After you've finished assembling a brand new guitar amplifier you can move on to the next and most important step, tweaking and testing. I know it doesn't sound like the most glamorous job but it is, with out a doubt, the most important part of building a guitar amplifier.
It's critical to point out that tweaking an amplifier requires a fundamental knowledge of how guitar amplifiers are designed and how the actually do the things we want them to do. Because every aspect of our amplifiers are built on the principle of overdriving the amplifier to intentionally distort the sound, without understanding how overdrive and distortion work inside of the amplifier, you are essentially throwing darts at a dartboard in the dark. There can often be multiple different ways to get the desired changes, but changes in a guitar amplifier can often cascade downstream and produce other changes that might not be as desired. Before you go on to read the rest of this post, check out some of my other writing on overdrive and distortion in guitar amplifiers.
Recall what I said in part one about a guitar amplifier being the sum of all its individual parts. I also wrote in that post that each electrical component has a tolerance, a small variation in the value of that component. If we have a capacitor and the value of that capacitor is supposed to be .1 uF with a tolerance of ±10%, the real value of that capacitor could be anywhere from .09uF to .11uF. Now if we take a tally of all the parts in an amplifier, even a simple guitar amplifier, you'll see all of those tolerances will very quickly add up to make some really big differences. This doesn't even take into account the parts of the amplifier that are distinctly unique, such as your preamp tubes. Take a working amplifier and switch out one of the preamp tubes, probably a 12AX7, with another 12AX7 and see how dramatically the sound will change.
When we tweak or dial in a guitar amplifier, we're going through the amplifier and changing some of these sensitive components to better suit other components and shift the way the amplifier operates to make it sounds the way we want. Now if you've been paying attention I said that an amplifier is the sum of all its parts, but if all of these parts are unique and they all effect the sound, where do we start? There's no point to making changes inside of an amplifier before you've even heard what it sounds like. You can assemble a guitar amplifier exactly the way it's written in the schematic and when you plug it in, it sounds amazing. But sometimes you test that amp for the first time and it has a very brittle treble response, also referred to as ice pick highs. Maybe the amplifier sounds boomy and bass heavy, which is usually not what we want from a guitar amp. With the right knowledge, swap a few parts around and your newly assembled amp can sound completely different.
Having a good baseline to work from is critical to tweaking an amplifier, and the key to creating a good baseline is picking some key parts to hold constant, and testing the amplifier with these constants.
I'll say it again because it may be the most important thing you need to know when going to tweak an amp, they are the sum of all their parts. So with that in mind, what parts do we use as our baseline and what parts do we single out to tweak and change? The answer to this question is more straightforward than it may seem. On the list of parts that drastically change the sound of your amplifier the vaccuum tubes are number one, typically followed by your speaker and transformers (yes, regardless of what anyone tries to tell you, the transformers do significantly affect your sound). Now the PT and OT aren't parts that you'll swap in and out like you would a tube or capacitor so in general, hold the PT and OT as a baseline and find a good set of tubes you like and hold on to those. That's your starting point.
One of the natural questions that may pop up from that last paragraph is; if the tubes can have such a significant impact on sound and preformance, why do we use them as part of the baseline? Remember that all we're trying to do is tweak the sound of the amplifier, so using one set of tubes then making a few tweaks and popping a new set of tubes in is gonna defeat the purpose of the changes you just made.
So now that you've established your baseline, you're gonna test. This is one of the fun parts because it doesn't invole any work inside the amplifier or math with the vaccuum tubes (which we'll get to later). All you have to do is play your amp... a lot.
You need to hear how the amp resonds to all the different settings and styles of play. Play it loud, play it quiet, high treble with low bass, high bass with low treble, especially play it the way you are actually going to play it. While it's important to test the amplifier from all perspectives, it makes very little sense not to test the settings you would actually use when you put it on a stage or mic it up in a studio. The key here is to get as good a feeling for how the amplifier is working with the current set of components and identify the problem areas or things you do not like.
I already listed two of the most common problems with amplifiers above; harsh treble response (usually sounds very brittle and cold) and overwhelming bass response (boomy even when the bass knob is turned down low), but these are just two examples. There are quite a few other problems like
| Test Number | Pickup | Guitar Volume | Guitar Tone | Normal Volume | Bright Volume | Tone |
|---|---|---|---|---|---|---|
| 1 | Neck | 10 | 10 | 5 | 4 | 6 |
| 1 | Neck | 10 | 10 | 5 | 4 | 6 |
| 1 | Neck | 10 | 10 | 5 | 4 | 6 |
| 1 | Neck | 10 | 10 | 5 | 4 | 6 |
| 1 | Neck | 10 | 10 | 5 | 4 | 6 |