Adding 4th, 5th, and 6th Harmonic Percussion

Adding the 4th, 5th, and 6th Harmonic to the Hammond (B3, C3, RT3, or A100) Percussion allows for much more percussion variation by making 3 more harmonics available in addition to the 2nd and 3rd standard harmonics. With the first 3 drawbars out (the usual setup) there is much more contrast between the drawbar setting and the percussion tones. The sharp percussive bite is now available much lower on the keyboard. To get the full effect of this harmonic addition EliminatingThePercussionVolumeDrop is very desirable since this will restore full volume control to the drawbar settings. Two miniature, four pole switches (one for the percussion on/off and one for the 4th, 5th, 6th harmonic selection) and some stranded hook up wire is all that is needed. The switches are available from Mouser electronics (part No.’s below). Note with this design, the 4th, 5th, and 6th harmonics are not available at the drawbars when the percussion is on. The usual percussion setting is some combination of the first three drawbars so this shouldn't be a problem.

DESIGN SCHEMATIC

The schematic below shows the switches S1, S2 and the associated hookup to the
AO28 preamplifier and the drawbars.
S1 is shown in the ON state and S2 is shown DOWN with the 4th harmonic selected.
The contact states for S2 DOWN, CENTER and UP are shown above the schematic.
The contact states for S1 OFF and ON are shown below the schematic.

<pre>
       .     .           .      .           .     .
      . \.  . \.        . \.  ./ .        ./ .  ./ .
       S2 DOWN          S2 CENTER          S2 UP

  |-----------------.   |-----------.
  |                  \  |            \
  |               S2-1\ |         S2-2\
  |               .    \.         .    \.---------|
  |               |               |               |
  |               |               |               |
  |               |               |               |
  |               |               |               |
  |               |               |               |
  .\    .---|     .\    .---|     .\    .---|     .\    .---|
    \S1-1   |       \S1-2   |       \S1-3   |       \S1-4   |
     \      |        \      |        \      |        \      |
      .     |         .     |         .     |         .     |
      |     |         |     |         |     |         |     |
      |     |         |     |         |     |         |     |
      H     HW       DB8W  DB8       DB7W  DB7       DB6W  DB6

     ’ /’  ’ /’  ’ /’  ’ /’       ‘\ ‘  ‘\ ‘  ‘\ ‘  ‘\ ‘
      ’     ’     ’     ’           ‘     ‘     ‘     ‘
            S1 OFF                        S1 ON
</pre>

S1   Percussion On/Off switch; Mouser Part No. 633-M204201-RO
S2   Percussion 4, 5, 6 harmonic selector; Mouser Part No. 633-M202401-RO
H    AO28 percussion amplifier input terminal
HW   Green wire that was on terminal H
DB6  Drawbar 6 terminal (4th Harmonic)
DB6W Blue wire that was on Drawbar 6
DB7  Drawbar 7 terminal (5th Harmonic)
DB7W Violet wire that was on Drawbar 7
DB8  Drawbar 8 terminal (6th Harmonic)
DB8W Gray wire that was on Drawbar 8

INSTALLATION--Minimal original wiring need be disturbed for this installation. Use the schematic above and the following information to complete the wiring.
Four wires need to be unsoldered:

<pre>
  Blue wire on drawbar 6
  Violet wire on drawbar 7
  Gray wire on drawbar 8
  Green wire on terminal H on the AO28.
</pre>

Eight stranded hookup wires need to be soldered to the following:

<pre>
  3 Drawbars
  3 Drawbar wires
  Green wire that was on Terminal H.
  Terminal H on the AO-28
</pre>

The other end of the Eight wires should be routed to switches S1 and S2 that are mounted below the front rail of the organ.
The wiring of S1 is straight forward since it is a standard two position four pole double throw (4PDT) switch. These wires should be soldered to switch S1 as follows:

<pre>
         ARM   CONTACT
   S1-1   H      HW
   S1-2  DB8W    DB8
   S1-3  DB7W    DB7
   S1-4  DB6W    DB6
</pre>

Use the six point terminal diagram below for wiring S2. S2 is a three position single pole 3 throw (SP3T) switch. Note the jumper wire from terminal 3 to terminal 5. There is also a data sheet available at www.mouser.com that has terminal numbering and switch contact configuration.

<pre>
     To S1-4<==.6  2.==>To S1-1
               .5--3.
     To S1-3<==.4  1.==>To S1-2
</pre>

THEORY OF OPERATION
When switch S1 is OFF, the 4th, 5th and 6th harmonic signals are routed to the drawbars and only the 2nd and 3rd harmonics are available for percussion. This is identical to the original factory design.

When S1 is ON, the 4th, 5th and 6th harmonic signals are routed to switch S2 for selection. Also when is S1 ON, the percussion input signal to the AO28 is routed to the output of S2.

The position of S2 selects the 4th 5th and 6th percussion harmonic.

<pre>
   When S2 is DOWN the 4th harmonic is selected.
   When S2 is in the CENTER position the 5th harmonic is selected.
   When S2 is UP the 6th harmonic is selected.
</pre>

The B3 percussion switch must be on for the 4th, 5th and 6th harmonics to work. When the B3 percussion switch is off, switch S1 must also be off so that the 4th, 5th and 6th harmonic drawbar signals do not go to the input of the AO28 percussion amplifier.

—JohnMihevic