Latest News 15/08/09

We are currently building some high quality Stereo 400w valve amplifiers from scratch, based on a  GEC design of the 1950s with an updated input/driver stage this revised input/driver stage allows more gain to be achieved with less stages of amplification, this in turn insures better closed loop stability than the original design.

The amplifiers are being built for the PA Market and fixed installations as valve amplifiers of this power offering very low distortion are simply not available.

DC Heater supplies will be used throughout eliminating mains hum.

Utilizing 10 KT88 beam-tetrode  power valves in the output stage, coupled to an ultra linear output transformer for each channel, this will ensure a long service life as each output valve will be working  @ 80w per device against a maximum for the KT88 of 100w.

Two different types of output transformers are being offered a toroidal type and traditional open frame, both having been specially wound for the application

The Beam tetrode  ultra linear output stage is preferable to the the pure tetrode or pentode connection for the following reasons -

Output impedance is much reduced from approximately 10k for tetrode and 1.2k for ultra linear. 

Distortion in the output stage is is almost independent of changes load in impedance that occur in a loudspeaker system. 

In a pure pentode connected amplifier using valves such as the EL34, distortion will consist of mostly odd order harmonics, for the KT88 in ultra linear connection distortion will consist of mostly even order harmonics, this distortion is cancelled out in the output transformer - or at least reduced and with the application of negative feedback distortion is further reduced to very low levels.

No additional screen supply is required resulting in higher efficiency.

Various types of KT88s will be tested the performance measured and the results will be published, there is a great deal of controversy regarding the merits of modern  types of valves now available most of it ill informed and not based on measured results.

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