![]() ![]() Hi Kevin, it sounds like the blocking you're talking about is the cathode bypass capacitor charging up? IMHO Autobias amplifiers to me usually (not always) sound more euphonic and less detailed, but for this reason may be more forgiving on bad sounding material. I have designed power amplifiers around everything from a 6BQ5 to K90 as well as dhts such as the 45/2A3/300B using fixed bias. (Larger cathode caps needed for better bass response exacerbate this due to the longer time constant of the bias network) Note this is a simplification and this phenomena can be made even worse by bad driver stage design, but in general fixed bias designs can and do recover more gracefully from overload. Good fixed bias designs recover much more gracefully from sustained clipping where the output tubes in a pp autobias design may be driven so far into class b that when signal drive is reduced to normal (not clipping) levels the output tubes actually cut off for some period of time - this is called "blocking" and sounds truly terrible. (The power dissipated in the cathode resistor is wasted energy and can be a very significant % of the overall power consumed in an output stage. This also makes more efficient use of the available B+ and wastes less power than autobias. I generally prefer fixed bias in my designs both SE and PP as it allows me to easily fine tune and find the "sweet spot" operating point for the output tubes I use, and perhaps more importantly it eliminates the need for high quality cathode bypass capacitors. ![]()
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