Duncan MacKinnon, founder of Stompnorth Pedals, knows a thing or two about valves in guitar pedals. Below, he dives into how they work and why we love them...
My introduction to building guitar pedals was a Build Your Own Clone – ESV Fuzz. The ESV stood for Extra Special Vintage on account of its Phillips Ac127/01 germanium transistors. Its sound is amazing and is indeed as old school as its name suggests, but I wanted to take "vintage" even further, so I started looking for valve kits and soon found a booster that used a single 12ax7 valve.
I loved the way it sounded and the response it gave, so I knew that valves were the way I wanted to go with my own projects. As a synth player first and foremost, I was used to manipulating waveforms and textures, but even from this hi-tech perspective I still found vintage hollow-state circuitry [i.e valves] offered more pleasing sounds and possibilities than the more modern solid-state, transistor and op-amp driven circuits - which, of course, are also tremendous.
The sensitivity and flexibility of a valve is, in my opinion, unrivalled in guitar pedal building where subtlety, response, transparency and sound quality (not necessarily fidelity) are the aim. A valve in low voltage "starved plate" mode offers a certain type of response and transparency which is different to the more full-on high voltage valve circuits. You can max-out a 12au7 pedal and still know which guitar you are playing. I also think they have a really pleasing and unique sound all of their own. This is the reason I designed one of my own pedals around a 12au7 running 9 volts.
Stompnorth's Clipshear Getter Drive is the company's flagship pedal, featuring a 12au7 valve along with MOSFET transistor and Silicon & Germanium Diodes
How a valve works
A valve can be thought of as a glass tube, with an anode (plate) at the top, and a cathode at the bottom. Electricity in a valve flows in the opposite direction to the accepted norm of positive to negative, and it does so in a wash of electrons called space charge; this is the bit I love the most! When you turn a valve on, the cathode has to warm up, and then space charge is produced. This flows from the cathode to the plate through a vacuum. Not carried by radio, or wire of any kind, but literally floating through space, a tiny bit of outer space, (with flying saucers and ray guns), right there in your pedal.
Your guitar signal enters through another internal component, the grid, which sits in between the plate and cathode. When a signal with positive potential enters the grid it gets caught up in, amplified by, and carried in the flow of space charge all the way to the plate, then on into your amplifier in an awesome shower of tone-bearing electrons.
Another fascinating thing about valves is their sheer theatre. They are huge in comparison to other components - you can see the internal workings through the glass. Also, each one is slightly different inside. The getter flash (silver cap) is another brilliant factor of valves. The science is fascinating and they look great, but this subject is best saved for another occassion.
In short, I think valves are magnificent, eloquent, and beautiful pieces of engineering that have un-reinventable qualities, which I think is why they are still so commonplace in guitar gear (and high end audio gear) today. They were not replaced by the transistor as we were told they would be - valves are here to stay.