A verified layout by Sebastian, thanks mate!
In his own words...
I forgot about this layout I did ages ago, built up (it works), and
never got around to boxing up: a Mesa Boogie Mk.1 preamp. I'm
personally partial to pre-gain tone stacks (e.g. '60s Fenders, Orange
Graphic MkII), so I was disappointed with the Runoffgroove Uno having
its gain control before the tone stack instead of after. So I knocked
together the schematic below, which is basically a straight JFET
emulation of the actual Mk.1 preamp if you plugged into Input 1 (i.e.
the input with the extra gain stage). This gives the user both pre- and post-tone
stack gain controls. I added a Marshall-style "treble peaker" bright
cap between the input and 2nd gain stages, just for some extra clang,
which can be engaged or disengaged with the "TP" switch. After the
master volume control, there's a mu-amp output stage to mimic the
push-pull amplification of the Boogie's phase inverter.
I
used BF245A JFETs just because that's what I have the most of, but I
tested it with PN4393s and J201s, and it worked just as well. The gain
boost can be either a toggle or stomp switch; as the original boost was
operated by footswitch, I opted for that in my build.
This
thing's pretty fun. Like actual Boogies, it takes a bit to dial in a
great tone—it's absolutely packed with bad tones, LOL—but it can do
everything from crystaline Fender cleans to Weezer "blue album" growl to
thrash grind. I hope other folks enjoy it!
Any demos of it? Very interested but I also remember making a Dr. Boogie and it just not being particularly versatile as a JFET based pedal.
ReplyDeleteOh, and if you have any particular voltages those trimmers are meant to achieve it would be useful to know (or are they just get it by ear)?
ReplyDeleteNo demos, I'm afraid; life these days barely affords me the time to build, let alone demo, pedals. Rest assured it does work, and it does have a wide range of tones. It's definitely not only for high gain.
ReplyDeleteAs for voltages, the usual 4.5V on the drain does the trick, though you can bump it up to 5.5V for more headroom. I also tried running the circuit at 12 and 18 volts, and it still worked just fine.