Adding reverb amount knob to Hammond L-100

[AlfredoMeister]

I'd like to add a knob to the front panel of my L-102 to control the volume of the reverb return signal, as "Reverb I" can be too much. Would an A100k volume pot on the output of the reverb tank do the trick? Thanks

[Jyvoipabo]

Hello,
I think this should work.
There are several possible locations for installing this potentiometer.
If I had to decide, I would try to insert it at the arrival of the white shielded wire right next to the reverb tabs.
And there are several possibilities for wiring it in series or in shunt (potentiometer mounting).
After a little trial and error, you should be able to get there.
Then all you have to do is drill a small hole next to the tabs in the pannel to secure it.
JP

[AlfredoMeister]

Awesome, Thanks!

[Jyvoipabo]

The Schematic

[kziss]

I'd like to add a knob to the front panel of my L-102 to control the volume of the reverb return signal, as "Reverb I" can be too much. Would an A100k volume pot on the output of the reverb tank do the trick? Thanks

Wiring up a 100K potentiometer as a Reverb Volume control will require the addition of a long shielded twin core cable to be wired up between the output of the reverb tank and the section on the organ control black metal panel next to the Reverb 1 and Reverb II tablet switches.

Many years ago I wired up a simple and fully adjustable Reverb depth control potentiometer on to someone's 1965 L-122 organ, and to achieve this I modified the Reverb I and Reverb II switching set up as described in great detail here below.

This Reverb Depth control potentiometer was wired on to the actual Reverb I and Reverb II switching setup on the black metal control panel so therefore all the wiring was short without the need for a long cable, and this modification does not create any hum or any high frequency oscillation issues.

With the stock circuit, there are two resistors wired on to the junction of the output side of the C302 20 nf capacitor which carries the pure (fully "wet") Reverb output signal coming from the plate pin 1 of the V6 12AX7 valve.

One resistor is the R405 820K resistor (.labelled as .82 M on the schematic), and the other resistor is the R406 270K resistor (labelled as .27M in the schematic).

When the "Reverb I" switch is turned on, the R405 820K series resistor is sent to the dry main organ signal thus producing the weakest reverb level.

When the "Reverb II" switch is turned on, the R406 270K series resistor is sent to the dry main organ signal thus producing a stronger reverb level.

When both the Reverb I and the Reverb II switches are turned on, then both the R405 820K resistor and the R406 270K resistor are activated in parallel thus resulting in the parallel resistance of around 203 K which produces the strongest reverb level, because there is now a reduced resistance between the wet reverb signal and the dry main organ signal.

I modified the Reverb I and Reverb II switches by unsoldering and removing the R405 820K resistor and the R406 270K resistor, and I replaced the R405 820K resistor with a 1 Mega ohms linear taper potentiometer which would henceforth function as the fully adjustable Reverb Depth control, and the Reverb I switch would henceforth now function as a "Reverb On/Off" switch.

I soldered two wires between the 1 Mega ohms potentiometer and the Reverb I switch contacts where the R405 620K resistor previously was wired up.

Wih this simple new set up, the desired reverb depth level is chosen by the setting of the Reverb Depth potentiometer dial, and the Reverb I switch then turns the Reverb effect on or off by connecting or disconnecting the reverb signal from the Reverb Depth potentiometer to the dry main organ signal.

In order for the 1 Mega ohms potentiometer to produce the weakest Reverb level at the fully anticlockwise position 1 setting, and the strongest Reverb level at the fully clockwise position 10, I wired up a wire between the potentiometer pin 3 and the contact on the Reverb I switch which has the signal coming from the C302 20 nf capacitor on it, and I wired up another wire between the middle wiper pin 2 of the potentiometer and the section of the Reverb I switch which sends the reverb signal to the dry main organ signal so therefore the Reverb Depth potentiometer now functions as the direct replacement for the R405 820K resistor on the contacts of the Reverb I switch.

The 1 Mega ohms resistance created when the Reverb Depth potentiometer dial is set at the fully anticlockwise position 1 results in a somewhat weaker reverb level than what is produced by the R405 820K resistor.

When the Reverb Depth dial is turned up to the fully clockwise position 10, then this results in a noticeably stronger, more intense cathedral-like or cave-like reverb level because there is now zero resistance between the C302 20 nf capacitor and the dry main organ signal, so therefore the Reverb Depth potentiometer allows a much greater range of reverb level settings from minimum to maximum compared to what is allowed by the three arbitrary reverb depth settings produced by the stock Reverb I and Reverb II switch settings.

If you consider the reverb level to still be too strong even when the Reverb Depth dial is set at the fully anticlockwise position 1, then you could either replace the 1 Mega ohms potentiometer with a 2 Mega ohms potentiometer, or you could wire up a resistor in series with the 1 Mega ohms potentiometer.

You could experiment with different resistor values in series with the 1 Mega ohms potentiometer in order to find what resistance value produces the weakest desired reverb level.

PLEASE NOTE: The ideal situation would have been if it was possible to connect the pin 1 of the Reverb Depth potentiometer to the ground so that the Reverb Depth potentiometer could then ground out and fully silence the Reverb effect when the dial is set at the fully anticlockwise position 1, however if the pin 1 is wired to ground, then the dry main organ signal would also be sent to ground and silenced when the dial is set to the fully anticlockwise position 1, so therefore the Reverb Depth potentiometer needs to be wired up as a simple variable resistor ( a rheostat) without any ground connection on the pin 1.

When I modified the 1965 L-122 organ with the Reverb Depth potentiometer, I physically removed the now redundant Reverb II switch so that I could then physically install the Reverb Depth potentiometer in the empty space previously occupied by the Reverb II switch.

I drilled a hole on the middle of the empty slit (where the Reverb II switch tablet previously protruded through) so that the potentiometer could then be inserted and bolted in place in that spot.

[AlfredoMeister]

You've done it again Kon. And by "it" I mean providing a clear and in depth explanation of a modification you've done in the past, complete with warnings, part numbers, and alternative modifications, Bravo. Thanks!

[LMF]

I had an L122 that had the deepest reverb I have heard on a Hammond. My M102 and C3 with Trek II reverb didn't come close.

[kziss]

You've done it again Kon. And by "it" I mean providing a clear and in depth explanation of a modification you've done in the past, complete with warnings, part numbers, and alternative modifications, Bravo. Thanks!

Thank you.