From bruce@ashbysolutions.com Fri Nov 09 14:41:03 2001
Subject:Re: Speak Easy Vintage Tube Pre/Chorus Pedals

John,

>****On a side note, the Chorus/Vib on my BX is dead. Has
>anyone ever tried a guitar pedal as a substitute for a clones' less
>than passable on-board effects? I have been looking at the
>Fulltone Deja Vibe and Sweet Sound Ultra Vibe. Any comments
>or suggestions would be appreciated.

Those units will give you nice vibrato, but I doubt that they will make an
accurate a copy of the Hammond scanner vibrato -- which is technically not
real vibrato at all. It is a variable-delay processor of sorts: in short,
a chorus generator.

I've tried using a number of guitar and pro-audio chorus effects over the
years, with varying success. The only device that I ever found to REALLY
simulate the scanner vibrato well was the Alesis Quadraverb 2. There may
be other devices out there that can do the same thing, but they will likely
be similar: complicated, highly flexible, multi-effect products. Here's
why; it gets a little technical, so bear with me if you're not a tech-y
kind of player:

The sound of scanner vibrato is not just made up of the chorus effect. It
also has some tone coloring that is a by-product of the only practical
method to make an electronic chorus effect in the late 1940's: coils
(inductors) and capacitors. If you look at the B-3 schematics, you'll see
that the vibrato uses a bunch of coils, connected to a rotating capacitor
that acts like a rotary switch, connecting each of the coil circuits in its
turn. In addition to the short time delay, the overall effect of this
circuit is a kind of low-pass filter -- bass comes through, treble
doesn't. In order to compensate for the muted result of the circuit,
Hammond engineers boosted the high frequencies prior to their journey into
the scanner. The final result was a (relatively) flat response, but the
frequency boost means that the higher frequencies dominate the overall
timbre of the chorus. This is what accounts for both a) the fact that
Hammond vibrato has more effect on brighter drawbar settings, and b) the
slight treble boost when the vibrato is added. "Normal" chorus units don't
provide this tone adjustment, and so the effect doesn't quite sound like a
Hammond vibrato.

What I did was put a high-pass filter (HPF) in front of the chorus block,
and a corresponding low-pass filter (LPF) after the unit. If you search
WAY back in the archives, I explained it all in painful detail, if you're
interested. The result was very good, but I used 60-70% of a $500 effects
box to get there!

Regards,

-BW

--
Bruce Wahler
Design Consultant
Ashby Solutions™ www.ashbysolutions.com
CloneWheel Support Group and HiNote moderator
978.386.7389 voice 978.964.0547 fax
bruce@ashbysolutions.com