Amp Valves: What Are They & What Do They Do?

Guitarists will already be aware of the unmistakable effect the humble valve can have on their sound. While there are at least 25 different kinds of valves found inside various different guitar amplifiers, there are four that you’re most likely to come across. Here, we cover each of them so you can get to know exactly what’s shaping your tone.

Driven Over the Edge

The thing that’s loved by all guitarists and hated by all audiophiles is the point at which the valves (also known as tubes) are driven over the edge and the sound takes on that tasty, distorted quality. The compressed distortion and the dynamic response that results is down to an exchange between the preamp valves and the power amp valves.
Preamp valves are almost always going to be a 12AX7. Now often made in China, these tubes were manufactured in Europe until 1980 and are sometimes referred to as ECC83 valves. Over in the guitar universe, a heated discussion still rages over which type of 12AX7/ECC83 is the best, because while vacuum tubes are actually pretty old tech these days, more than enough of them are still being made. Mazda, Tungsram, Mullard, RFT are all big names in the tube-production industry and are part of the discussion, as well as where the best tubes are made: China vs. Russia? Or is it Hungary, the Czech Republic, Slovakia, or Germany? And so on.
The discussion mainly revolves around the point at which the valve hits overdrive and how that overdrive sounds. Of course, this is really a matter of taste. If you swap out your valves for a different type, it could actually also be an ECC82, or an ECC803, or a 12AY7. Each adds its own distinct flavour, and you’ll only find out which flavour you like by trying them all. Since slotting a valve into your amp is a 30 second job, and valves don’t cost all that much, it’s an adventure worth taking.
Back and forth across the pond, preamp valves are given various different names. For example, in the USA, the ECC83 is almost exclusively referred to as the 12AX7 – which has slowly become the worldwide standard when it comes to identifying your preamp tubes. But when it comes to preamp tubes in general, there are still a lot of slight variations out there. As such, you might find that the 12AX7 tube in your amp is marked one of the following:

Other preamp tubes come with their own specific tipping point when it comes to overdrive. While round and warm overdrive is not likely to suddenly evolve into the sound of a jar full of bees, a fresh set of preamp valves can completely transform a less-than expressive amplifier, or tame and sweeten an overly ‘hot’ amp. When it comes to guitar amplifiers, it makes sense to stick to the same ‘group’. So, the 12AX7 (and all of the letters that usually follow) is a good replacement for any ECC-types, or the 7025, 5751, and 12AY7. But if you’re thinking about swapping out your tubes, where exactly do you find them? Take a standard amp cabinet – it can be a combo or amp head made for guitar or bass – once it’s opened up (sometimes you can do this without having to open it up), you’re likely to see both larger and smaller tubes fitted, and some might be ‘standing’ and others suspended. The smaller tubes are the preamp valves, and the larger tubes are the power amp valves. Some amplifiers also have built-in analogue effects like spring reverb or tremolo that are driven by valves. This will usually be a 12AY7. You might also find a rectifier tube – which deserves an entire blog of its own.

Note: preamp tubes can be swapped without the need for any technical knowledge, but power amp tubes should only be changed by a qualified technician. Fortunately for the more adventurous, tinkering guitarists, the tubes of one section usually won’t fit in the other. But if they do, here’s a little advice: just don’t do it, unless your amp has a bias control. We’ll cover bias functions in the next bit.

Hotter Tubes Have a Lower Distortion Threshold

The overdrive point is also described as how ‘hot’ a tube is. Some amplifier manufacturers use a colour coded or numbered system (1 to 10) to describe how ‘hot’ a tube is, and you’ll often find that power amp tubes have these ratings.

Why is this information useful? Amplifier builders set the bias of an amp in the factory. Sometimes the bias is fixed, as with Mesa Boogie models. ‘Bias’ means that the voltage is set at a certain point to ensure that the amp functions optimally. With valve amplifiers, this is important, and this is why it’s also recommended to replace power amp tubes in pairs that have been matched for a specific bias point. Generally, you can say that Fender ‘white’, Groove Tubes 4 to 7, and Mesa-colours best match most factory-set bias points. You can put them in pretty much any amplifier and they’ll deliver a versatile sound, enough gain, and enough headroom.

To make it easy to remember: lower numbered valves (or the Fender colour classification, ‘blue’) hit overdrive earlier than higher numbered valves (the Fender colour, ‘red’). So higher numbered valves stay ‘clean’ for longer.

Tip: For the purists out there, it’s worth noting that some Fender/Groove Tube valves state whether they were made in China or in Russia/Czech Republic/Slovakia.

More on Power Amp Valves

While the preamp valves take care of the overdrive, the power amp tubes shape the character of an amplifier. The clean tone of a Fender Twin Reverb, the ‘Californian Tweed sound’, or the classic ‘British drive’ are all considered iconic guitar sounds, and this has a lot to do with one type of valve. In short: Marshall made their name because of the EL-34; Vox owe their renown to the EL-84; and American amp builders in general seem to have patented the 6L6 valve. This is still largely the case, although Fender also uses 6V6 valves in their power amps; Marshall also uses the KT-66; and Mesa Boogie fit their Transatlantic Series with both English and American tubes.

What do the valves actually sound like? How do you describe the sound of these different valves? This is a tough one, because the sound of an amplifier is the result of the interaction between many different elements (valves, power, wiring, the speaker, housing, etc.). What we will say is that valves always sound better the more you push them, so heat that amp up!

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