"Sweep tubes" refers to types of tubes that were designed for the horizontal-output circuit of tube-type TVs. They generally have reasonable current ratings, high voltage ratings, good gain, and used to be cheap and widely available. The classic DIY amplifier application was not for audio, but for amateur radio as a linear RF amplifier.
On the downside, they're desinged for high-impedance place circuits, so an output transformer to utilize them in a traditional audio amp would have to have a very high turns ratio, which would compromise performance. Now for a very high-impedance application they can work well, such as the Acoustat direct-coupled electrostatic-speaker amplifiers, which indeed used sweep tubes.
But there's also the reason that since it's been what, 35 years since tube TVs were common? So they're not nearly as cheap and plentiful as they used to be, meaning that there's no longer much of a reason to find uses for them so tangential to their original application.
On the downside, they're desinged for high-impedance place circuits, so an output transformer to utilize them in a traditional audio amp would have to have a very high turns ratio, which would compromise performance. Now for a very high-impedance application they can work well, such as the Acoustat direct-coupled electrostatic-speaker amplifiers, which indeed used sweep tubes.
But there's also the reason that since it's been what, 35 years since tube TVs were common? So they're not nearly as cheap and plentiful as they used to be, meaning that there's no longer much of a reason to find uses for them so tangential to their original application.