Amp tube changes can make dramatic changes in guitar tone from a single part swap within our amps other than replacing speaker. Unless you know about the difference in tubes and how they perform, that change will be very random. We’ll try and provide info to further the knowledge of players who are already have experience with these amps.
Vacuum tubes were once used in everyday life for decades. They were used in our TVs, car/home radios, hi-fi systems, and guitar amplifiers, and were important components in the military for applications like radar tech to weapons guidance systems and more. Over time though they have been replaced by other forms of more compact and more stable technology, except in guitar amps, where they maintain their existence over all kinds more advanced electronics. Electric guitar amps with tubes simply sound better. There are some great sounding solid-state amps and digital modeling amps have also made strides into the industry, but most working musicians continue to use tube amps for both recording and touring. These amps still define the fundamental tones of rock, blues, and country music.
Tubes amplify your electric guitar because of the way they distort. Crank a simple transistor circuit hard, and it clips/distorts in a really noisy way. Crank a tube into clipping and it distorts more gradually and more smoothly. It rounds off into distortion and usually creates better tones because of this.
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. (When talking about distortion, I also mention the things that affect your clean tone.)
Most tube amps, even when set to clean levels are still distorting a little, and that distortion creates layers of harmonic depth that sweetens and fattens up our tone even when we’re playing straight and clean.
Most amp tubes have at least four parts within their vacuum-sealed glass bottles:
- A cathode
- A grid
- A plate (also called “anode”)
- 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 not important in the categorizing them. Pentode tubes, which account for most output tubes and a few preamp tubes, carry two further grids:
- A screen grid
- A suppressor grid
These help to overcome capacitance between the control grid and the plate.
In laymans terms: a tube is to designed to make a small voltage such as a guitar signal into a larger one. Strum a string on your guitar and the pickup sends a small voltage to the input of your amplifier, where it passes to the grid of the first preamp tube. The increased voltage at the grid causes electrons to boil off of the cathode and onto the plate at a correspondingly increased rate. As a result the sound gets bigger. This slightly bigger signal from the preamp is passed along to the output stage where the output tubes or power tubes make it even larger and carry it on to the speaker using the output transformer.
Preamp tubes and output tubes do essentially the same thing. Tubes are the heart of your amplifier. They do the real work. Everything else inside is there to help them run them efficiently and to help pass along the signal. In addition preamp tubes are also used for other functions within the amp. Functions include driving reverb or tremolo stages or splitting the signal and reverse the phases of the two legs that are fed to the output tubes depending on the amp.
Preamp tubes are easily identified as the smaller tubes in your amp and are usually positioned in your amp’s inputs and early gain and tone stages. Sometimes they are covered with metal shields which are easily removed. Since the mid-50s, preamp tubes have mostly been of the smaller 9-pin variety. Some older amps will still have bigger 8-pin or octal tubes that fit the same sockets used by many types of output tubes. The most common type by far is the 12AX7 (also known as ECC83 in Europe).
Some other types you may see look the same other than the numbers printed on them. These are:
- The 12AT7 (often used in reverb-driver and phase-inverter stages)
- The 12AY7 (original equipment in the first gain stages of many legendary Fender tweed amps of the mid/late 50s
- The 5751 (a lower-gain replacement for the 12AX7)
All of these are what we call dual triode types, because they contain 2 independent tubes within the same bottle. They are separated 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 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 it can be directly substituted in a pinch.
The only pentode preamp tube seen with any regularity in amps today is the EF86 (or 6267). These tubes were used 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 pentode preamp tube is the 5879, notably used in Gibson’s GA-40 Les Paul amp of the late 50s. 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 12AX7s, but aren’t prone to distorting the way that dual-triodes can, and instead pass their fattened-up signal on to the next stage.
They also have a reputation for handling effects pedals very well. Drive a 12AX7 hard and it will induce quite a bit of sizzling, slightly fizzy-voiced distortion of its own. This can be great if you’re looking for a super overdriven tone that’s hot at all stages, but not wanted at all if you want more clarity and headroom, or the fatter distortion that’s generated in the output stage of the amp when a cleaner preamp signal is driven into clipping at the output tubes. It will depend on what you are going for tone wise.
Some modern high-gain amps are designed specifically to create hardcore 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. This way preamp tubes can produce a scorching, harmonically saturated lead tone that sustains all day. In amps they really rely 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 (including some very high-end boutique tube amps), preamp tube distortion becomes just a part of the amp’s overall distortion characteristic, blended with clipping at the phase inverter and output stages, and sometimes at the speaker too.
Many players have learned to create a bigger tone by using lower gain preamp tubes. By lowering 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. Many players think the last thing they want to do is lower the gain of a preamp stage, but in doing so you can often prevent your signal from dirtying up in the preamp, and thereby pass a beefy, full sounding signal along to the output stage when the amp is cranked. This generates more output tube distortion, which results in a fatter, fuller tone in many simpler tube amps. This tip doesn’t usually apply to high-gain type tube amps, in which these amps generate preamp distortion for their tone. This 5751 swap is a trick that was used by Stevie Ray Vaughan to help generate his signature tone, and it has also been employed by plenty of other great blues players. If you’re trying to get less 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.
Even tubes 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. For one thing, while tubes are manufactured under fairly rigorous conditions, they are still imperfect devices. Every little fluctuation in assembly or materials results in a slightly different sound and performance from each tube that comes along.
That’s why good tube distributors need to test tubes they sell:
Put even two high-quality “new old stock” (NOS) preamp tubes from respected American or British manufacturers on a tube tester like a pair or RCA 12AX7 preamp tubes that came out of the factory in 1963, and they will most likely have slightly different readings for gain and other factors. Put enough of them up on a tube tester and some will even fail to meet required minimum standards. Unfortunately that’s the way it is. Aside from the different readings, these tubes will each sound just a little different and other makes both NOS and current will sound different again.
For different sounds or finding the right tone, you should get your hands on as many different makes and types of tubes as you can reasonably afford. Try swapping a few to see which ones help you to best achieve the tone you are seeking. The first preamp tube position usually affects the tone of that part of the amp the most. Read your amp’s tube chart or owner’s manual to make sure you know how to change tubes safely, and are changing the right tube.
Before changing anything, please don’t touch any hot tubes! Let them cool down first. The tubes and the amp itself can be damaged when changing the tubes. Not to mentioned the potential to burn your paws in the process.
Search the internet and read up on what other players consider to be the best current- manufactured tubes coming out. Also see if you can find any affordable NOS tubes, or perhaps you can pull some used but functioning examples from old junker radio or hi-fi systems that you find at garage sales and swap meets. Experiment a little, and see which ones work for you. Preamp tube swapping can be fun and it’s also a great way to find your tone. As always play well and play on!