Les projets en cours

845 20W A1 class monotriode amplifier

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 ..)