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feeshta

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About feeshta

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  1. Sorry to revive a zombie thread, I just stumbled across this again. Amplifiers are amplifiers. Some are specialized for different purposes of course, but they still do the same job. Again, sorry to revive an ancient thread. I guess I forgot to click the notify button last time. We could go round in circles on the semantics of tube amp vs tube hybrid all day. You could call it a tube hybrid, but most amplifiers referred to as tube hybrid do not function in the same way. Most are what is referred to as a "tube-preamp", where the tubes just step up in input voltage to what amounts to an otherwise normal, solid state amplifier. The designs I mentioned are not related to the "tube-preamp" style that people are used to seeing, though. The big change in the amps mentioned is that the solid state output stage does not function like a normal solid state output stage. It isn't actually amplifying the voltage of the signal, just the current. They essentially replaced the output transformer with a solid state circuit that does not step down the voltage in order to convert it to current. Instead it passes that voltage along cleanly, and adds in the matching current to follow the input signal. The main benefit is the elimination of the coloration and damping effect of the transformer, though in the case of a guitar amp, there are obvioous other practical benefits such as the removal of a huge chunk of steel and the associated weight. They are not using the tubes to "color" the sound in this case, they are using them for their accuracy, and ability to limit the number of components in the signal chain. An interesting side note, is that in the TU-600 at least, this enabled them to eliminate the AGC circuit. AGC, short for Automatic Gain Control, is basically a feedback elimination loop circuit that attenuates the input when the output starts to get too high. Solid state amps are prone to what is called a cascade effect overload. The long chains of transistors, each amplifying the signal from the former and passing it on to the next, can sometimes run amok when too large an input signal is applied, and end up destroying the weak link in the chain. The AGC is like a compression circuit, that cuts the peaks down to prevent an overload. Unfortunately, this also limits dynamics. Elimination of the AGC yields an amp with much more impressive attack and dynamics. Again, sorry for the zombie thread, I just came across this again today while looking for something else.
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