Ace's
Cool Tube Amp Book!
...whereupon the Rightous Dr. Pepper
breaks it down for ya and tells you whatchoo kneed 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!"
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
who was thinking 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.
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...
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