Tubes come in many types and variations. Designers select tubes based on the specific application, availability, and cost. Commercial tube technology dates back to the 1920s. Over a period of the next 40 years tubes became more specialized and in many cases more compact for a given application or power rating. Modern tube amplifier designs can reach all the way back to the 1930s for a simple triode (three element) tube such as the Western 300B, or spring forward to the early 1960s when some of the most advanced pentode (five element) power tubes were introduced.
In any tube amplifier there is a rectifier that converts AC voltage to DC voltage (this function can be performed by either a rectifier tube or solid state diodes), a driver tube, and an output tube. The driver tube is needed between the input signal and the output stage to boost the voltage of the low level input signal prior to the final output stage. If the amplifier is two channel (stereo), then at a minumum, two driver tubes (or the equivalent) and two output tubes are needed.
In the interest of saving space and cost tube manufacturers, over time, have been able to combine two triodes into a single tube package, with each triode half functioning independently. That is the case with the 6SN7 and also the 12AX7, 12AU7, 12AT7, and 5751 family of tubes. These are known as twin triodes. Similar packaging was done to combine two different tube types into a single tube, such as the 7199 (triode/pentode) and 6GW8 (triode/pentode).
A single triode or pentode tube can be used as an output tube, with fairly limited power, or tubes can be combined in even numbers (pairs) for more power. This design is commonly known as a push-pull (P-P) amplifier and is typically used where more than just a few watts of output power is required.
For the push-pull circuit, the negative half of the audio signal must be phase inverted and sent to the one half of the P-P tube pair. One twin functioning tube can perform the both the driver function and the inverter function for one channel. So in a P-P amplifier there will typically be a twin triode tube (such as a 12AX7) or a triode/pentode tube (such as the 7199) used as both the inverter and the driver for one channel, and a second twin triode tube or triode/pentode tube used as the inverter and the driver for the second aaudio channel. In addition, four (or more) output tubes (usually pentodes) will be used in pairs as the output tubes.
If an amplifier has only two tubes, it is most likely using solid state rectification and one triode/pentode tube (such as the 6GW8) per channel with each tube performing the driver and output functions for one audio channel. As this is not a P-P amplifier no inverter tube is required, however, power will generally be approx. half of the equivalent P-P design.
Fans of triode amplifiers have to go to significant expense to find full-range loudspeakers that are of sufficient sensitivity to be able to be driven to realistic levels with just a few watts of audio power. Headphone enthusiasts can enjoy the same highly regarded smooth midrange reproduction that triode amplification can provide with less concern about the limited power of most triode amplifiers. The AKG K1000 headphones aside, most if not all other dynamic headphones require far less than 1 watt of power to achieve full output. The very low power needed for headphone listening makes headphone amplification an ideal application for a triode amplifier.
