This month Thomas Mayer focused on the high mu dual triode. The 6SL7. Due to it's popularity in the 1990’s prices went up for some particular exotic versions. With some luck they can still be found at reasonable prices as 6SL7 has been produced by all the major manufacturers and NOS are still around in large numbers. The reason for the popularity was it's good linearity.
The 6SL7 has also been used as driver tube for small triodes like the 45 or 71A and its possible to build a simple two stage amplifier. Thomas prefer prefer 6SL7 in small signal applications like phono stages.
After being overused and implemented in not so well suited applications their popularity went down.