Professor
Tweed
The Fender Princeton adapted for use as a distortion
stompbox
by runoffgroove.com

This project is rated 2 out of 3 for the level of
complexity.

One of the often overlooked Fender amps is the tweed-era Princeton. They are
excellent guitar amps, especially for blues playing. The major drawback was
the 4.5 watt output wasn't enough to keep up with a band. Again, using the
technique pioneered by Doug
Hammond, we decided to try to capture some of the sound of this little amp
in pedal form so it can be used with a band.

The circuit we chose is the 5F2-A Princeton, one of the so-called "narrow
panel" amps produced from 1955 to 1960. It was a very simple amp, using one
12AX7, one 6V6 power tube and a 5Y3 rectifier. Since we used 9V DC power, we
left out the 5Y3. We used JFETs to replace the two halves of the 12AX7 and the
6V6 power stage.

We took the 5F2-A schematic and copied it part for part using MPF102 JFETs in
place of each tube stage. Each tube Grid was replaced by a JFETs Gate. The tube
Plates were analogous to a JFET Drain. Finally, a tube Cathode was replaced
with a JFETs Source. We used a 100k trimmer for the plate resistors on the
schematic due the fickle nature of JFETs and the much lower power supply
involved.

Our "artistic license" is in the choice of the last JFETs output cap and the
setting of the dual Low Pass Filters. Our goal was to produce the sound of a
Jensen speaker. These are typically a little broader in frequency response than
something like a Celestion.

Using all MPF102 JFETs, there is a great range of sounds available. No, you
won't be playing "South of Heaven" with this pedal, but some classic blues,
country, surf and rock sounds live here. The cleaner sounds are compressed and
have an edge to them. As you wind up the Volume knob, you'll hear the dirt
level increase. At full Volume, there is plenty of nice overdrive that never
loses its dynamic feel. Even at full tilt, the circuit can clean up very
well.

Possible mods:

Try using a J201 in the first stage. You'll notice the
overall available gain increase and the sound will darken a bit. We preferred
the sparkle of all MPF102s, but if you use sockets (as is always suggested) for
the transistors, you can experiment until you find the combination you like
best.

Socket the cap in the feedback loop. It is the 1uF cap connected to the
output cap on one end and the 22k feedback resistor on the other. In this
socket, try any value from 2n2 to 1uF. The sound will "open up" most around
10n and will "close up" as you get closer to 1uF. Try different values here to
find the sound you like most.

Another possible area for modding would be the Tone control, specifically
using different values for the 4n7 and 470p caps. The Tone control works well
in its stock form, but you may find something interesting by substituting
different values here.

Listen
to it!

Here are pin voltages from the official runoffgroove.com build
to provide a baseline (supply voltage from a 9.1v battery):

Q1 MPF102
(J201)

Q2
MPF102

Q3
MPF102

D

4.60 (4.60)

D

4.56

D

4.61

S

1.14 (368mV)

S

1.16

S

.891

zero voltage on
all gates

Q1
2N5457

Q2
2N5457

Q3
2N5457

D

4.55

D

4.63

D

4.61

S

1.055

S

1.089

S

.907

zero voltage on
all gates

Pablo De Luca (aka Gringo) contributed a PCB
layout for Professor Tweed (PDF, 156k)

Professor
Tweed by runoffgroove.com is licensed under
a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported
License.
Permissions beyond the scope of this license may be
requested.

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