Jeff,
In the June issue “Axes Bold As Love” article,
there are two photos of Jonny Lang’s Deluxe Reverbs. The caption states, “He
plugged into both the Normal and Vibrato
channels simultaneously for extra thickness
and texture.” Indeed, the photos show four
guitar cords, connected to the “1” inputs
of both channels on both amps.
However, the common wisdom concerning
Fender amps with vibrato is that jumpering
the channels doesn’t work. Unlike a four-input
Marshall or a Bassman, a Fender amp
equipped with vibrato has an extra triode
amplification stage in the vibrato channel—
the normal channel has two stages, but
the vibrato channel has three. Since each
stage inverts the signal, the signals from the
two channels are out-of-phase when they
arrive at the phase inverter. Combining out-of-phase signals like this should result in a
thinned-out, weaker sound—pretty much the
opposite of “extra thickness and texture.”
So what gives? Is there something special
about Lang’s amps? Is he perhaps using
some sort of external splitter that provides
an intentional phase reversal for two of the
four channels? Or is Jonny just a weirdo who
happens to like a thin, nasal guitar sound?
Rich Stevens
Potsdam, New York
Hi Rich,
Nice catch and great question. While I don’t
believe that Jonny is a big fan of “thin, nasal
guitar,” you are correct in why this shouldn’t
work. (Nice technical explanation too. Boy, I’d
better watch my job!) First, I’ll tell you exactly
what’s going on in Jonny’s rig and then give
you a couple of cool alternatives.
In the picture of Jonny’s rig that you’re referring
to, you do see cables connected to input
1 of both amp channels, but he only uses
one channel at a time. Not shown are the
other ends of those same cables connected
to an A/B switcher. If you look closely, you’ll
see that the channels are set differently. He
uses the amp’s Vibrato channel for his rhythm
sound and switches to the Normal channel
for his solo tones. Using the amp this way,
the phase of one channel is not affected by
the phase of the other because the signal is
not passing through both channels simultaneously.
Mystery solved. Now let’s get to a couple
of cool alternatives that will indeed allow
you to use both channels simultaneously.
The first method is a simple modification
that will place both channels in phase with
each other, as well as enable both channels
to have reverb and tremolo. Although I’m
sure there are a few variations on this type
of modification, this is a very simple version.
I’ve done this on many original black and
silverface models over the years. Doing this
modification on the current reissue models
may be a bit more challenging due to the
use of circuit boards, but it’s nothing that a
good, experienced tech shouldn’t be able to
handle. And we all know that a good, experienced
tech should always be the person
executing any amplifier modification because
of the risk of life-threatening injury! Fender
would also like to remind readers that modifying
an amp’s circuitry will void the warranty,
and that repairs should only be performed by
an authorized technician.
Locate the capacitor that is the output of the
second gain stage in the Normal channel.
This would normally be a .047uf capacitor
and on the schematic of the reissue Deluxe
Reverb (available for download in the online
version of this article atpremierguitar.com), it
is noted as C5. In the original black and silver
versions of the amp, this cap was connected
to a 220k resistor via a long wire. Remove the
wire connection from the 220k resistor. Now
locate the capacitor that is the output of the
second gain stage in the Vibrato channel—
typically a .022uf capacitor. On the reissue’s
schematic it is listed as C12. This capacitor is
connected to the junction of a 10pf capacitor
and a 3.9Mohm resistor, as well as a 500pf
capacitor (C13, R25, and C14, respectively).
Connect the wire from the output of the
Normal channel capacitor to this junction.
That’s it! You have now sent the output of the
Normal channel to the input of the reverb
and tremolo circuits along with the Vibrato
channel. Of course, all modifications to an
amplifier will change some aspect of it, and
this is no exception. This one may change
the gain structure and feel of the amp a bit,
but I’m sure it’s nothing you couldn’t quickly
become comfortable with. Once this is done,
you will be able to run the guitar into both
channels simultaneously, each with different
EQ settings. This will also give you the option
of using two different guitars, one connected
to each channel. This will enable each guitar
to have reverb and tremolo, as well as having
its own channel volume and tone controls.
Another option is to use an external device
that would supply you with two output signals,
one of which would be 180 degrees out-of-phase with the other. This would allow you
to simultaneously use both channels of any
black or silverface amp you might run into.
One such device is the Barber Electronics
Launch Pad. This device not only gives you
two phase-reversible outputs, but also a
variable 20 dB of gain for when you need to
really push those amps over the top.
There you have your two channels demystified
and reenergized!
Jeff Bober
Jeff Bober, one of the godfathers of the low-wattage amp
revolution, co-founded and was the principal designer
for Budda Amplification. Jeff has just launched EAST
Amplification. He can be reached atpgampman@gmail.com.