MY GUITAR PLAYER HAS BEEN JUMPERING THE INPUTS OF HIS
blackface Fender Super Reverb by plugging his guitar into the Vibrato
channel’s first input and then putting a patch cord from the second
input to the Normal channel’s first input. Then to compound the
situation, he runs a patch cord from the Normal channel’s second
input to a tuner. Does all this cause any damage or reduction in
I don’t think he will do any damage to his amplifier, but I doubt
he’ll be winning any awards for thick tone either. To understand
why, we must have a short discussion about phase relationships.
A tube amplifier such as a Super Reverb has a preamp
consisting of mostly 12AX7 preamp tubes. Depending on the
number of gain stages found in each channel of the preamp section,
the preamp’s output signal will either be in phase or outof-
phase with the input. On a preamp with an even number of
gain stages—such as the Normal channel of a Super Reverb—
when the input receives a positive signal voltage, the amplified
output is also a positive voltage. Conversely, on that same channel,
when a negative signal voltage is applied to the input, the
preamp’s output is a negative voltage—again much larger than
the input, but still negative.
However, the Vibrato channel of the Super Reverb (as well as
many other Fender amplifiers) has an extra gain stage that is used
to “mix” the reverb. So with three stages of gain, when a positive
signal is applied to the input of the channel, a negative amplified
voltage comes out of the preamp. And when a negative signal
voltage is applied, a positive output voltage occurs. Now, let’s
say a positive signal voltage is sent to both channels simultaneously.
The Normal channel’s preamp section puts out a positive
amplified voltage while the Vibrato channel’s preamp section
puts out a negative voltage. These two signal voltages are opposite
and will cancel each other when both signals arrive in the
power section of the amplifier. You will hear this as a loss of
volume, and especially a loss of bottom-end. And when you vary
the two volume controls, the phase of one channel will overwhelm
the other channel, creating a thin, out-of-phase sound.
Some amps, such as the earlier Marshalls, have the same number
of gain stages in each channel and can be jumpered for an
improvement in tone. Just remember, though, that when you
jumper the inputs, the signal actually driving the amplifier is cut
in half, resulting in slightly less gain per channel.
Phase is very important also when hooking up multiple amplifiers.
When using two or more amplifiers, care must be taken
so that all of the speakers are moving the same way at the same
time. An easy test is to listen the sound of two amplifiers side
by side. Then turn one of the amplifiers around facing backwards,
but still side by side. Play and listen again to determine
which way sounds better. If it turns out that the sound improves
with one amp facing backwards, you can change the phase in
one of the amps (it doesn’t matter which one) by reversing the
leads that connect to the speakers. This will ensure that you’re
getting the fullest sound when you face both amps forward
onstage. —Gerald Weber, Kendrick Amplifiers
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