Like a fine wine, some musical instruments get better with age. Sometimes, due to age, neglect and abuse, older guitars needs some work and TLC to be returned to playable condition. This blog documents some of the work I have performed on musical instruments for myself and others over the years.
I make my living off the evening news
Just give me something
Something I can use
People love it when you lose
They love dirty laundry
Playing with the mosfet a few nights ago was fun, and I'm sure I could squeeze a little
more clean gain out of it by a few value tweaks. I still might come
back to it for my Jamtastik! pedal, but something reminded me of a few two-stage jfet preamps I've been meaning to try. I came across one awhile back in an Instructables post:
As much as I like the relative warmth of the Fetzer Valve, it pales in
comparison to the LPB and TL071 in terms of gain. For my Jamtastik!, it
really just needs some more power. Here's the Fetzer Valve Revisited as built for my Jamtastik! pedal.
So, with two stages, the question
becomes how much clean gain can I get from it with a 9V power supply?
Taking the Instructables circuit and applying some of what I see in the
Fetzer Valve Revisited, I swapped out the Rd=2k2 for 50k trim pots.
Lacking 750R and having the wrong style of 2k trim pots for
breadboarding, I went with Rs=820R for starters. Since I'm plugging in a
guitar and bass, I also reduced the input and output capacitors to
0.1uF. And, wanting to have some level control, I added a A100k voltage
divider on the output. After biasing both engines at idle to 4.5V, we
have a simple, not totally clean, super low-noise gain monster. I give
you, Pinch of Dirt.
But, and this is a most important but, it cleans up very nicely by
turning back the guitar volume and still has plenty of gain! So, the
next step is to add a minimal shunt resistor plus some attenuation on
the front with an inline resistor, in front of the first input
capacitor, a la the Fetzer
Valve revisited. After I find the sweet spot for that inline resistor,
I'll play around with values of Rs to find the limit of how much clean
gain and dirt I can get out of this circuit.
We both know
What could have been
On Sunset strip
Our broken dream
So, this actually has little to do with Robbie Williams and
bears little resemblence to Jack Gantos or The Trouble in Me. But, my first foray into the world of mosfet devices was a success. Due to
availability of parts on hand, I ended up with a cross between
the AMZ and Blue Dragon Mosfet Boosters:
my homebrew Mosfet Booster has a 2N7000 mosfet, 2.2K fixed for Rd and Rs, a 10M3 fixed biasing the Gate, a 0.1u output
cap, and 2/3 of a 2N2222 acting as a protection diode. I left off
the tone control and simply put a 100k fixed across the output to ground. The lack of a
resistor across input and ground was noticeable when plugging and
unplugging, so I'll put something hefty in for controlling the pop next time. About
5V was the closest I could get to the low end of Jack's suggested bias
voltage of 4.5-5.5V at point A=Drain, and Max Greco did not offer any
suggestions here other than to leave it to the user to play with the
100k for biasing the beast. The circuit seemed to sound cleaner when I
pushed the voltage at A up to about 8V. That put Vref at about
2.4V, and it was too late in the day to play loudly enough to tell how clean it really was. I
was using a B5k instead of A5k for a gain pot, and found the 4 o'clock
position, about 0.7k, to be the limit before the
clean boost started slipping into a somewhat understated but tasty overdrive. This
calls for more experimentation at higher volumes to find reasonable Vref and bias voltage levels, and try it along with a master volume plus tone control with a clean recovery stage.