Marshall Amplifier Customization

Marshall Overdrive Mods

I design Marshall mods to bring out the the best that the amplifier can deliver. I have discovered ways to make a 50 watt head head sound as big as a 100 watt. I retain clarity and punch even at high gain levels by carefully tailoring each stage and preventing mushy bottom end that fuzzes out.

My drive channels of my Soloist mods are considerably hotter than the stack JCM800 circuit and in recent years I have cut back the gain and offer foot-switchable drive and fat boosts that you can kick in when needed. That way there is no need to compromise your sound by reducing the gain. Just kick off the boost and get that perfect, just over-the-edge AC-DC sound for rhythm. Kick in the boost for solos and get the right amount of drive with a slight volume boost to bring you up front. Perfect, no need to mess with hair trigger master volumes that get too loud too fast, or with lead channels that sound too different from your main sound.

Using my Drive channel along with the Fat-Boost and Drive-Boost foot-switches you get four different drive settings without having to readjust a single control. Chance are that's plenty of gain but if you're serious about your soloing then my lead Channel is for you.

The lead channel kicks in an additional 12ax7 gain stage with separate gain control and Master volume. Used with the Fat-Boost and Drive-Boost switches it adds four more high gain modes to the rhythm channel. That 8 levels of drive controllable by four foot-switches plus a clean channel.

Ok, sorry, I didn't mean to scare you, we can do it very old-school as well. My Studio mod has no overdrive mods and no masters. What it does have is every combination of 100 watt JMP variation from the Plexi to the mid seventies Super Lead and Super Bass heads. All in one. Check it out.

My Marshall Jaguar 20 watt head originally had only a tone and a volume control. Check out what happened to it. Am I insane or what?

Reverb, Tremolo, Cathode-Biasing FX loop, everything can be done to a JCM800 or or old JMP head. To keep things from getting confusing and since, obviously there is just too much here for a single amp I have some standard combinations that will serve you well if you are a gigging musician or different ones if you are a recording engineer

Drive Boost

I find that one setting of gain for the drive channel is not really enough. Sometimes you want an in-between sound that's not too dirty and then later you need more kick. So, the Drive Boost was born. Hi gain/ low gain - controlled by foot-switch. Simple!

Fat Boost

Ok, by now you get the drift. Nice to control your gain as you need to but there's nothing like a really Honking Tele. So, this is the Honk foot-switch. Again Simple.

Together with the Drive Boost it will provide four distinct drive channel settings, all chosen for graduated increases in gain and fatness.

Marshall FX Loop

An FX Loop makes a Marshall come alive. Add a good reverb or delay processor by hooking it up to an all-tube FX Loop and listen to the difference. I install send and return jacks and an FX return volume control as well.

Solo Boost

When used in conjunction with the FX Loop the Solo Boost will provide an easy to control increase in volume. No added drive, just a bit more volume, foot-switchable of course!

Marshall Dual Concentric EQ

You can't beat having two separate EQ's for the Clean and Drive channels.

With this mod you get separate Treble, Middle, Bass, and Presence controls for each channel by using Dual Concentric ("Stereo") controls. Use the rear controls to EQ the Clean channel and the front set of controls for the Drive channels

• No extra holes to drill


• Completely reversible back to original if you so desire

It's really like having two amps. Good for JCM800 and Jmp 50 and 100 watt amps as well as the Plexi reissue

Variable Mid Sweep

Dial in super fat mids or dial out the mids for a scooped out tone. This additional control is used with the existing mid and treble pots to provide extended range of control over the all important mid range frequencies. Not a parametric eq, it is never peaky or nasal sounding.

Instead of a variable control a pull-pot can be used to switch between two presets.

Bass Resonance

The Bass Resonance mod enables you to control the bottom-end of the amplifier. This circuits operates in the power stage, after the preamp and FX Loop in a similar fashion to the Presence control, except that it Boosts the Bass instead of the Treble frequencies.

With the setting at minimum you get the original JMP sound from the power amp. Boosting the control a bit will bring it closer to the JCM800 heads and bring it all the way up to make a 50 watt head thump like a Super Lead.

• Makes an open-back combo sound like a closed-back cab


• Works on power stage - great when using FX Loop for external preamps such as JMP1


• No added EQ circuitry to color your tone


• Variable control pot can be installed in one of the speaker jacks if desired - no holes need to drilled


• Great for dropped tunings (low D)

Class A Cathode Bias

Loud punchy amps like the Marshall benefit greatly from the added smoothness and compression that this mod provides. Best of all, however, is that you can switch back and forth from the original fixed-bias mode to the Class A mode with the flick of a switch. No need to go to standby first or even stop playing.

Marshall Power Tremolo

Add Tremolo without any extra circuitry in your audio signal chain that could possibly affect your tone with this Power Tremolo mod. It works by modulating the neg bias signal to the power tubes, a brilliant Leo Fender design.

Plexi Conversion

As Marshall evolved their 100 watt designs from the Plexi into the Super Lead series, the Plexi EQ was abandoned for a newer one with more prominent midrange. Bright caps were added to the second channel and the bottom end was cut. The presence circuit changed as well.

This mod will convert a Super Lead or JCM800 Master Volume head into a Plexi model

• Remove unwanted brightness


• Increase bottom end of second channel


• Fatter presence control


• Decrease noise and hiss

Super Lead Conversion

On JMP amps lots of people used to remove the word "Bass" from the rear of the chassis in order to sell Bass heads off as more-popular Super Lead models. If the serial number of your JMP head says "SB/A xxxx" instead of "SL/A xxxx" then you have a Bass head.

Transform early JMP's from Bass to Super Lead:

• Brighter second channel - great when linking both channels together


• Increased upper mid frequency definition


• Brighter presence control

Channel Linking

By now everyone know sabout jumping a wire from the low input of one channel to the high input of the other Channel when using a Super Lead or Super Bass head. Since the low input is attenuated you are adding extra series resitance to the signal that is being fed to the other channel when using this method.

You can avoid this side-effects by internally bridging the two inputs

• Eliminates need for jumper cable


• Quieter operation- less noise and RF


• No added resistance to alter tone


• Completely reversible for amp resale value

RF Elimination

All of my Drive and FX Loop mods now include RF elimination as a standard feature. If you have a stock Marshall JCM800 with bad RF problems then this will completely eliminate the problem.

Output Tube Conversion

For several years during the Bosnian war Marshall stopped using the EL34 and was equipping their amps with Russian 6L6GB/5881 tubes. Also during about the same period the US edition was equipped with American 6550 tubes to provide a more metal tone popular at the time.

For the classic Marshall EL34 tone this conversion is a must.