This section details the various
electrical modifiations that were done to the amp. These mods were
added to improve some of the tonal characteristics of the sound.
These were the 4 main things I wanted to address:
1) Low end response was weak overall. Was hoping to have the low
frequencies improved upon. More bass.
2) Noise in OD channels was a bit excessive. Very hissy and there
was a tendancy for the amp to start squeeling uncontrollably when
driven too hard
3) Negative Feedback mod - this modification was designed to improve
or "tailor" the sound of the amp. I'll explain what that's
all about in the following section
4) Replace the original Marshall tubes. They were 13-14 years old,
figured it was worth the investment to replace them. I used J&J
Electronics tubes. Output tube circuits were bias to 70mv
The following modifications were done by local amp guru Kevin
Bourke. I'm using terminology that was common practice in the
on-line user forums to describe each modification.
1) Choke Mod
This modification was designed to
reduce noise in the amp and improve the bass response. It involved
the installation of a Mercury Magnetics 10 henry choke (p/n MC10H).
A choke is an inductor used to block high frequencies and pass low
frequencies. The choke is mounted to the top of the amp chassis.
Three holes had to be drilled in the chassis: two to mount the choke
and the 3rd for the wires which pass through the chassis and attach
directly to the main printed circuit board.
As long as we're here, I'll point out
the tubes. I replaced the original Marshall tubes with brand new
J&J Electronics tubes. Those are EL34's and 12AX7's. The EL34's
were ordered as a matched quad set.
Getting back to the choke mod, this
picture shows resistor R106 (that big white thing). That resistor is
removed from the printed circuit board and the wires from the choke
are attached in it's place.
|Here you can see
R106 removed. The two wires from the choke are soldered
directly to the circuit traces.
2) Plexi cap mod
This modification reduces the amount
of hi-gain squeeling from the OD channels. A .68uf/63V capacitor
soldered in parallel with resistor R97 (that little blue
boxy-looking component soldered above R98)
Negative Feedback mod
This modification involves replacing
resistor R58 with a different value (second resistor from the
bottom). The purpose of the modification was explained this way:
"Changes the way the power supply delivers power and
provides more traditional power tube breakup at lower volumes".
After I heard the final mod, I understood the meaining of that
description. Consider it a another tone shaping option. That's the
easier way to describe it.
The general idea here was to chose
one of the following values which were meant to emulate a certain
type of Marshall amp sound:
47k ohm - late 60's Plexi
74k - early 70's MkII Plexi
82K - stock JVM (the original stock component value)
137k - EVH's Plexi
176k - JCM800 2203
177K or higher - a Marshall with a Vox amp's lack of headroom
Which brings us back to that mysterious NFB knob on the rear panel.
What Kevin Bourke did, instead of replacing resistor R58 with a
specific value, was to install a variable potentiometer, mounted on
the back panel of the chassis. Now you can sweep though a whole
range of impedance values. It seems almost like another volume
control except as the sound gets louder, it's also changing the
tonality of the amp in special ways.
Here's an interior shot showing the
NFB potentiometer mounted to the back plate of the chassis.
Cathode Bypass Capacitor
On the outside front panel, just
below the Crunch Mode button, there is a simple toggle switch which
allows you to switch in and out capacitor C83. (I = capacitor
engaged, O = cap disengaged)
From the inside, you can see a
twisted pair of blue wires which are attached to that front panel
switch. C83 is removed from the printed circuit board and attached
to the back of the switch. The switch is then wired to where C83
was. The switch simply provides an in-circuit or out-of-circuit
option. This mod makes a huge difference in the output tonality,
especially in the crunchy and hi-gain channels. Bright on one side,
little darker on the other side. A cool tone shaping option.
There were a number of other
modifications, mostly involving resistor and capacitor changes meant
to balance out the preamp circuits, replacing aging parts, sonically
What I walked away with was an amp
that far exceeded my expectations. It went from being ok to great,
literally overnight. A really good recording amp. I had a couple
friends (Marshall fanatics) come in and take it for a test drive.
Glowing reviews! That was good enough for me. This was a cool
Closing: sorry I didn't get a "before" recording done.
The amp sounded good. I wouldn't have bought it if it didn't. But
the issues I mentioned at the top got resolved and the
"extras" were just icing on the cake. This is a link to a
video I produced here demonstrating the sonic capabilities of the
Also: a link to the user forum where I got most of my information
from. There were also a few YouTube videos which helped.