Basically a Swiss Army knife of tone, the THD UniValve is a Single-Ended Class A amplifier head with a single output tube that can be switched at will among many octal-based power tubes... including 6L6, EL34, 6550, KT90, KT88, KT77 and KT66, for different tones without re-biasing the amp. Likewise, the two preamp tubes can be any combination of 12AX7, 12AT7, 12AU7, 12AY7 or 12AZ7. This amp features built-in attenuation for super huge tones at low volumes. Perfect for practice or performance. It comes with two extra THD Yellowjacket Tube conversion sockets. $1199