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The Harp of New Albion
A somehow poetic name relates to the rich harmonic content of tube amplifiers (and to the piano piece by American composer Terry Riley).
I wanted to build one for years and it came to life a decade later (with the know-how). It came to life with a realization that it is all the truth (the musicality of the vacuum tube).
Amplifier was built around two broken down monophonic power amplifiers by the Chech(oslovakian) firm Tesla. Probably they were built in the sixties. They used the dual parallel pentode push pull principle, and one of the power supply transformers was missing. I used the two output transformers and a power supply transformer. Instead of pentode connection I used the triode connection to the output transformers and a very low amount of feedback loop. Surprisingly the electromagnetic hum was non-existent!
The four power pentodes (EL 34) – two in push pull per channel are located in front – behind the aluminium rods (also parts of original amplifiers). Behind these tubes is a metal heat reflector, since the quiescent current through the pentodes is quite high.
The driver circuit is something that I devised myself – a hybrid transistor differential/cascode amplifier serving as symmetric phase splitter driving the double preamp triode ECC 85. The driver stage works with relatively low voltage (150 – 170 Volts). The power pentodes work with the usual 450 Volts. This should supply around 30 Watts per channel (I didn’t measure it), which is perfectly enough for the home use.