# Valves in a Marshall JCM 900

By bright\-koala\-cc9d3551

Topic: Art, Music & Film

[Open the interactive Jig](https://ask.jig.com/j/7YyFDdMxsPNN)

## Guide

### 1. Meet the JCM 900

This is a Marshall JCM 900, a famous rock guitar amplifier from 1990\. Hidden inside are seven glass valves, also called vacuum tubes\. They take the tiny signal from a guitar string and make it big enough to shake a stage\. Drag anywhere to turn the amp around\.

This head is 74 centimetres wide and weighs about 19 kilograms\.

### 2. Inside the box

Let's lift away the vinyl\-covered box\. Underneath is a steel tray called the chassis\. The valves stand up from it in a row, with heavy transformers at the end\. Each part has a job in growing the guitar's sound\.

On a real amp you can see the valves through the metal grille at the back\.

### 3. Seven glass valves

Three small ECC83 valves shape the sound\. Two make the signal stronger, and the third, the phase splitter, makes two mirror copies of it\. Four big 5881 power valves then drive the speakers\.

ECC83 is the European name for the 12AX7, one of the most common valves in guitar amps\.

### 4. Warm them up

A valve only works when it is hot\. Inside each one, a thin wire called the heater glows orange, like the wire in a toaster\. Press Switch on and watch the valves light up\.

Real valves take about half a minute to warm up; here it is sped up\. That is why amps have a standby switch\.

### 5. Inside a valve

Here is one small valve with its front cut away\. The heater warms a metal tube called the cathode, which gives off tiny charged particles called electrons\. The metal plate around it is charged positive, so the electrons fly across the empty space towards it\.

There is no air inside: a valve is a vacuum\. We drew the electrons as blue sparks; real ones are far too small to see\.

### 6. The grid is a tap

Between the cathode and the plate sits a wire spiral called the grid\. The tiny guitar signal on the grid lets more or fewer electrons through, like a hand on a tap\. The plate copies the wiggle, much bigger and upside down\. Slide the guitar signal and watch\.

A change of just a few volts on the grid controls a flow driven by hundreds of volts\.

### 7. Crank it up

Turn the gain right up and the wave grows too big for the valves to follow\. Its tops get squashed flat\. That squashed wave is the crunchy, growling distortion of rock guitar\. Switch between clean and cranked to hear and see the difference\.

In the JCM 900's lead channel, a pair of tiny diodes squash the wave too, for even more crunch\.

### 8. Push and pull

The four big power valves work in two pairs\. The phase splitter hands each pair a mirror copy of the signal, so one pair pushes while the other pulls\. Together they drive the output transformer, which turns their high\-voltage flow into a strong current for the speaker\.

Together the four valves make up to 100 watts, loud enough for a big stage\.

### 9. Rock on

So the valves never make the sound themselves\. They control power from the wall socket, shaping it into a giant copy of the guitar string's wiggle\. Warm, loud and a little bit wild: that is the sound of a valve amp\.

Power valves slowly wear out, so players replace them from time to time, a bit like guitar strings\.