Valve Guitar Amplifier – Pre-Amplifier Tube Operation Explained

A quick and easy pre-amplifier valve change in the front section of you your valve guitar amplifier can make the most dramatic tonal alteration achievable from a single component swap, other than perhaps a speaker change. This article has ben created as a guide to Pre-Amplifier Tube Operation and will explain the basics of how valves perform and the effects on the overall sound quality, and tone, achieved from your valve amplifier. There are a number of pre-amplifier valves on the market the 12AX7 (or ECC83) being probably the most common found in guitar amps. Below you will find information on the gain of each prexamp valve type.


Guitar Amplifier Valves Explained

Once upon a time, vacuum tubes or valves were used in many applications, television sets, car and home radios, hi-fi systems, and guitar amplifiers, and were crucial components in myriad military applications, from radar technology to missile guidance systems and more. With the invention of solid state transistors gradually valves have been replaced including in guitar amplifiers. This is due to the solid state transistors being cheaper to produce, reduce space requirements in circuits and lower power consumption.

Valves have still continued to be used in high end guitar amplifiers, where despite the cost, guitarists agree that tubes sound better than anything else out there. There have been some quality solid state amplifiers produced and even new modeling amps but overall 99 or of 100 professional guitarists still continue to use valve amplifiers for touring and recording. Above all Valve Amplifiers have defined the tones of rock, blues, and country guitar.

Distortion
Distortion

Distortion

The key to the valve amplifier tone is how the valves distort the incoming guitar signal. If a transistor circuit is driven hard by imputing a signal larger than specified the waveform becomes clipped – chopped off or squared – this creates a very harsh distorted sound. With a tube circuit the signal is gradually distorted or becomes rounded and this creates a smoother sound tonally pleasing to the ear.

This is why any decent sounding solid-state amp requires a lot of extra circuitry to do what a very simple tube amp circuit can do naturally.

Guitar Amplifier Basics

All amplification valves have four main elements within a sealed glass tube: a cathode, a grid, a plate (also called “anode”), and a filament (or “heater”). The most basic tubes are called “triodes,” named for the first three of these elements (a filament is always present, so it’s ignored in the naming process). Pentode tubes, which account for most output tubes and a few preamp tubes, carry two further grids—a screen grid and a suppressor grid—that help to overcome capacitance between the control grid and the plate.

Inside a 12AX7 Valve
Inside a 12AX7 Valve

In simple terms, a valve will a small voltage (guitar signal) into a bigger one. This is done by first picking a string on the electric guitar. The pickup convert the sound to a small voltage which is passed to the input of the amplifier, where it is passed along to the grid of the first preamp tube. The increase in voltage at the grid causes electrons to pass from the cathode onto the plate at a correspondingly increased rate and the sound gets bigger. This slightly bigger signal from the preamp stage valves is then passed along to the output stage, where the output valves make it even bigger. This greatly increased signal is then passed to the speaker via the output transformer.

 Pre-Amplifier Tube Operation Explained

Pre-Amplifier Tube Operation or valves and output valves do essentially the same thing however the output power is different. Pre-amp valves are responsible for the early stages of the sound amplification and to perform other functions like drive the reverb or tremolo stages before the main, modified signal is passed to the output or power valves. Pre-amp valves are easy to identify within the amplifier circuit as they are generally smaller and are positioned near the inpuyt and tone controls of the amplifier. The most common type of pre-amp valve used within guitar amplifiers is the 12AX7 (known as ECC83 in Europe, or the high-grade US alternative 7025).

Some other types can be found which look the same apart from the numbers printed on the casing.

  • 12AT7 – often used in reverb-driver and phase-inverter stages.
  • 12AY7 – low gain compared to 12AX7. Original equipment in the first gain stages of many legendary Fender tweed amps of the mid and late fifties. Can be used to reduce the pre-amp gain when swapping out the 12AX7.
  • 12AU7 – medium gain compared to 12AX7. Original equipment in the first gain stages of many legendary Fender tweed amps of the mid and late fifties. Can be used to reduce the pre-amp gain when swapping out the 12AX7.
  • 5751 – an alternative lower-gain replacement for the 12AX7 – the choice of Stevie Ray Vaughn who modified his guitar amplifier with these.

All of these are what we call “dual triode” types, because they contain two independent tubes within the same bottle. They are mostly differentiated by their gain factor— the degree with which they increase the signal they are given. The 12AX7 has the most gain of the bunch, and the 12AY7 and 5751 are direct substitutes with less gain, which in many cases means they’ll distort the early stages of the amp less. The 12AT7 also has less gain than the “AX,” but requires a slightly different bias voltage for optimal operation.

The only pentode preamp tube seen with any regularity in amps today is the EF86 (or 6267), which appeared in early Vox amps and has more recently been used in models from Matchless, Dr Z, 65amps, and a few others. Another less frequently seen, but much admired, pentode preamp tube is the 5879, notably used in Gibson’s GA-40 Les Paul amp of the late fifties. Both of these pentodes fit the same 9-pin bottle as the dual triodes but require very different circuitry, and are known for their thick, robust sound. Both have higher gain factors than even a 12AX7, but aren’t prone to distorting the way that dual-triodes can, and instead pass their fattened-up signal on to the next stage.

Some modern high-gain amps are designed specifically to create extreme yet controllable preamp tube distortion by cascading multiple gain stages, one into the other, with gain and master volume controls between them to control the drive levels at each stage. Used in this way, preamp tubes can produce a scorching, harmonically saturated lead tone that sustains all day—what we usually hear as a classic shred or contemporary rock tone—in an amp that really relies on its output tubes just to amplify this sound, rather than to add further distortion to it. When driven into distortion in a simpler, more basic amp with fewer gain stages (a category that might nevertheless include some very high-end, “boutique” tube amps), preamp tube distortion becomes just a part of the amp’s overall distortion character, blended with clipping at the phase inverter and output stages, and often at the speaker too.

Once an understanding has been gained of preamp tube distortion, many players have learned to create a bigger tone by using lower gain preamp tubes. To lower the gain of a preamp stage a little, you can swap a 5751 into any socket that carries a 12AX7. To lower it even more but retain the same performance characteristics (other than gain) you can use a 12AY7.  This 5751 swap is a trick that was used by Stevie Ray Vaughan, for one, to help generate his signature tone, and it has also been employed by plenty of other great blues players. If you’re trying to achieve less of what you hear as preamp distortion and more output-tube distortion, you can also try using a 5751 in the phase inverter position, which is usually the last preamp tube before the output tubes.

Replacement Guitar Amplifier Valves

Even valves of exactly the same type can sound quite different, depending upon their manufacturer and small changes in their design and production. The fact that tubes distort so organically also means that no two tubes distort or even amplify exactly alike. Even replacing a valve by the same manufacturer and type can give varying results in terms of tone and distortion. Minor fluctuations in assembly results in a slightly different sound and performance from each valve. Most distributors sell valves which have been tested and selected for basic characteristics.

Experiment and find your “tone”

As a guitarist we all seek the optimal sound which we can hear in our heads. To do this it may be required to buy a few different valves fro different manufacturers and try swapping them around within the various positions within the amplifier – as long as it is a like for like swap. Only use a 12AX7 preamp valve if your amplifier specifies that valve be used, valve sockets are universal and will accept many different valve types and not all valves are the same. Fitting the wrong valve could damage the amplifier. It is normally best to change first pre-amp valve as this can affect the input tone the most.

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