1. Guitar amps
In general, only very low-cost guitar amplifiers (and a few specialized professional models) are predominantly solid-state. We have estimated that at least 80% of the market for high-ticket guitar amps insists on all-vacuum tube or hybrid models. Especially popular with serious professional musicians are modern versions of classic Fender, Marshall and Vox models from the 1950s and 1960s. This business is thought to represent at least $150 million worldwide today, perhaps more (it's hard to determine, as most of the makers of vacuum tubes and vacuum tube amplifiers today are private companies who are secretive about their sales.)
Why tube amplifiers? It's the tone that musicians want. The amplifier and speaker become part of the musical instrument. The peculiar distortion and speaker-damping characteristics of a beam-tetrode or pentode amp, with an output transformer to match the speaker load, is unique and difficult to simulate with solid-state devices, unless very complex topologies or a digital signal processor are used. These methods apparently have not been successful; professional guitarists keep returning to tube amplifiers.
2. Professional audio
The recording studio is somewhat influenced by the prevalence of vacuum tube guitar amps in the hands of musicians. Also, classic condenser microphones, microphone preamplifiers, limiters, equalizers and other devices have become valuable collectibles, as various recording engineers discover the value of vacuum tube equipment in obtaining special sound effects. The result has been huge growth in the sales and advertising of vacuum tube- equipped audio processors for recording use. Although still a minor movement within the multi-billion-dollar recording industry, tubed recording-studio equipment probably enjoys double-digit sales growth today.
3. High-end audio
At its low point in the early 1970s, the sales of vacuum tube hi-fi equipment were barely detectable against the bulk of the consumer-electronics boom. Yet even in spite of the closure of American and European tube factories thereafter, since 1985 the sales of "high-end" audio components have boomed. And right along with them have boomed the sales of vacuum-tube audio equipment for home use. The use of vacuum tubes in this regime has been very controversial in engineering circles, yet the demand for tube hi-fi equipment continues to grow.
Eric Barbour, Senior Editor VTV METASONIX