Hi Tony,

> Anybody know what is the maxium frequency of modulation a Vactrol

wil be able to respond to

That will depend quite a lot on just exactly what type of Vactrol you
use. Take a look at the second page of this datasheet, and at the top
you'll see two 'response time' graphs, which you will see have quite
different scales for the 'C3' and the 'C4':

< http://optoelectronics.perkinelmer.com/content/Datasheets/DTS_vtl5c3c
4.pdf>

That we are talking 10's to 100's of milliseconds shows they are
pretty sluggish - this might (and I only say 'might', as I have no
experience with these devices!) be good enough up to a couple of
Hertz or so. I skimmed the A-101-9 pages, but couldn't see any
mention of just what device is used inside that module - thus if you
go this route, be prepared to have to juggle some component values
along the way (although this may be made easier by having the LFO
range switch to play with too).

Since using a Vactrol will mean providing support circuitry to drive
it, it might not be much more work to replace resistor R4+the
frequency pot with an OTA, and do it that way: off the top of my
head, the scaling for the input to the OTA should be pretty straight
forward; harder will be how to drive its control current, as this
would probably need some sort of expo converter, and since you need
to source current for the OTA, you won't be able to use the simple
pnp+npn transistor combo, I think it might need an op amp or two
(although I guess the pnp+npn route with a simple current mirror
might do it, it doesn't really need to be super-accurate after
all...).

In conclusion, lots of possibilities: the deciding factor is likely
to be how much time you are prepared to spend on it! (At least it
should be relatively easy to graft anything onto the 145 by merely
removing resistors etc,. i.e. without having to cut any tracks.)

Tim