The solid-body guitar, magnetic pickup, loudspeaker, output transformer and electron tube are the stuff of rock ‘n’ roll and rock music—without these cornerstones guitar tone as we know it would not exist. Moreover, tubes are considered by many to be the heart of any self-respecting guitar amplifier as they perform the task of boosting (amplifying) the small signal from the guitar pickup so that it can drive a loudspeaker to push air around and be heard by an audience. The tube is the component that makes this happen: all other electronic components—resistors, capacitors, inductors and transformers—serve to feed the correct currents (electrons) and voltages in and out of the tube.
So, it seems there is something to this beating heart analogy, but what keeps that heart beating and pumping electrons around? Well, if you look closely at a 12AX7 tube you’ll see, within its glass envelope tiny metal parts. These are known as electrodes. The electrodes, the cathode, grid and anode, are the working parts of the tube, just like the cylinders, pistons, and other moving parts within a car engine or chambers and muscles in the heart. In the tube, the grid controls the flow of electrons from the cathode to the anode, so that a larger (amplified) version of the guitar signal on the grid appears on the anode. Simple, eh? That’s the beauty of tubes—the simplicity of operation—however, how well the tube amplifies a signal is more involved.
For instance, the electrodes must be tightly packed in the glass envelope so that they are rigid relative to one another otherwise the tube will pickup external vibration. This is called microphony and can cause the the tube to ‘squeal’ or self-oscillate when the guitar amplifier is cranked up. Also, tubes can generate noise and hiss if there are issues with the fabrication of the cathode. Stretching the beating heart analogy a little further, the cathode can be considered as the pacemaker, in that it is this critical, delicate structure that dictates the ultimate performance of the tube in terms of lifespan and sound quality. If any part of the tube were to be considered of vital significance to guitar tone then this is it.
The devil really is in the detail when it comes to tube cathode manufacture, but there’s practically no information to be found about it on tube vendor websites or even guitar magazines. In fairness, books on the subject aren’t that easy to come by and are strictly for hardcore tone anoraks, however, if any guitarists or sound engineers are mad enough to want to delve in and explore this rabbit hole I can highly recommend a couple of excellent books written by engineers at Telefunken—if anyone knows about how to blend Barium-Strontium-Calcium electron emissive oxide-coatings and apply them to cathodes made from exactly the right kind of nickel alloy, it has to be these guys! These books are the right stuff and definitely worth a read if you want find out more about how the inner workings of tubes affect guitar tone. There are two volumes which can be downloaded from tubebooks.org
The Oxide-Coated Cathode, Volume One: Manufacture by G. Herrmann and S. Wagener
The Oxide-Coated Cathode, Volume Two: Physics by G. Herrmann and S. Wagener
“The designer starts with something that exists, then modifies it to improve it, or makes a new product. He or she must understand the principles behind the operation of the product and work with, not against them. The designer should observe how a thing “wants to work” and make it that way. The more he or she knows about materials, manufacturing processes, and the intended application, the better the designs can be. The product should be as simple as possible, reduced to its essentials, and made the easiest way.”
—W. A. Dickinson (engineer with Sylvania)