"I could use a really huge resistor to bias the tube on just a little and be close enough, right?" If there is no tube current, there is no bias voltage across the cathode resistor. Obviously, the other side of the push-pull output picks up the slack.īut Voltage = Resistance * Current. That way, the whole positive signal cycle causes plate current to flow, while the tube is cut off for the negative half-cycle.
To do this exactly in accordance with the definition, you bias you tube right at the point plate current cuts off. Let's go to the other extreme: textbook-definition class B.Īccording to the definition, in class B the tube passes current for exactly 50% of the input signal cycle (class A is 100%, class AB is >50 but <100%). So I think we understand class A is easy to cathode bias, because the average tube current doesn't change much. Called LooseChange by the wrong name (before edit), and also forget to prove why deep class AB is hard to do with a cathode resistor. But you'd have to have a very compelling reason to go through all that trouble to use a bunch of extra circuitry instead of a single resistor. I suppose you could use servo-biasing to build safety into fixed-bias, so that it could sense and counteract an overcurrent situation. If there's a short, the cathode resistor will burn up before something expensive.įixed-bias will cheerfully let your tube burn up, along with taking out an output or power transformer. If the tube draws too much current, bias voltage increases and turns the tube more-off.
The old-school rule of thumb was use cathode biasing eveywhere you could, because it helps prevent catastrophic failure of the tube. I'd suggest you shouldn't pick biasing method based on tone differences. I think I recall him saying that the sonic difference was minimal I'm guessing that's because there wasn't a big shift in the operating point when going back and forth between the biasing modes. Think about it: no one ever complained their preamp was squishy and compressed, yet almost every guitar preamp is 100% cathode biased.Īsk LooseChange how different the sound is between fixed/cathode biased in his Standard. You might use a technically too-small bypass cap along with cathode bias in an amp trying to push deep into class AB that will cause a difference in distortion and compression, but it's almost more about a misuse of cathode bias.Īs a result, I think a lot of the difference is a comparison of two amps with the different biasing methods which have too-different operating characteristics to make a fair comparison. Cathode biasing the same tube into deep AB doesn't work too well, because it's harder to turn the tube more-off with a resistor after you reach a certain point. Fixed biasing makes it easier to run the tubes with much higher plate voltage, while idling at much less current.
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