Ace's Cool Tube Amp Book!

...whereupon the upright, but not uptight, Dr. Pepper breaks it down for ya and tells you what you need to know leaving out what you don't wanna know or don't care about yet.
or
"...how I learned to stop worrying and love the tube!"

WARNING!! DANGER DANGER!!! IMPENDING DOOM!! SHOCK HAZARD!! EMINENT PERIL!!

Tube guitar amps run on lethally high voltages! Some of the parts inside can hold lethal voltages long after the device is turned off! I take no resposibilty for your safety. Abandon all hope ye who enter here...

What I'm tryin' to do is some articles to 'splain tubes guitar amps in a manner that normal guitar player-types will understand. Now there's LOTS of books on guitar amps. You should totally read them! This is for my lame-ass buddies that didn't want to read any of those books. This is not all you need to know to build a guitar amp. But hopefully after you've read my stuff all the other stuff will make more sense.

The introduction:

I was talking to a friend the other day about having me build him an amp. Like many folks he didn't know much about amps at all. It occurred to me that I really should start from the top when explaining the wild world of guitar amplifiers.

So I'm gonna assume, because you're reading this, you're an electric guitar player. You've already figured out that you need an amplifier to amplify the small electrical signal from your guitar to drive a speaker for the world to hear. Here's a quick FAQ:

"What's the difference between tube amps and solid state amps?"
Tube amps use old-fashioned vacuum tubes to amplify voltage in the amp. Solid state amps use more modern transistors and IC chip amplifier cicuits.

"Why are tube amps better?"
The short answer is: there's a lot of electronic magic/voodoo/happy-accident that happens very easily and naturally in vacuum tubes and tube amp circuits that's very hard to duplicate with other kinds of electronics.

"What's the difference between combo amps and a head?"
A combo has the amplifier and the speaker in the same cabinet. A head is just the amplifier, an electrical device, that must be connected to a seperate speaker cabinet.

"Which one is better?", "Which one should I get?"
An age old question, my friend.
Combos
The upside - are handy 'cuz the amp and speaker are together, just one thing to grab. Many folks also dig the way they sound. The open-back cabinet allows the speaker sound to fill a room nicely.
The downside - Larger combos, like Twin Reverbs or Bluesbreakers, can be very heavy to haul. Tubes are subjected to speaker/sound vibrations constantly, hard on tubes. Open cabinet can sound thin.
Heads
The upside - can be paired with a variety of speaker cabs. Tubes isolated from speaker vibrations. Head can be built to vent tube heat better. Speaker cabinet can be optimized, lots of cabinet designs to choose from for a variety of situations or sounds.
The downside - Two pieces (head and speaker cab) instead of one = more gear to haul. This can be a real issue if you're lookin at a 100 watt head and one (or more) 4x12" speaker cabs. However a 20 watt head is pretty small and real handy and can be plugged into a 1x12" cab, which is not that big either. And you can have another 2x12" cab for bigger gigs with the same head. So the head-and-speaker-cab setup is pretty versitile.

"Can you give me a quick idea of how guitar amps work?"
I think so.
In a basic guitar amplifier you have a preamp and a power amp.
The preamp takes the tiny voltage coming from the guitar pickup and amplifies it enough to drive a power amp. In a guitar amp this is where the tone shaping happens, like bass, mid and treble adjustments.
The power amp takes this signal and amps it up even more to drive speakers.
That's the basic setup.
As you might guess, there's lots of different kinds of preamps, power amps and combinations thereof. There's also features like overdrive, reverb, tremelo, and channel switching that might be in there. There's a few classic styles of preamps that you'll hear people refer to, like Fender-style or Marshall-style. But even amps built like the classics have an endless variety of tones by things like part values and circuit tweaks. And then there are designers still coming up with new designs. This is what gives all the different amps in the world their own sound.

There's also diferent types and sizes of power amps. From 1 watt recording/practice amps to 10 watt, to 20, 50, and 100 watt amps.

5-10 watt amps with a single power tube are called "single-ended" amps. There's lots of these, like the old Fender Champ, and newer amps, like the Epiphone Valve Jr. Gibson GA5. More every day. These amps are all Class A, self biasing and have a cool sound when cranked up loud. Very simple circuits.

15-20 watt amps use two small-type power tubes in a "push-pull" setup. In these amps a tube is wired as a "driver" to split the guitar signal into 2 signals, one in-phase, one out-of-phase, to drive the two power tubes. Amps like the Marshall 18 watt or Vox AC15 use the small EL84 power tubes in a self-biasing arrangement. Other amps like the Fender Deluxe Reverb use two 6V6 tubes for around 20 watts. These are real nice amps to jam with your buddies or to play small bars.

30-50 watt amps also use two tubes, push-pull-style. These amps use larger type power tubes, like 6L6s and EL34s, that are made to produce more power. They also run the tubes at higher voltages and use larger output transformers rated for the extra wattage.

80-100 watt amps use pairs of tubes for the push and pull. In amps like the 100 watt Marshall or the Twin Reverb you'll find four EL34 or 6L6 power tubes, two for push, and two for pull. You'll also find even larger power and output transformers to deliver the extra wattage. These amps have a wonderful sound but get very loud very quickly. The large power supplies needed also makes 'em really heavy.

"OK, now, what are power and output transformers? And why are they so important?"
The power transformer takes the power coming in from your wall socket and "transforms" it into tube voltage, what the tubes want to see. Larger power tranny = more power,
The output transformer takes the tube voltage and "transforms" it into speaker voltage, what the speakers want to see. Output trannys have tone, all your sound goes thru this to get to the speakers. Good ones really do sound better than cheap ones.

"How long do tubes last? How often should I change 'em?"
Power tubes are kinda like guitar strings; eventually they'll get dull-sounding and wear-out and need to be changed. If you don't play very often or very loud your tubes can last years. Like 5-10 years or longer. If you play every night and like your tubes biased hot you may change 'em once a year. If the amp used to sound good and just doesn't anymore, and nothing else is wrong, it may be time for new power tubes.
Preamp tubes don't get the same kind of use/abuse in an amp as power tubes and can last for years. Change 'em when/if they go bad, or if you just wanna try something new in the amp.

Well, if you made it this far without your eyes glazing over congratulations! Let's get into a more detailed breakdown of how these tube guitar amps go together.