The CTA506 power plant is based on the KT120 power valves. These valves are a new variant of the legendary 6550 / KT88 power valves. For more than half a century, these classic valves have been the foundation of numerous of high quality amplifiers and surely, they will continue to serve as one of the premium choices among manufactures of high-power valve amplifiers.
However, nowadays KT120 is the most powerful dedicated audio tube in production. In comparison with the KT88, nearly 30% more power can be drawn from these impressive valves. With a plate dissipation rating of 60W, a pair of KT120 tubes in push-pull configuration can deliver nearly 200W of audio power. When used in circuit designs of more modest power outputs, the KT120 can deliver incredible quantity of headroom and dynamic without overloading.
When output powers of more than 50W are going to be provided from a power amplifier, the input and driver circuits of the amplifier are confronted with a dramatic increase of current needed to supply the power devices of the amplifier. This is a reality regardless if the amplifier is based on bipolar transistors, Fets, or vacuum tubes technology, and no matter if single high power output devices are used for the design or if the amplifier employs multiple low power devices in a parallel configuration.
Even though Fets-semiconductors and audio valves in theory are voltage controlled devices, these also require relatively low impedance driver circuitry with current capability, partly due to the increase of input capacities and partly due to the often neglected fact that also audio valves are drawing current through their input terminals.
