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Notice, by the way, that I’ve labeled each gain stage as G1 or G3, with the phase inverter and power tubes also labeled. The G1-G2 thing is just my way of keeping track of where the gain stages are in the signal path, because I need to know which ones come first and who listens to whom. We’ll talk more about technically labeling gain stages in a much later article. Check it out…..I have complete control of nearly every gain stage in the amp! It’s a tone machine, once you see how it works. So let's figure out the different ways we can make this amp distort.
You start by going into the first gain stage (G1) straight from your guitar (no pedals yet). Guitar signals are not strong enough to drive a tube gain stage into saturation--not even close. So, all G1 does is give your signal a nice clean boost. Also note that you can control how hard you drive G1 with your guitar volume or your pick.
At this point, if all you did was feed that straight into G2, your second gain stage would be just starting to hit max output, and hence just shy of starting to clip. Still "clean", with no clipping to speak of, yet it still has respectably distorted sound…..what with those fat clean harmonics and all. But if you look at the picture, you’ll see that your signal won't get anywhere close to maxing G2 out because it's going to get reduced by several attenuators, namely, the tone and volume knobs. Those typically take a lot of strength out of your signal. The upshot is that the weakened signal isn't really powerful enough to drive G2 into saturation either…..just the clean-ish flavors of distortion. Pretty, but not dirty.
If G2 could feed yet another gain stage, G2's signal is easily strong enough to push another gain stage well into saturation...so that's just what Paul Rivera soldered in here for us: a third gain stage. Now, if you smacked G3 with a full strength signal hot off of G2, this thing would put out dirt for days, plus a ton of compression, a drastically altered frequency response curve, lots of various harmonics, and several other things. Cool when you want it, but you don't always want it.
So Paul gave us a way to control how much signal G3 gets and, therefore, how much G3 distorts: another attenuator. And…..just to confuse everybody, he called it a GAIN knob. As you might expect, you can drive G3 into dirt oblivion with this knob, but there's a very important notion you need to think about. That wicked strong signal is going right on into everything else left in the circuit path, and it's easily capable of driving every single remaining gain stage into saturation as well. Now you see why we number the gain stages and draw the map…tells ya’ who’s talkin’ to who. More on that in a minute.
What's left? At this point we hit the phase inverter (PI), a unique type of gain stage that makes the peaks and troughs in the signal wave identical for paired power tubes and prevents phase cancellation, intermodulation, and all sorts of noisy things. But, phase inverters of all types are also very capable gain stages in their own right…which means that you can take that hot signal from G3 and saturate the phase inverter very nicely, and pile the tone right on top of whatever distortion you decided to get from G3. See how all the harmonics, EQ shift, compression, clipping, etc., add up as you go? As a matter of fact, even a fraction of the signal from G3 will cause saturation in the phase inverter. What does Rivera do to give us some control? He solders in another attenuator, of course! This time, again to confuse us, he calls it a MASTER VOLUME.
A lot of people get seriously derailed here, and end up despising master volumes because they "suck tone"……and some of the notoriously improperly-wired early ones really did eat tone. Good Master Volume attenuators are electrically transparent. The trouble is, a lot of folks don't understand that what the Master Volume attenuator really does is control how much distortion and saturation occur in both the phase inverter and the power tubes (see diagram).
Think about that distortion onset thing. Turning the Master Volume down doesn't just turn down the volume. It actually takes the phase inverter and the power tubes right out of saturation and most distortion too. So your dirt sounds different because it only comes from the preamp tubes in G1-G3, (the preamp) and you don't get that legendary power section (phase inverter and power tube) flavor that everybody talks about. You also lose all of those cool power section harmonics, the unique brand of compression you'd normally get from the power section, and the changes in the frequency response curve, plus all those subtle saturation effects like swirl. So, yes, trying to use a Master Volume to simply turn the volume down is a gimongous tone suck. The big moral here is: don't use your Master Volume for a volume control. Use it to control for distortion.
Whew! At this point you know that clipping, compression, EQ coloration, and lots of other useful things happen when you have a strong enough signal to push a gain stage into distortion. It’s also nice to know that gain simply means a stronger signal, not distortion or saturation. Armed with all that, you can look at the diagram and see how each knob lets you control for distortion in each gain stage. Controlling for all those effects at each point in the signal path creates your gain structure.
I think you can see why gain structure is such a powerful concept—it literally describes the way you’ve sculpted saturation in your amp. For pro audio (aka…gig), gain structure is much more technically useful, describing your signal management from guitar string to eardrum. You can see where your newfound horsepower is going.
Now that we’re thinking about our entire audio path as one big gain structure, let’s take a quick look at one more step to treasure map nirvana: cascading effects. We touched on this already, but it’s worth a separate mention all its own. As you revisit the diagram, you'll note that whatever you do to your signal quality and strength flows downstream to all the gain stages in succession, piling up distortion effects along the way. You may have also noted that the tone knobs not only control signal strength and, hence distortion, they control which frequencies distort. Finally, you may now have noticed that if you turn one knob up early in the circuit, you can still run the later gain stages as clean as you want, because you can turn all the other knobs down. But, here’s a kicker, and it’s a biggie that bites many an unwary audiophile…whatever you take out of your signal is gone forever. No amount of EQ or FX can ever re-create lost detail, so think twice before you attenuate. Clarity and presence are better than blistering loud or slashing wildly through a tattered mix any day, at least in my humble opinion. Guard that signal like it’s gold.
The picture is getting clearer… you have nearly complete control over where you get your distortion effects in your rig (depending on which flavor you like), how much, and in what combinations. You also have massive control over the quality and detail of your signal, and the degree to which you hear the player through the rig. You just became a pretty powerful cook.
You can see that you have enough control to make almost any single gain stage in the entire amp either clean or dirty, and exactly how clean or dirty. You can blast a single stage or balance/distribute moderate distortion between every stage in the circuit. You like power section harmonics better than the preamp flavor for your power chords? No problem. You simply balance the harmonic content exactly the way you want. In other words, you have gigantic tone shaping power--and that's why audio engineers are so hot on the holy-grail subject of gain structure.
Go back to your amp and draw yourself a diagram like mine. Think about which gain stages are clean or dirty as you dial your controls. Distribute the distortion in all the different ways you can think of. Begin to see how your hands, the guitar controls, the amp controls, the mix, and the room itself are at your disposal to get the signal to the eardrums just the way you want. We’re going to talk more about all the contributing components in future articles, from pickup height to transformer saturation, and even room effects. For now, just take a fresh look at your amp and fiddle with this stuff—we’ll explore more about what it’s really doing after you’ve had the chance to get re-acquainted. I bet you never dreamed you had so many cool amps hidin' in that one little box.
PS... Now to really work this circuit, draw yourself a boost pedal in front of the amp. That's right, another gain stage. Not a dirt pedal, just a clean boost. Remember how G1 and G2 always run clean, and you have only G3, the phase inverter, and the power tubes makin' dirt? Not any more...
Now for the sources and links for further reading...
What is gain?
Power tube vs. Preamp tube distortion
In pursuit of the real deal on mv
Gain structure in hi-gain amps
The thread that this article came from: (my thanks to all the contributors)
Oh, and one link not from TGP...tons of good info in a small space, and an excellent glossary, all courtesy of TGP’s own Randall Aiken...
Aiken amplification's tech pages