Welcome! Thus far, we’ve looked at the Preamp and FX Loop sections of an electric guitar amp. Next up is the Power Amp.

I’m sure you’re well aware that there are a few different types of rock guitar amps out there right now — digital/modeling, solid-state, and of course, tube.* Plus there are a few hybrid amps lurking too. What’s a hybrid amp? Well, as the name suggests, it’s basically a preamp and power amp mix ‘n’ match. For example: a tube preamp with a solid-state power amp, or a digital preamp with a tube power stage. Make sense?

*NOTE: At the risk of stating the beyond obvious, a tube amp is one that has tubes in both its preamp and power amp sections.

At the end of the day, though, the goal for any and all electric guitar amps is to sound (and allegedly “feel”) as close to an all-tube amp as possible. Why? Because the vast majority of guitar tones we consider both classic and desirable were created with all-tube amps — that’s why! Consequently, that’s what we’re going to look at here: tube power amps and why some (not all — tubes alone do not guarantee a great amp, more about this in a later episode!) are so highly revered and often emulated.

At this point, I feel I need to explain/point out a very important characteristic of a desirable tube power amp — it doesn’t merely amplify the signal fed into it; it will also make it sound, and invariably feel, even better. Here’s the skinny:

I’m sure you’ve heard of and own hi-fi (high-fidelity) units, such as home stereos, MP3 amps, and home sound systems. In a nutshell, the job of these units is to accurately amplify (make louder) whatever signal is being fed into them — be it CD, TV, or MP3. Their job is to do so without adding any unwanted colour (Editor’s note: These Brits and their colourful spelling!), compression, noise, and certainly NOT, distortion!

Good tube guitar amps are the exact opposite and are an audiophile’s worst nightmare! They’re essentially lo-fi devices that, when turned up to a Nigel Tufnel, Spinal Tap-approved “11” (that’s one louder, innit?!) not only make a guitar’s puny signal strong enough to move speakers, but can also contribute smatterings of gloriously musical compression and harmonic distortion, which are essential elements for a rock guitarist’s tonal bliss. What’s the signal-to-noise ratio at such settings? “Really awful” is the technical answer in audiophile-speak, but a necessary evil in the holy quest for a great rock guitar tone.
Furthermore, a good, tube-driven power stage (e.g., as found in a Marshall 100W Plexi, Mesa/Boogie Dual Rectifier, Vox AC30, Friedman BE-100, Diezel VH4, or an EVH 5150, to name but six), when pushed, is a valuable sonic contributor to the aforementioned lo-fi glory that guitarists lust for — not only in terms of “trouser-flapping” volume but also tone and feel. Yep, the power amp is pivotal, my friend. Consequently, understanding what does what, how, and why is kinda important.

As stated at the very onset of this series of articles, my goal is to (hopefully!) make these as “guitarist-friendly” and easy to understand as possible. For this very reason, since the power amp is so pivotal, I’m going to chop this segment into several, bite-sized chunks so it’s as digestible as possible. Right, now back to the plot.

What exactly does the power amp in a guitar amp do?

As mentioned in Part 2, the power amp is responsible for taking the beefed-up, boosted, and shaped preamp signal and giving it sufficient muscle to move a speaker or set of speakers with ease.

Here’s a pictorial synopsis:

Make sense? Despite the left-hand guy’s impressive physique, he’s not yet strong enough to drive speakers. The right-hand guy is!!! You can think of the power amp as an intense gym where steroids are not only allowed, they’re mandatory!!

Power Amp Anatomy Basics

In very simple terms, the power amp can be thought of as having three basic but essential elements:

Phase invertor or driver tube
Power tubes: not only the type, but how they’re used
Output transformer
In a nutshell, here’s what these essential things do:

1. Phase Invertor/Driver Tube

This is a small, preamp-type tube (e.g., a 12AX7 in many amps) that — because of its preamp “size” — is often lumped into the preamp section. However, because its sole job is to feed the power tubes, it’s really a power amp family member. Don’t believe everything you read on the Internet, my friend!
Exactly what this bad boy does and how it does it is dependent on the exact nature of the power amp it is driving, and that’s a subject we’ll be digging into in the next episode or so. For the purpose of this overview, all we need to understand is that this bad boy is the gateway to the power amp.

2. Power Tubes

As their name suggests, the job of these tubes is to add mighty muscle mass to the preamp’s already-toned signal so that the attached speaker(s) can be driven. Pretty common power tubes include EL34, 6L6, 5881, 6550, KT66, KT88 & EL84, and 6V6. When driven, all of these types have their own unique and sought-after (if used correctly!!) tonal characteristics — including musical distortion and compression. In guitar amps, there are two types of tube power amp — Class A and Class AB. What’s the difference? Stay tuned for the next Power Amp Episode, and all will be revealed!

Mesa/Boogie Dual Rectifier Power Transformer and Tubes

3. Output Transformer

ALL tube amps with a power amp have an output transformer, and as the second word in its name implies, a big part of its job is to literally “transform” the high-voltage, high-impedance output of the power tubes to a high-current, low-impedance signal that will drive the speaker or speakers attached without turning them into burnt toast!! Needless to say, this big brute is a vital part of power stage.

Now that you know the basic lay of the land, get ready for a deeper dive next time out. Until then — to quote one of the many great lines from Spinal Tap (if you haven’t seen this classic movie — please right this horrible wrong immediately!): “have fun…all the time!”

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The Guitarist’s Guide to Electric Guitar Amps – Part 4: The Power Amp

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