Chapter 3

The Pre-Amp Gain Stage

Another essential feature of an amp is the pre-amp stage, also known as the gain stage. We will address the term gain in a moment, but first, know that some volume must exist at the start of any guitar amplifier to grab a signal to amplify in the power stage. The pre-amp stage, therefore, comes before the power-amp stage.

A 50W EVH 5150III high gain 3 channel amp head.

For example, the EVH 5150III 50W 6L6 insert on the far left is where your guitar or pedalboard cable will go using a ¼” jack. The dial next to it is labeled Gain and is the pre-amp gain stage receiving the signal. You can increase the gain.

The next three dials are for equalization. These are low, mid, and high. Then there is the volume which is the power-amp stage. This whole lot appears repeated on the right-hand side but with the addition of a Presence control. Why? The answer is they are separate channels and sound differently.

The left grouping forms at least one channel (we shall explain why it is two channels shortly). The right grouping is another channel. Amps that we have shown you already have at least one main channel, but many modern amps can have more than one channel. In our EVH 5150III 50W example, the amp has a clean channel one. A button next to it turns it into the second channel, which has more crunch. The third channel is the high-gain overdriven-sounding channel. It does not switch to another channel, like channel one into channel two.

Shared channels are labeled on the amp (channel one/two). Multichannel amps usually start with the clean channel first and generally get more distorted with subsequent channels. Higher gain is sometimes called a crunch channel and or an ultra gain channel. It is all relative. So it is not uncommon for amps to have shared channels. The EVH 5150III 50W, in our example, has a shared channel. That means instead of a whole new channel for channel two, pressing the channel two switch button uses the same settings on channel one but the tone changes to a distorted crunch because of a fundamental channel change. This lot is repeated in another channel on the right side of the EVH 5150 50W, but this is not a shared channel. It still works the same way. At the end is a presence dial that creates a sense of the sound being more punchy and closer to you or duller and further away if you turn it down. All the channels share it.

Some amps only have one channel, such as the Marshall JCM800 head we showed in the chapters back. Others have three or more.

Next up is a 100W EVH 5150III showing three separate channels. It has the same channels as the 50W versions, but two are not shared. The other significant difference is that it has four power tubes instead of two.

EVH 5150III 100W. Three separate channels.

Notice the small square buttons at three intervals on the face of the control? Each switch selects a new channel, meaning they don’t have to share settings with another channel like the 50W channels one and two. In addition, newer EVH 5150 III 50W versions have new inner concentric ring pots to help segregate the channel features more. It helps prevent large jumps in volume when you switch the channel. Sometimes for clean, you need the volume on high, but when you switch to the distortion channel 2, the volume is too much and can give you a sudden jump. That is solved with inner concentric rings for channel two settings.

Each amp has characteristics and even quirks like this. It is how the engineers have put things together. It is half the fun of playing electric guitar to find an amp you have not used before and discover for yourself how to use them.

Why not take the time to download some EVH manuals to confirm what we have been discussing. https://www.evhgear.com/support/product-manuals

The point is that amps can be quite operationally different, even between wattages of the same amp. Don’t assume that because a 50W has a MIDI or line out, the 100W version will have them also. Always look at the specs.

Pre-amp tubes

There are a few pre-amp tubes makes you should know. Pre-amp types are interchangeable, unlike power-tube amps requiring specific power tube types for that amp. However, it would help if you stuck with the ones recommended by the manufacturer until you learn more about pre-amp tube differences.

They also have different names for the same pre-amp tube. The most common types of pre-amp tubes are: 12AX7 (ECC83), 12AT7 (ECC81), 12AU7(ECC82), 12AV7(5965), 12AY7(6072), 6CG7(6FQ7), and 5751 tubes. There are many more. Out of those, we listed the 12AX7(ECC83) is the one you will come across most and the 12AY7(6072).

Here are the tube layout schematics with the shared channels for the EVH 5150III 50W version. It’s like looking down on an amplifier from above with the top off.

Look at the circles labeled with a V and a number after them. Those are the vacuum tubes. You can easily see the V7, and V8 tubes are bigger and labeled 6L6GC. These are power tubes. V1 to V6 labeled 12AX7A are pre-amp tubes. V9 is a unique pre-amp tube that uses a power amp phase splitter, but you don’t have to know anything about that for now. I want to highlight how pre-amps V1 to V6 are assigned to channels under the schematic list.

In this amp, Channel one is clean, Channel two gain, and Channel three ultra gain. Clean, medium gain, and high gain. These are called CH1, CH2, and CH3 in the key under the tube layout schematic.

Notice that V1 (pre-amp tube 1) is a shared tube between all three channels? Also, notice that V5 and V6 are shared by both channels one and two? V2, V3, and V4 are only used by channel 3.

So shared channels here are sharing parts of the same tube system. That means if a valve were to fail in V1, it would impact all three channels. However, if a valve were to fail in V2, V3, or V4, only channel three would be impacted.

So there are ways to determine which tube could be a candidate behind a tube failure if you have schematics explaining associations like this. First, you unplug the system, pull out a tube, and replace it. Try the amp. If the problem is still there, pull back out the new tube you put in. Put back in the old one. Then repeat the process with the next tube to check if it is that one that has failed. You only need one pre-amp tube to find the culprit this way. It is highly improbable two tubes would fail naturally at the same time. Multiple tubes failing simultaneously is a sign something has gone wrong with the amp.

Like the power stage, the pre-amp stage takes a signal and processes it so you can listen to a result. Like the power stage this gain increase will color the tone a bit. This color is also an identity of the amp and is part of its character.

The pre-amp stage comes before the power-amp stage. Master volume increases process a signal that the pre-amp section has treated. A pre-amp section combined with a power amp section makes up the basic features of any guitar amplifier sound. The tone is raw and harsh without a guitar speaker cabinet, like an electric saw, requiring filtering, which speakers do.

Amps should never be switched on without plugging into a cabinet because you can damage them. Think of it as power trapped in the amp and overloading electrical components. That is why amps can blow parts and smoke, especially the power transformer (PT), which you can also see in the EVH 5150III 50W amp schematics diagram (top right of the diagram). If you incorrectly hook up an amp or something is wrong with it, the PT is usually the first to smoke. The good news is that PTs are easy to replace by a reliable tech if yours should fail. However, they are not cheap.

The speaker shapes those raw guitar amp sounds into various electric guitar tones. It acts as a moving filter for pushing large volumes of air. The filtered frequencies are what produce sound. Speakers come in different types and each with a different filtering design to give a different response. Microphones hugely impact the tone for recording, even down to single-centimeter movements in positioning. Microphone placement is an art unto itself.