Overdrive unit
punch
If a guitar amp was a body, the overdrive would be the brain. It's the most important bit. Even 'clean'
channels have to incorporate a bit of overdrive, because the dynamic range of the guitar is far higher
than, say, the power amp. If you insisted on playing with no distortion at all you would need a massive
amp to be heard in a band. So it's just as well that with valves, overdrive is almost impossible to
do wrong. Almost any combination of valves will give you a great distorted sound, as long as you follow
these guidelines...
- Small inter-stage coupling capacitors (1n to 4.7n)
- Small cathode bypass caps (0.1uF) or none at all
- Some sort of low-pass filter to remove the highest harmonics.
crunch
The overdrive section has now been updated, and there are now three types of distortion (I still wouldn't
call it a three channel amp mind you) 'Super clean' uses a soft limiter to clip the loudest bits off
clean guitar playing without you really noticing. This has to be done, because clipping the op-amps in
the EQ stage sounds unpleasant and could be bad for them. If you really overdrive this channel
(with the input gain control in the pre-eq section) you get
an interesting sort of mellow fuzz tone. The overdrive and buzz knobs have no effect.
The next setting up runs the signal through one half of the 12AX7. This was the old clean channel before
I fitted the input gain control. With my guitar, it sounds good clean with the input gain at minimum.
The overdrive and buzz knobs still do nothing.
On the next setting it goes through both parts of the 12AX7. Now you get lots of distortion, adjustable
with the overdrive knob. The input
gain and overdrive now work like the 'volume 1' and 'volume 2' on some of Boogie and Marshall's machines.
I find I get all the distortion I want with the input gain at minimum and the overdrive at three o'
clock. Turn both up full and we are doing our Peavey 5150 impression, not sure if it would be 'brown'
enough for Sir Edward though.
The 'buzz' control now comes into action too, and can be used to improve the brownness by removing
treble. I usually put it about halfway.
buzz
When playing with the overdrive controls be sure to fiddle with the pre EQ too. Adjusting the bass
changes the fatness of the distortion. Too little bass and it will sound thin, too much and it will
be farty. The right amount depends on your guitar and pickups. The effect of the treble control is quite
hard to describe, so you'll have to build the preamp and try it out. Yes, it does have an effect!
SCHEMATIC
e-mail me: "steve at scopeboy dot com"