Leslie model 122 switch speed delay reduction modification.
I got this mod from Matti Adolfsen several years ago. I have posted it several
times in the past, but I don't think Matti would mind me posting it
again.
There is a simple mod that I have done on both of my 122 amps that works
well to reduce the switch delay time to be so short that it is
unnoticed.
Do not change the 10uf cap to a smaller value, this will reduce the bass
throuput.
Matti Adolfsen wrote:
Some theory for background info:
1) The relay pulls, when the control voltage at the grid of the control
tube exceeds about 25 volts ( or.. if the E.I.S solid-state relay is
used, 27 volts).
2) You cannot change the "phantom" voltage on the balanced line too
fast, as there is no input transformer in the Leslie, and too fast
changes will drive the 12AU7 preamp tube to saturation or cut-off. =
short silence when changing speed.
3) And you cannot change the decopling capacitors, as this will decrease
bass response.
4) the time between operating the switch to when the control voltage
passes this 25V threshold depends on the time constant of the delay
circuit (10 uF capacitor and some resistors) and the starting voltage.
And we cannot change the time constant, so...
The mod:
You need 2 inexpensive components to reduce the switching delay to about
50%.
Ok, here they are:
one zener diode, 30 volts / 1 watt.
Connect in parallel with the
half-moon switch (or whatever switch you are using).
The cathode of this
diode to the positive terminal of the switch.
One resistor (10 to 15 k / 0.5 watt).
Connect in series with this
switch/zener combination.
What does this modification do?
Originally, the phantom voltage swings from 0 volt (fast) to 70..90 volt (slow).
This modification limits the swing to about 10V... 40V.
As the time constant remains the same, (10 uf capacitor and the resistors are the
same), the delay will be about 50% shorter.
I have this mod installed in my system, and it works ok. the delay is so
short it does not matter anymore. This requres no modifications to the
leslie, so the organ is fully compatible with any 122 type leslie!
This modification too is fully compatible with all organs wired for 122
type leslie.
c/o
--------------------------------------------------------------------
Matti Adolfsen, Ouluntie 66, 94700 Kemi, Finland
Sound engineer, teacher - College of art and Media (Tornio)
Musician (B-3 + 251), Audio & RF service technician
mailto:matti.adolfsen@llaky.fi http://www.llaky.fi/audiocrw/madol/
---------------------------------------------------------------------
John Jacob