Marshall JVM410H guitar amp
The JVM410H is a four-channel, 100 watt guitar amplifier. The four channels are identified as Clean, Crunch, OD1 & OD2. Each channel has 3 switchable "modes" which provide additional tone-stacking/gain-staging options.
Front panel close-up: Clean/Crunch channels and input jack
To the right of each channel is a "MODE" button. Each channel has
three switchable voices or "modes" which provide additional
tone/gain-staging options. You select a channel by pressing the mode button once. Then you can cycle
through the 3 different modes by repeatedly pressing the mode button. There's
a small LED on the front of each button that turns from Green to Orange to Red and
back again. Green is the cleanest setting. Orange and Red add additional gain staging or "heat", resulting in more overdrive/distortion sounds.
The mode you select for a particular channel is remembered when you switch to a different channel and then switch back.
Front panel close-up: OD1/OD2 channels
Front panel close-up: Master/Reverb section
I have a Marshall footswitch which can be used to control/program the amp for session work if needed.
Rear Panel - speaker outs, FX Loop, NFB knob
The NFB knob is part of the electrical modification that was done to the amp. I'll comment more on that in the following "Mod" section.
Rear Panel - Preamp and power amp I/O's, Direct XLR
(balanced) line output
This line out incorporates a cabinet emulation circuit which gives the signal the tonality of a miked speaker cabinet.
Rear Panel - Footswitch, MIDI I/O's and mains input.
Amp serial number
Speaker cabinet - inside
I acquired this amplifier, and the matching 4x12 cabinet, in 2020 as an investment for the studio business. The amp was built (and purchased by the original owner) in 2007.
The amp was completely stock. It even had the original Marshall tubes. I decided, after doing some recording with it, to have it modified. Details regarding the electrical modification work are presented after the technical specification section below.
This section details the various electrical modifiations
that were done to the amp. After doing some recording and listening, I
decided I wasn't completely happy with the way the amp sounded. So I
looked into having the amp modified and serviced.
I spent some time on the web searching for any
information related to Marshall amp modification and found plenty related
specifically to the 410H. There were 8-10 (or more) detailed
"mods" including step by step instructions on how to install the
1) Choke Mod
This modification was designed to reduce the noise in
the amp and improve the bass response. It involved the purchase of a
Mercury Magnetics 10 henry choke (p/n MC10H). A choke is an inductor used
to block high frequencies and pass low frequencies (hence the name
"choke"). Or something like that. The choke is mounted to the top of the amp chassis. I
had to drill 3 holes, two to mount the choke and the 3rd for the wires
from the choke which pass through the chassis and attach directly to the
main printed circuit board. The tube sockets for those tubes are attached
directly to that 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. The tubes were biased to 70mV.
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 passed through the chassis and soldered directly to the circuit
trace. This was the final step for the Choke mod.
2) Plexi cap mod
This modification was to reduce the amount of hi-gain
squeeling from the OD channels. A .68uf capacitor soldered in parallel
with resistor R97
Note: that cap was eventually removed and replaced with a smaller component as shown in the following pic (that little blue box above R98)
Negative Feedback mod
This modification involves replacing resistor R58 with a
different value. Which I did (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". That sounded cool. Ok I'll do that
lol. 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:
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/On = 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.
Finally... Kevin made a number of other modifications, mostly involving resistor and capacitor changes meant to balance out the preamp circuits, replacing aging parts, sonically better parts...
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 learning
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