PIGNOSE G40V AMP MODS BY JEFF BOBER
In last month’s column [“Dusting Off a Pignose
G40V,” June 2011], we began
digging into a Pignose G40V—
an all-tube, 40-watt combo.
The owner hadn’t used the amp
in five years, so I outlined what
he should do to make it operable and reliable. In this installment, I’ll address his question
about adding a line out jack
and suggest a few other possible mods. To view and print
the G40V schematic, go to the
online edition of this column at
premierguitar.com.
If you own an amp that
doesn’t offer a line out for direct
recording or sending a signal to
a house mixing board, I believe
that instead of modding the
amp, you’d achieve much better
tonal results by using a Red Box
or some similar unit that you
patch between the amp output
and the speaker. Because these
external devices have speaker-simulation circuits, they sound
much better than any basic line
out. And because the G40V
actually has a 1/4" output
from the chassis to the internal
speaker, it would be very easy to
connect an external box.
However, if you wish to
use a line out simply to send a
signal to the power section of
another larger head or power
amp, then a very basic line out
will do the job.
Warning: Working inside an
amp is very dangerous—it can
even be lethal. So if you are not
familiar with the inner workings
of a tube amplifier and the possible
hazards involved, please have this
work performed by someone who is.
Mount a 1/4" jack on the rear
of the amp close enough to the
speaker output jack that you’ll be
able to install a resistor between
the two jacks. Connect a 1.5k
resistor to the tip connection
of the speaker jack. Orient the
jacks so that the other end of the
resistor will connect to the tip
connection of the new line-out
jack. Before soldering this end
to the line-out jack, install a 100
Ω resistor in parallel with a 0.1
µF capacitor between the tip and
ground connections of the line-
out jack. Now what you should
have is the signal coming from
the speaker output jack through
a 1.5k resistor feeding the 100
Ω and 0.1 µF parallel network
across the line-out jack. The two
resistors serve as a voltage divider
to reduce the signal level, and the
capacitor serves to slightly attenu-
ate some of the high-end content.
Without it, this signal can sound
pretty harsh. Feel free to experi-
ment with different cap values to
find something that matches your
taste. Larger values will attenuate
more high end and vice versa.
The Hughes & Kettner Red Box Classic turns any amp’s speaker output
into a frequency-compensated line out.
with humbucker-equipped guitars, you’ll need to reduce the
amplification of the lower frequencies in the first stage. To do
this, locate the 220 µF capacitor
(C8) in parallel with the 2.7k
resistor (R11) in the cathode of
the first gain stage. Replace this
cap with a substantially smaller
value. I’d suggest values as small
as 1 µF or 0.68 µF.
Now, if you need to bring
a bit more fullness back into
the amp, we can visit the next
amplification stage. Increasing
the value of the .047 µF cap
(C9) in the cathode of this gain
stage will cause it to amplify
more low frequencies. Even
increasing it to a value of 0.68
µF should make a significant
difference. But hey, if you’d like
to be a bit more daring and raise
the full range gain level of this
stage, try decreasing the value of
the cathode resistor. Experiment
with a 4.7k or possibly a 2.7k
and see if you like the results.
Now, let’s move over to the
tone stack. In looking at the
schematic, I’d guess that the
amp’s Treble control affects a
bit more of the frequency range
than it should. This would be
due to the exceptionally large
value of the treble capacitor
in the tone stack. A 0.001 µF
(1000 pF) capacitor (C12) here
can make an amp sound a bit
nasal. Try replacing this cap
with a 470 pF, a typical value
that works well in higher-gain
amps. This should yield a bet-
ter-sounding tone stack.
JEFF BOBER, one of
the godfathers of the
low-wattage amp revolution, co-founded and was
the principal designer
for Budda Amplification.
Jeff launched EAST
Amplification in 2010, and he can be
reached at pgampman@gmail.com.
