The Hughes & Kettner Red Box Classic turns any amp’s speaker output
into a frequency-compensated line out. |
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.
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.
Click here to download a PDF of the schematic.
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 attenuate
some of the high-end content.
Without it, this signal can sound
pretty harsh. Feel free to experiment
with different cap values to
find something that matches your
taste. Larger values will attenuate
more high end and vice versa.
Next, looking at the schematic,
I noticed a few other
areas where you could personalize
the amp with some simple
mods. This is the fun part. I
should note that these amps
sport typical low-cost printed
circuit boards, so you should
be a bit careful when working
on them. Since you’ll probably
be turning the board over more
than a few times, make some
quick notes of where all the
wires are attached, in case one
decides to disconnect itself!
First, if you’d like to give
this little piggy a bit more gain,
locate the 510k resistor (R3)
that attaches to the CW side
of the volume pot. This resistor,
along with the volume pot,
forms a voltage divider, and as
such prevents access to half of
the signal level. You can either
remove this resistor and replace
it with a solid lead, or simply
solder a short jumper wire across
it. (The latter makes it easier to
reverse this change.) This will
allow access to the full output
level of stage one, enabling more
saturation of stage two.
If this turns out to sound too
mushy on the low end, especially
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 better-
sounding tone stack.
Last, let’s address the Presence
control. In parallel with the
Presence control is a 4.7k resistor.
I’d suggest removing this from
the circuit. Since the Presence
pot is connected directly to
ground on one side, this resistor
is not needed as a DC path to
ground, and removing it should
noticeably increase the Presence
control’s range. There is a better
method for wiring this presence
circuit that would require this
resistor, but as always, I attempt
to keep these modifications as
basic as possible without altering
the circuit board itself.
Now you’re ready to enjoy
your rip-snortin’ little piggy.
In closing, thanks to all the
readers who sent me links to
the Bassman 50 schematic I
was searching for [
“Rockin’ a Bassman 50,” May 2011]. I
appreciate it!
Jeff Bober is one of
the godfathers of the
low-wattage amp revolution,
co-founded and was
the principal designer for
Budda Amplification. Jeff recently launched EAST
Amplification, and he can be reached at
pgampman@gmail.com.