Gain staging and tone shaping refers to the fact that not just the amount of gain, but also where in the circuit it’s generated, has a huge impact on the tone and playing response. I’ll demonstrate this in the accompanying video.
My amp of choice for these examples is the THD Flexi 50. This amp has the somewhat unusual attribute in that it has two gain controls on its single channel. The first one is the typical input volume, or gain control. The other one is located later in the circuit in a switchable boost function.
What’s the Point?
So, what’s the point of two gain controls? Why not just build extra gain into one control? Well, apart from some technical limitations concerning how much gain you can get from a single tube, there’s also the issue of control: where in the circuit gain is produced has a profound effect on the final result.
Gain Early or Late?
Let’s start with the first gain control. Labelled “Volume” this controls the output from the first preamp tube. Turn this up, and progressively more overdrive will be added to the signal. It’s a thick, fat and compressed overdrive, which – depending on what kind of guitar and pickups used – can sound kind of bloated and loose. At the top of the pot’s travel, you’ll get sag. Sag is a particular kind of compression that lets the initial attack of the note through, but then ducks the volume momentarily before coming back up, letting the note sustain normally. It can sound good, and I firmly believe that it’s a question of application – it’s neither good nor bad in itself – but for the most part it’s a bit too rich for my taste.
The second gain control is a part of the boost circuit, as I’ve mentioned. This circuit also has a single tone control that works a bit like a tone control on an overdrive pedal would. As the boost circuit is placed further downstream, it behaves differently from the input volume control. The tonality is different, and the dynamic response is different. Tonally it’s a lot brighter and it sounds more aggressive. It doesn’t get smooth in the way that the input gain does, and its response is more of a “kerrang” than an “ooh” (sadly, onomatopoeia has its limits – these were the best I came up with). The input gain evens out volume differences, whereas the boost is more dynamic and has a more sudden transition from clean-ish to overdriven.
Tone Control Interaction
A small (not really) caveat: what I’m about to say is a
massive speculation qualified guess. Unfortunately, schematics for the THD Flexi are thin on the ground, so I have no way of knowing if what I’m saying is correct (apart from ripping the amp open, which I’m not about to do). Suffice to say that the way the tone controls respond leads me to believe that they are placed before the first gain stage. The reason for this is that turning up the bass control makes the low end quite soft and flubby, and somewhat indistinct. Turning it up with most of the gain coming from the boost circuit, results in a tighter and more assertive bass response. This is somewhat confusing, as the boost is even further downstream from the tone controls, so you’d expect even more flubby bass. Even so, this isn’t the case, and my guess is that the tone controls aren’t connected directly to the boost circuit, but may run in parallel (yet another massive speculation). Let me know if you happen to have the facts here.
Knowing the inner workings of the amp shouldn’t be a prerequisite, and I’ve just come to think of the input gain control as a “warmth and fatness”-dial, and the boost gain control as an “aggression and clarity”-dial. As is often the case, these differences are easier to feel than to hear. The sagging is especially noticeable when playing, but a lot more subtle for the listener.
My advice would be to be careful with the bass control if you’re running the input gain high, as this can cause an unfocussed and smeared low end that is impossible to get rid of by any other means. I would be very careful with the Cut control, as it adds a lot of upper midrange that is very unpleasant. At the same time, I think one needs to accept that the basic voice of the Flexi is bright, so don’t turn down the Treble control too much, as this also robs you of some clarity.
In trying to wrap things up, I’d say that gain staging and tone shaping are two sides of the same coin. The way overdrive and gain are generated, and the way it interacts with the tone controls, defines a lot of the basic voicing of an amp. In the Flexi you have opportunities to experiment with gain staging at different points in the circuit, which gives this little one-channel amp a lot more flexibility than you’d expect. Conversely, this also adds to the complexity of the amp. I’ve been the owner of this amp for close to ten years, and I‘m still finding new stuff out all the time. Hopefully, recognising the way the amp responds and interacts with the tone controls will provide a good starting point for further exploration.