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Re: I'm glad no one said ....

209.214.21.146

It is hard to explain properly in a short amount of words.

A tube is baised to have a certain voltage difference between cathode and grid. Let's say that is 50 volts. Now we know our tube could handle up to a 50 volt postive peak music signal but no more assuming claas 1 operation. The question then becomes what happens when it sees that negative 50 volt swing? If the tube goes into cutoff we have a class AB amp. If it stays in conduction we have a class A amp. The sooner we reach cutoff on the negative swing (=biased for less milliamps) the sooner we reach class B. When we reach class B all of a sudden the tube that is in conduction works into a different load. This point of transistion and the working of the tube into a different load may be audible.

To me any talk of bias point needs to take into account the actual voltage the tube sees and from that the actual wattage the tube dissapates can be calculated. For a class AB1 push pull tube amp I'd suggest a bias point of 2/3 to 3/4 of the tube's rating. I would also point out that when in class A DC current cancel in the output transformer.

So no, you would not reach clipping sooner or have less wattage. If anything you would have more! But you would reach class B mode sooner....but if that is beyond your normal listening levels who cares???


Russ
P.S. Wattage sets the limit on what the tube can do but current is what shortens live....So a tube with 450 volts cathode to anode and 50 ma of current is likely to last longer than the same tube biased at 250 volts and 90 ma.


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  • Re: I'm glad no one said .... - Russ57 15:12:52 03/30/06 (0)


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