Les projets en cours

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).

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.

Evolution 3 ("Orphée")

Last evolution of the basic schema, it has been initially developped has a "bread board" for testing a driver stage used in the TB3-1000 amplifier (see below). The driver tube is now a 211, while a WE 300B is used in input. Here again, two interstage transformers are used. This amplifier has the highest transparency compared with the previous ones. Be careful that this is by no means an easy realisation.

Important!

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.

833A/TB3-1000 75W/100W A2 class monotriode

The evolution 3 allows us to test the 211 as a driver tube: in this realization, a 845 will be preferred. The TB3-1000 (or 833A) tube is a very powerful triode that allows about 100W (75W for the 833A) output power in single ended configuration. With such a power, any speaker can be drived. Unfortunately, the price that must be paid for that is a heavy grid current and so a specially designed driver stage. The 833A or the TB3-1000 are directly heated power triodes (10V 10A for the 833A and 12V 8.5A for the TB3-1000), with toriated tungsten filaments, and are designed to operate under 2500 to 3000V of plate voltage. Their mu coefficient is relatively high at 35: their plate curves are mid-range between thos of a low-mu triode and those of a pentode (or power tetrode). The output transformer must be designed to withstand about 7kV on the primary.

Push-Pull 845

Last (?) amplifier using the 845, it is the conclusion of a work aimed at producing a sufficiently powerful amplifier with a very good musicality. Unlike previous descriptions, the full schema of amplifier and power supply will be given, along with building tips, as the project will take shape. Let's begin the story ... Basic structure is that of the evolution 3 amplifier with WE300B as input tube and 211 as driver. Increased musicality was obtained by some modifications. Fist of all, use of an input transformer yielding a better gain distribution between stages, all interstages transformers having now unity ratio. Bias currents have been substantially increased with respect to those of Evolution 3. This gives, specially for the 211, a better linearity. The only drawback is an increased power consumption and a worse overall energetic yielding. Bias point of the 211 is the most important thing (and difficult one) in all the design : 1000V/75mA has been chosen after many experiments (this point is the maximum operating condition for some NOS 211). Power supply uses exclusively valves, except for filament heating, which is rectified by a silicon bridge. Each stage has its own power supply. (to be continued ..)