Getting the Marshall Tone & JCM2000-Newer Repairs

Marshall JCM2000 Bias
Marshall, reissues, JCM series, and newer. The output jacks oxidize very easily making them not pass signal. Exercise the jacks by plugging into all of them 20 times each. The amp should be off.

The power tube PCB develops carbon due to shorted and arcing tubes. If a new set of tubes blows the fuse, this is a good place to look. Remove the carbon with a razor blade or drill out the affected area. Filter caps need to be discharged, check pin 3 (plate) for any DC voltage.

Be sure to bias the amp whenever replacing the power tubes, these amps can be tough on tubes quickly otherwise. Marshall recommends 45mA.

The first couple preamp tubes will make a big impact on the sound. Chinese tubes are bit lower output will help when feedback is excessive. Russian tubes will provide the most gain.

The power tubes brand will not make much of a difference in tone and feel unless the amp is played at stage volume.

Recommendation: if you are looking for that Marshall tone get a boutique amp ($1200-2500) as it will be superior in almost every way. A vintage Marshall will cost more than its purpose serves between initial purchase and restoration. If you own a Marshall that is 1980’s or newer I can tell you replacing the output transformer with one from Mercury Magnetics or Heyboer will open up the amp with more clarity.

Tube Amp Wiring and Layout

I spent a lot of time changing component values up or down just slightly to make sure the sound and feel was good as well as the shape of the sine wave. A good clean sound will help the over driven tone better.


The 100 watt amp project has been pretty successful, now it just needs a cabinet and front and back control backing plates. I ended up with a Gain, Bass, Treble, and PPIV Master Volume.


My workbench: borrowed oscope, light bulb limiter, aluminum heat sink with resistor load, the Fluke 87 (love it), and Guitar Amplifier Preamps by Richard Kuehnel.

Bench Testing

The amp is kind of a combination of an Orange style power amp and phase inverter and a simple Fender two tone control preamp.

Here is the PDF schematic if you’re curious. Schematic 2012-09-03 100w

The Problem with Digital Music Equipment

I spend a lot of time discussing electronic repairs with customers. We talk about why the problem happened, what needs to be repaired, the cost to repair versus replace, and how to prevent it from happening again. The issue they face a lot of the time is many pieces are not worth fixing. Digital equipment is even worse when it comes to “cost of repair versus replacement” because of the proprietary parts.

The bottom line, many digital devices are overly integrated and force the user to discard the them rather than repair them.

The Solution
Computer powered systems are more modular and allow the user more flexibility. It’s far more advantageous to use a software guitar amp simulator than a digital all-in-one system, just as one common application. As the user you can choose the A/D converter, computer specs, and as new versions of the software are available you can update as needed. Best of all, you can use your system for a number of other things.

With computing power increasing all the time it just makes sense to use the processor you already own than have it integrated in a piece of electronics that will be outdated in a year. Then when you need, simply upgrade your computer on your terms.

Tubes and More Tubes

Tube Testers • These are awesome, if you ever wanted one, get one.

Testing preamp tubes • See if both triodes are even enough to be used as a long tail phase inverter.

JJ KT66 • I think someone needs to make a guitar amp that uses only one of these. When the bassist in the band gets new tubes you would be able to use their old ones.

Mixer Repairs

Yamaha PM3000 • The groups, auxes, and some channels were out on it. The power supply didn’t work either, a bad power rail. Lots of cleaning, resoldering, and general TLC is in store for it.

Mackie CFX20 • Just needed to replace a jack on this one.

Tracking down the issue is a bit easier with the PCBs out of the chassis.

Output level control • It gets people every time, taking it out should take care of this. Most of the time it just operator error.