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845 20W A1 class monotriode amplifier

845 20W A1 class monotriode amplifier

This project is finalized, that means it is no longer experimental. This amplifier, altough limited in output power (20W), has been designed to give the best listening pleasure.
No compromise has been made during components choice: the 845 output triode is one of the best tube available today in that range of power for single-ended applications, the driver stage uses a 300B, a long revered audio tube (note that you must use a WE 300B or a Sovtek 300B). This technical choice has been made in some commercial (like Marantz T1) or amateur realisations and can be questionnable since the 300B is itself difficult to drive. Nevertheless, the exceptional transparancy of the 845 makes the driver stage proeminent in global tonality and thus must be of the highest possible quality: that explains why we’ll still use the 300B for that purpose. Using an interstage transformer allows a sufficient gain and a good impedance matching with the 845 grid, (it must be noticed, however, that a good interstage transformer is as complex to construct as an output transformer and costs about the same price). In that configuration, the input stage needs only to provide a low excursion (18Vrms). Input and driver tubes are biased by a cathode resistor while the ouput tube has fixed biais (by increasing its plate voltage, it is possible to use automatic biasing: take 1155 V and a cathode resistor of about 2k4).


Previous realisations have bias point computed for WE 300B or Sovtek 300B. Cetron and chinese versions of that tube are not electrically equivalent: bias point must be recomputed.

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