularly big fan).  It also cannot 
be said to be inexpensive, probably another reason why only the 
well-heeled pros use them.

I have a schematic of the TriAxis around here somewhere that looks like 
a scan of a factory drawing.  It's definitely legit (their factory 
schematics are almost always drawn by the same person and have a 
distinctive style); it looks like they were making changes right up to 
the product ship date.  There are hand-drawn corrections all over the 
document.  The vactrols are used in the "Fender tone stack" (as the 
popular Fender tone controls are called) to make these controls 
programmable.  The LDRs are specified in pairs for the treble control 
and I believe the midrange as well.  The bass control can use a single 
LDR because of the way it's wired in a Fender amp.  There is also a 
weird control called "dynamic voice" which basically interposes a 
vactrol into their graphic EQ circuit in such a way that you can fade 
in and out of a particular EQ setting.  The graphic EQ, which is a 
peculiarly Mesa design, uses inductors and discrete transistors (I've 
been told it dates to an old Radio-Electronics circuit) and is not 
easily made programmable--at least not without a huge number of 
additional vactrols.  Anyway, the main EQ settings use a 4051 analog 
switch to give eight factory presets.  That appears to be the one 
compromise they made in the design.

Unfortunately, Mesa/Boogie schematics seem to be few and far between on 
the web these days; I don't know if it's because of company policy or 
not.  If I can locate my copy, I'll send it if you'd like.  I have no 
links to Mesa schematics anymore--sorry.

To be sure, vactrols are used all over the place as "soft" switching 
elements as well (to prevent pops and clicks).  You are correct that 
this is their main use in Mesa/Boogie products.  I think the TriAxis 
may use more than 40 vactrols; it's been a long while since I've looked 
at the circuit.

Earlier, I mentioned Tim Caswell of Studio Electronics re: the 
Soldano/Caswell preamp that uses the servo-motor pots.  Caswell also 
designed a switching system some years ago that used a vactrol as a 
remote volume control "VCA" (any volume pedal could be plugged in and 
would function as a CV pedal).  I think he used only one LDR in that 
application, but I'm not sure.

You might also look at JH's Poly-Korg clone; I'm fairly sure there's a 
lot of LDRs used in a similar manner to the TriAxis.

-PRH

On Saturday, Dec 3, 2005, at 21:39 US/Central, harrybissell wrote:

> Most of the Mesa/Boogie designs that use pairs of vactrols are doing
> switching, not linear control.  I haven't seen them do a real 
> potentiometer
> with a pair of vactrols. (I'd like to... and schematics or links ???)
>
> H^) harry