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

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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/

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John Jacob