Marshall JVM410H guitar amp

 

The JVM410H is a four-channel, 100 watt guitar amplifier. The four 

channels are identified as Clean, Crunch, OD1 & OD2

 

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 byutton that turns 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. 

 

When the Clean channel mode switch is set to Green, the channel volume is removed from the circuit. If the mode is set to Orange or Red, the volume control is activated allowing for more overdrive type sounds. It kinda mentions this under the Volume knob (Orange & Red). I have to tell you, based on what I've heard, the clean and the crunch channels are almost all you need to get that classic Marshall rock sound.

 

The last mode you selected for a particular channel is remembered when you switch back and forth between channels.

 

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.

 

TECHNICAL SPECIFICATIONS

GENERAL
Model
JVM410H
Range
JVM
Technology
Valve (vacuum tubes)
Channels
4 (Multi)
ELECTRONICS
Output wattage
100W
Outputs
Speaker outputs: 5 x 1/4" jack sockets (16Ω load / 8Ω load / 4Ω load), Emulated Line Out (XLR), MIDI Thru
Inputs
1 x 1/4" jack instrument input, MIDI In, Footswitch
Controls
Channel and mode select (button), gain (per channel), treble (per channel), middle (per channel), bass (per channel), volume (per channel), reverb (per channel), presence, resonance, master volume select, master volume (switchable for each channel)
Effects
Digital reverb
Effects loop
2, 1 x series parallel, 1 x series
VALVES
Preamp valves
4 x ECC83, 1 x ECC83 (phase splitter)
Power amp valves
4 x EL34
ACCESSORIES
Footswitch
PEDL-91016 included
Cable
Detachable power cable included, speaker cable included
DIMENSIONS (INCLUDING FEET AND HANDLE)
Weight
22 kg / 49 lbs
Width
740 mm / 29.1"
Height
310 mm / 12.2"
Depth
215 mm / 8.5"

 

Electrical modifications

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 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 experience. 

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 amp

https://drive.google.com/file/d/10iXFmzy-2UvEtCm8WQeQ_1MT0qunkubF/view?usp=sharing

Also: a link to the user forum where I got most of my information from. There were also a few YouTube videos which helped.

https://www.marshallforum.com/threads/jvm-mods.101251/

 

 

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