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Tube Amp Basics

Tube Amp Basics


The preamp is the “front end” of the signal path in your amp.  It determines most of the tone characteristics of your sound.  This is the section of the amp that the EQ knobs control.  It is also the part that determines the amount of distortion in your tone, usually controlled by the “Gain” knob.  Preamp distortion is characterized by words like “grind”, “crunch”, etc.  The tubes that are generally used in the preamp are 12AX7s, which are the little tubes in your amp.


EQ Section…

EQ stands for “Equalization”.   These are the knobs on the amp that let you shape your sound, and the “center” of your signal path.  Some small amps only have a Tone knob, which simply rolls off high end as you turn it down.  Other amps have what is known as a Baxandall Tone Stack, which consists of separate bass and treble knobs (turning the knobs past halfway scoops mid frequencies, while turning the knobs less than halfway accentuates the mids).  The most common EQ is the traditional TMB (Treble/Middle/Bass) Tone Stack, which is sometimes also paired with a Presence knob, and less frequently, a Resonance knob.  Treble controls high frequencies, Middle contols midrange frequencies, and bass controls the low frequencies.  Presence accentuates high-mids, while Resonance accentuates low-mids.


Power Section…

The power section is the “back end” of your signal path.  This is the part that actually AMPLIFIES the signal sent to it from the preamp, then sends it to the speakers.  The “Master” knob controls the power sent to the power tubes, which are the larger tubes in your amp, thereby determining the overall volume.  The EQ knobs do not affect this section, but the “presence” knob determines the midrange voicing of the power section. 

As the volume increases, the power tubes will begin to distort.  THIS is the magic of tube amps.  Power tube distortion is rich in complex, musical harmonics.  It’s not crunchy, but sweet.  It gives an amp “guts” and “oomph”.  You HEAR preamp distortion, you FEEL power amp distortion. 



Power Tubes…

Different amps use different kinds of power tubes, and they all have a different musical character.  Play them all to determine what suits your musical style best.  Certain tubes are associated with certain amps, so find out what your favorite bands play.  Almost everything is some variation of a Fender, Vox, or Marshall.  Here is a short list of power tubes…

6V6: Low power Fenders, such as Champs and Princetons, up to the medium powered Deluxe.  6V6s distort quickly, and have a growly midrange.

6L6/5881: Mid and high powered fenders, such as the Bassman, Super, and Twin.  Mesa Boogies also use these for high gain.  6L6s have a glassy clean sound with a scooped midrange.  When they are overdriven, they have a very aggressive, tight character.  This is the classic “American” power tube.

EL84:  These are closely associated with the British VOX sound of the AC30.  For cleans, think of the chiming sound of the Beatles.  Distorted, think of the warm, boxy midrange of Brian May’s rhythm guitars in Queen.

EL34/KT77:  British muscle, the sound of a wide open Marshall.  EL34s are warm in the midrange, and loosey-goosey and forgiving in the bottom end.  The sound of ROCK.  Even the clean sound has a little hair on its chest.

6550/KT88:  The most powerful power tube.  Very even harmonic response, with the tightest low end of any power tube.  When overdriven they have a beautiful, woody character, but they are DEAFENINGLY loud.  These can be used in some Marshalls, but they are most associated with the Ampeg SVT, the mother of all bass amps.


Master Volume…

The Master Volume control is incredibly useful.  It has two parts; the “gain” knob, and the “master” knob.  Think of the “gain” knob as the distortion control for the preamp.  When it is low, the sound will be clean.  As you turn up the gain, the sound becomes increasingly distorted.  The “master” knob controls the output volume of the power section.   By using these controls together, you can achieve distorted sounds with lowered volume, or very loud clean sounds.



The transformers are the imposing metal cubes next to the tubes on the amp chassis.  Most tube amps contain an output transformer (the big one), a power transformer (the smaller one), and sometimes a choke (the tiny one).  The weight and size of the transformers can be a visual cue to the build quality and ruggedness of an amp.  Old Hiwatts, Oranges, and Ampegs have MASSIVELY oversized transformers that weigh a ton, and you could drop those amps off a building with no harm done.  Anyway, without going into a dissertation on electrical engineering, here are the basic functions of each transformer…

OUTPUT:  The output transformer is very important, because its build quality and how well it is matched to the circuit affects tone substantially.  Companies like Heyboer and Mercury Magnetics make upgrade transformers for many common tube amps that will make almost any modern amp sound better.

POWER:  The power transformer doesn’t affect tone, but it is responsible regulating the power to your amp.  It is always good to have an overbuilt power transformer to make sure the guts of your amp are getting solid, consistent power.

CHOKE:  Honestly, I have no idea what the choke does.  Some amps have them, some amps don’t.  There are some amps that people modify to have chokes.  Whatever.  If you find out what they do, holla at me.




The rectifier is the part of the amp that transforms the AC power from the wall into the DC power used inside the amp.  Tube amps can use either a solid-state rectifier, a tube rectifier, or both.  The type of rectifier in the amp has a lot to do with the “feel” of the amp.

 Tube rectifiers have a delayed note response, known as “sag”.  When you hit a note on the guitar forcefully, the rectifier tube is temporarily overloaded, which reduces the volume of the attack slightly.  As the tube recovers from the overload, the note will get louder.  This phenomenon is known as “bloom”.   Amps with tube rectifiers are described as “smooth”, “spongy”, and “forgiving”.

Most amps since the mid-sixties have solid-state rectifiers.  They have a more direct, immediate response than their tube counterparts.  They are more articulate, and more aggressive sounding.   They tighten up the low-end response considerably.

Some amps have both features, such as the Mesa/Boogie Dual Rectifier.  It lets you assign either a tube or solid state rectifier to either channel, which offers a lot of flexibility.

One is not better than the other.  They are just different.  Play a bunch of amps to see what you prefer.



Effects Loops…

As a rule, all time-based effects (reverb, delay, chorus, flange) should come AFTER your source for distortion.  On a pedalboard, this is pretty easy.  You just put your time-based pedals after your dirt pedals in the signal chain, and all is well.  But what if you are actually using the overdriven preamp section of your amp as your distortion source?  The answer is simple; use and effects loop.

The effects loop consists of two jacks on the back of your amp.  The “send” jack, which takes signal from the preamp and delivers it to the effects, and the “return” jack which delivers the signal from the effects back to the power amp.  A passive effects loop is simply that, two jacks that let you patch stuff between the preamp and power amp.  An active (or “buffered”) effects loop is powered by the amp, and lets you raise or lower the input or output of the loop to compensate for varying volumes on certain effects devices.  Again, one isn’t better than the other, they’re just different.  See what works best for you.


Speaker Outputs…

On the back of the amp, there will be either a series of jacks with different impedance markings (4 Ohm, usually two 8 Ohm, and a 16 Ohm), or one or two general speaker output jacks and an impedance selector knob or switch.  Burn the following sentences into your brain:


This is incredibly important.  You can do some serious damage if you mess this up.  Also, don’t ever use a speaker cable for an instrument cable.  BAD BAD BAD BAD BAD BAD BAD BAD.

Impedance (Ohm’s law) is really difficult to keep straight.  An impedance mismatch between your amp and speakers can seriously damage your tubes, transformers, and the voice coils in your speakers.  Most cabinets will either have their impedance clearly marked, or they are switchable to different impedance settings.  Just make sure that your head and cab are running at the same impedance, and you’ll be fine.  If you are not sure about the cabinet or the amp, have someone with some electronics know-how check them with a multi-meter.  If you’re not absolutely sure, don’t plug in!!!



Speakers deserve a workshop of their own, but we’ll do a short overview here.  Speakers come in infinite variations, and your speaker choice can shape your tone immensely.  Guitar amps commonly use 8”, 10”, 12”, and sometimes 15” speakers, and each size has different sonic characteristics.  The magnets are either Alnico (warm, chiming, expensive), ceramic (bright, cutting, more affordable), and neodymium (neutral sounding, extremely light weight, expensive).  The cone material and construction also affects a speaker’s sound.  Speakers each have an impedance rating, and depending how they are wired together, that rating changes when you use more than one speaker.  Always defer to a qualified amp tech when you are unsure, unless you are willing to arm yourself with the necessary knowledge to understand all this stuff.  Again, different speakers work in different applications, so check out as many as possible.