20W Power Tube 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).

Tube sockets are ceramice with gold plated pins. Power supply uses only polypropylen capacitors (special order) after the filter chokes and filaments are DC heated.


Evolution 1

In this version of the 845 amplifier, the input stage is modified in order to suppress the global feedback loop. The two parts of the 6sl7 tube are wired in a local feedback configuration, with a small amount of positive feedback. The schema has been used on many preamplifiers and amplifiers in the 50s-60s, and is well suited for our application. Its distorsion is less than that of the whole driver+power chain. The 300B is now capacitively coupled, for DC matching and for allowing a lowering in plate voltage, which in turn makes a multifilar interstage transformer possible (BP of 300khz!).


Evolution 2 ("Erato")

In this variation on the same output tube, all feedback, either global or local is suppressed. The 6sl7 is no longer suited as an input tube and a directly heated triode must be used. The choice has been made to use the 45, a very good medium power triode, still available as NOS. The interstage transformer between the 300B and the 845 has now a double output impedance, while a second interstage transformer is used for coupling the 45 and the 300B. Here again a multifilar winding is possible (and recommended!). More difficult to tune than the Evolution 1, the filament of the 45 must be DC regulated (NiCd batteries can be used). There is a gap in listening performance between that version and base amplifier, but it is much more expensive.