Q: “I learned a lot from your tip the other day on watts and ohms as it applies to guitar amps. It made me wonder why there are so many different tubes are used on one amp. Fender’s ’65 Twin Reverb comes to mind. I think it has four 12AX7 tubes, two 12AT7’s and four 6L6’s. I assume this is to achieve its particular sound, but I’d like to know more.”
A: Unfortunately a course in amplifier design is well beyond the scope of inSync, but I think I can shed a little light on this. Tubes are one of a number of different types of components that may be used in an electronic circuit. You also have things like resistors, capacitors, coils, transformers, transistors, diodes, etc. Just like there are a number of different types of tubes in a Fender amp, there are also a number of different types of capacitors and other components. They each have a specific job to do. Some may be part of a power supply circuit that supplies regulated power to the rest of the components, some may be part of the tone controls (which are really just amplifier circuits with filters in them), some may be part of the circuit that takes care of what any footswitches do, etc. The list goes on.
In the case of the Twin Reverb the 12AX7’s and 12AT7’s are low power tubes used in the preamp stages. They are both twin (dual) triode, hi-mu tubes, and certainly are part of what gives the Twin Reverb its signature sound. The 6L6 is a high efficiency, high power tube that’s used in the “power amp” section of the Twin. These tubes are part of the reason why those amps are known for their really “loud and clear” sound.
But there are many devices that have some or all of these different tubes in them so there is much more to the given sound of an amp like the Twin Reverb than selecting a few components. The overall design, including rest of the components used and the way they are setup to interact with one another, the circuit layout (or topology), and things like bias settings and other operating levels all play a part in the final sound.