Greetings, amp fanatics, and
thank you for reading my
column and submitting your
questions and comments. While
I typically pick one of your
questions to discuss here, this
month I’m breaking with tradition.
I recently had an unusual
combo come across my bench,
and I felt it would make a great
topic for a column. The amp—a
Sound X-305-R—belongs to
a friend of mine named John
Ingram, and it’s a model I had
not previously seen.
Since the amp was already in
good working order, though a
bit lackluster in the performance
category, John brought it in for
some basic service and also asked
me to “turn it into something
cool.” That sounded like a good
idea, so that’s exactly what I did.
In case you ever come across
your own Sound amp, I’ll explain
these simple but effective modifications.
But first, let’s investigate
the history of Sound amps.
While doing research for
this column, I read a comment
regarding Sound amps posted
by Andy Fuchs of Fuchs Audio
Technology. So I contacted
Andy and asked him what he
knew about these rare birds.
“I know that they were made
on Long Island—Mineola to be
exact—by John Daugherty,” Fuchs
told me. “Daugherty may have
worked for Ampeg and/or Oliver
[Amps]. I don’t know how long
Sound was in business, but I know
Daugherty ended up working at
Marlboro Sound Works—along
with Ed Finger, a former Ampeg
sales person—selling solid-state
amps of various sizes and shapes.
Musical Instrument Corporation
of America owned Marlboro,
which eventually folded. I’ve seen
and heard some Sound amps that
actually sounded pretty good.
Every one I ever saw was covered
with gray vinyl and had aluminum
accents on the box. I don’t
know much else about them.”
Well, as you can see in the
photo, this amp is indeed covered
in gray vinyl with a brushed-aluminum
faceplate, aluminum
knobs, and even narrow aluminum
rests on each side of the
cabinet in lieu of feet. The combo
sports a single Jensen C15N with
a manufacturing code of 220629.
The reverb-tank date code of
6649 and the mid- to late-’66 date
codes on the pots suggest the amp
was manufactured in late 1966.
The X-305-R has two channels,
each with two inputs and
Volume, Treble, and Bass controls.
There is also a Reverberation control
to select reverb for Channel
1, Channel 2, or both, along with
Depth and Rate controls for the
tremolo. The tremolo is a bias-modulated
circuit, which means
it affects the output stage. Because
the amp only has one output section,
the tremolo works for both
channels. Other front-panel controls
include rotary switches for
Power and Polarity (ground).
The amp’s tube complement
consists of four 12AX7s, one
12AU7, and two 7868 output
tubes, the latter of which appeared
in some ’60s Ampegs. Inside, the
similarity to ’60s Ampegs was so
strong that I wondered if Ampeg
produced Sound amps as an in-house
brand, perhaps for a chain
of music stores. I couldn’t confirm
this, so let’s stick with the “former
Ampeg employee” story.
Testing the amp, I found it
a bit on the weak side, with no
appreciable gain or overdrive, and
both channels sounded identical.
The tremolo was decent, but even
at the slowest setting it was a bit
fast. Here, you see the capacitors
that form the tone stack for
Channels 1 and 2.
The two blue caps, 100k
resistor, and tan ceramic cap to
the right of the blue wire are the
components for Channel 1’s tone
stack. These consist of a 0.1 μF,
a 0.039 μF, and a 330 pF capacitor—
an atypical set of values that
definitely gives the amp its own
sonic character. The components
to the left of the blue wire are for
Channel 2’s tone stack, which I
decided to replace with more traditional
“British” values. I changed
both capacitors to 0.022 μF and
the ceramic cap to 470 pF, which
yielded two different-sounding
channels. Using an A/B footswitch,
John could have channel
switching on the cheap!
This next photo shows a 1.5k
resistor attached to a cathode of
V4. This resistor was originally
not bypassed (that is, it had no
capacitor in parallel with it), so
I bypassed it with the 2.2 μF
25V capacitor shown here. This
greatly increased the gain in that
part of the circuit, which allowed
the amp to develop substantially
more overdrive.
This last photo shows a pair
of brown 0.047 μF capacitors
paralleled across an existing pair
of blue 0.047 μF caps. This is the
tremolo’s oscillator circuit. As previously
mentioned, even with the
Speed control fully counterclockwise,
the speed was originally far
too fast to be practical. Increasing
the capacitance by paralleling these
caps slowed the oscillator enough
to provide a nice, swampy feel.
Because this is a bias-modulated
tremolo, the louder and
harder you play, the less the effect
is apparent. But you can definitely
use this to your advantage. For
instance, you can play deeply
tremolo’d chords and then crush
into a solo with almost no trace of
tremolo, only to have the tremolo
fade back in as the notes decay or
as you reduce your Volume control.
Very cool.
So there you have it. More
sounds from the Sound!
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.