Knowing more is the other half of the battle ...

In my last post, I discussed the basic workings of the guitar, its electronics, and did my fair share of step exercises by climbing on and off my soapbox.  That's what boredom will do to a guy - or at least what happens when I don't have a real project to play with.  Happily, my parts should be in for my new amp build and then I can discuss real stuff.

Today, my plan is to unveil some of the mystery surrounding amps themselves and give another high-level view to these mythical beasts.

A little of the basics

There are probably as many amps and types of amps as there are types of guitars and players.  It's impossible to cover everything with any accuracy because there are just too many variables.  However, for this, I'm going to cover some concepts that have been present since the beginning with the Fender Bassman or the early Marshall amps.  These concepts do tend to be the common benchmark for many amps today, so it's a fair representation, as far as that goes.

I can't say this enough, this is just a high-level educational overview.  Some of the material covered will not be perfect or even completely technically accurate except to provide a teachable example of the concept.  This means, so go blowing yourself up if you decide to poke around in an amp because of something you thought I was telling you here.

Gain Stage 1

A Fender amp is a very cool thing.  I've owned many over the years and still have one now.  It's a classic and the early ones had a small bit of magic in them - probably from being such a prominent part of the dawn of rock and roll.  But, for as much as I worship at the altar of Leo Fender for many things, including his business acumen, his amp designs, although legendary, were not original or novel by any stretch.  Many of the concepts brought out were simply adaptations of earlier radio amplifiers.  This means that for the amp enthusiast, there are many authoritative works that can be explored that predate the 1959 bassman 410.

Here is a basic schematic of the Bassman 5F6, which is almost the archetype by which many amps model themselves.

But, let's start at the beginning.

Our signal has left our guitar and has hopefully traversed a very good quality cable.  Good quality is very important because a bad cable will let in noise, interference, and just poorly transfer signal from the guitar to the amp.  Now, this is important to recognize the variables we have here.  Our output signal from the guitar is somewhere between 20 and 750 millivolts.  This is a very small signal that is very sensitive to outside influence.  A good cable with good shielding is critical to ensuring that signal arrives at the input jack of the amp intact.

So, assuming our signal arrives in proper fashion, our amp has to convert that signal to between 5 watts and 150 watts to power our speakers.  The output wattage depends on many things, but usually we are running somewhere around 20 - 50 watts given a normal combo amp.  The trick of our amp is converting our say 100 millivolt guitar signal into the crowd pleasing roar of the speaker output.

Stage 1

Upon arriving at the input jack, out signal runs to the first preamp tube.  This is our first gain stage that amplifies our signal into something the amp can use in further stages.  Generally, this is a tube in the 12A_7 series.  It could be a 12AX7, a 12AY7, or even a 12AU7.  Each of these tubes are roughly of the same family and essentially convert signal to voltage at different levels.

As the signal enters the amp, it is pushed through various resistors and capacitors to modify the expected sound.  Think of the "Normal" and "Bright" or "Bass" inputs on a typical amp of this type.  The tonal differences of the inputs are achieved with similar but slightly different signal paths winding through various resistors and capacitors to "bleed off" certain higher frequencies to ground.  (For a more complete discussion of this concept, see the first part of this series that talks about the function of capacitors in guitar electronics)

Gain Stage 2

As the now amplified signal progresses through the preamp circuit, it encounters some variable resistors.  As we know, variable resistors are potentiometers or pots.  In this case, we encounter the volume pots for the channel.  Along with this are some additional signal modifying capacitors and resistors.  This helps to "shape" the raw tone both before and after the volume pots so that is it more reflective of the individual inputs.

Stage 2

For example, in the Bassman circuit, the signal for the "bright" channel progresses through a .02 picofarad capacitor into the 1M ohm volume pot and then onto the 12AX7.  The "Normal" channel goes through a .02 picofarad capacitor, hits a 1M ohm volume pot, and then the signal goes through another .0001 picofarad capacitor before progressing onto the 12AX7.  This simply means that the "bright" channel is actually more pure than the "normal".  The additional cap on the "normal" channel bleeds off very high frequencies (the same as it does on a guitar's tone pot when turned off).

At this point, the two channels' signals converge into the next stage.

Gain Stage 3

After leaving the second 12AX7, the now unified signal enters the tone stack.  In later Fenders, there are distinct tone stacks for the different channels, especially as one channel had reverb and vibrato while the other didn't.  In the case of the classic Bassman, there was one tone stack.

Funny enough, there isn't too much difference between the concept of the simple guitar tone control and this stage of the amp.  Pots adjust level while various values of capacitors bleed frequencies off to ground.  Put the different values together in a series and voila, you have tone controls that allow subtractive shaping of the tone by signal modification.

Stage 3

Subtractive Shaping? … What are you talking about?

When we look at the amp controls, we usually see our pots labeled from 1 - 10.  Naturally, we assume that if we start at 1 on the Bass control and twist it to 10 that we are adding bass into the mix.  However, in most cases, this is exactly the opposite.  Instead of adding anything, we are actually moving the signal closer to natural by simply removing any frequency attenuation.  That means that when all of our tone controls are on 10, that is really what the "raw" signal sounds like.  Obviously, "raw" after the internal modifications I talked about earlier.  Note that some amps do employ active tone controls where signal is added to the raw signal to boost frequencies, but it is more common for this subtractive shaping.

Gain Stage 4

Once our signal has passed through the tone section, it hits one last 12AX7 where the final signal is modified, balanced, and sent off to the power tubes.  I'll cover the power amp stage sometime later, but for now, let's recap quick.

Stage 1:  Input Jack to first preamp tube
Stage 2:  Preamp tube 1, through volume controls to preamp tube 2
Stage 3:  Preamp tube 2, through the tone stack to preamp tube 3
Stage 4:  Preamp tube 3 to the power amp

Stage 4

So where does preamp dirt come in?

One of the coolest things about tube amps is driving them hard.  Hard enough to create very pleasing distortion tones.  In general, this happens by driving successive stages harder and harder until they distort.

For example, if we have a very hot signal coming in to the amp, say from a very powerful pedal or active pickup system, we take the chance of overdriving our input stage.  This happens because we are feeding more power to the first stage than it was designed to expect.  Of course, the problem comes because the rest of the amp is designed to buffer distortion from early gain stages through the use of resistors and capacitors and signal routing.

However, there are techniques for driving successive stages harder and harder until they distort in very nice ways.  On Fender amps like this, one trick is to change out preamp tube 1 (V1) from a 12AX7 to something like a 12AU7.  The 12AU7, while a lower powered tube, actually tends to send a hotter signal down the line, which causes a harder signal to hit the second tube (V2).  This will cause an earlier break up because as the volume is turned up, more voltage passes through at a lower volume.  For an example of this, check out my posting concerning the mods I did to my Carvin MTS3200 or some of the changes I made to the tubes in my Fender Twin Reissue.

While some of that probably is clear as mud, the important thing to take away is that through creative use of voltage, we can drive the preamp harder as the signal winds its way through the circuit.  This will create preamp overdrive.

Next time around I'll discuss some of the relevant output stages including power tube distortion, negative feedback, and other fun stuff.  Or my parts will get here and I'll have something else to do instead of talking theory.  (The joy of waiting for custom wound transformers that are on back order)