TR-606 DIY part 1: SUB-SONIC DRUMS
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The first modification I made was to change capacitors 19 and 20 (see fig.
1, bd1.GIF) from 56 nF to 68 nF.  This dramatically lowered the pitch of
the drums and made them sound more sub-sonic.  I also thought the decay of
the drum was far too short, so I used a bit of 808 technology and added an
extra filter network into the drumoscillator.  This made the drum oscillate
all the time and totally distorted the output, so I added a variable
resistance to the new capacitor to ground. This gave me total control of
the decay time.  By doing this you can quite easily get decay times of up
to five seconds.  It will boom even longer and deeper than an 808.

I was really pleased with these results, but felt that a pitch control was
lacking, so this was next on the list.  Resistor 58 (33K) controls the
pitch of the bassdrum; decreasing this resistance makes the pitch go up,
so adding the pitch control was fairly easy.  I added a 47K resistor and a
100K pot to the existing circuit (see fig 2, bd2.GIF); this gives just
over an octave of shift.

If you like, you can also add a BC108 transistor and a 100K pot to allow
external CV control from a sequencer or envelope generator.  The sequencer
or envelope generator can also be triggered from the 606's trigger outputs
on the toms, which will give you instant drum and booming bass lines.
[the CV will only respond to 0-5V - ed]