Pre/Power Amplifiers

ARC’s founder, the late Bill Johnson, had a soft spot for the ‘sound’ of the 6550 power tube, a longstanding design developed by New Jersey-based Tung-Sol back in 1955. This tube, alongside the EMI/Marconi KT88 ‘kinkless tetrode’, was one of the first valves designed specifically for the guitar and hi-fi audio market. Fast-forward some 70 years and the KT170 power output vacuum tube is the latest Tung-Sol branded design from the New Sensor Corporation, which owns the Tung-Sol brand name alongside the familiar Sovtek and Electro-Harmonix marques. The KT170 is manufactured by a US-linked factory in Russia whose tube production was initially subject to export restrictions following EU/US import sanctions in response to the invasion of Ukraine. Self-evidently a solution has been secured, albeit at a cost.

In practice, the KT170 is a development of the KT150 and the KT120 which was launched in 2009, the latter substantially more powerful than the longstanding and now less commonly-used KT88. Today, the KT170 is billed as ‘the most powerful octal beam tetrode ever produced’, with a greater plate voltage headroom, maximum cathode current and plate dissipation than any of its antecedents. Specifically, the KT170 has a plate (anode) dissipation of 85W (sufficient for a pair to support amplifiers rated over 300W), against 70W for the KT150, 60W for the KT120, or about 40W for a KT88. Its ‘original shaped’ bottle has been retained to optimise vacuum retention and maximise heat dissipation under high working loads. The tubes are driven in the REF 330M via SiC (Silicon Carbide) J-FETs to optimise both the auto-biasing and ‘low-end control’. PM