Just curious whether these amps are self-biasing.
Alex (who can't wait to get started on this project!)
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Cathode biased?
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Jason
Alex,
Yes, cathode biasing can be thought of as being 'Self biasing" to a certain degree. In a tube, plate current and cathode current are almost the same amount when operating [for practical purposes!]. Since there is a resistance in the cathode circuit, according to Ohm's Law , and since there is current drawn through this resistance, a voltage drop develops across this resistor. It is a positive voltage. Now, in order for the tube to act as a "valve", the grid needs to be negative with respect to the cathode. This is what fixed bias does. It injects a negative voltage at the grid and thus it is negative with respect to the cathode. With cathode bias, a positive voltage is developed across the resistor, and in turn makes the cathode more positive with respect to the grid. The tube sees this as the grid being made more negative.
The reason the grid must be negative with respect to the cathode is because AC audio signals are sine waves. In a sine wave, the average amplitude is 0--because 1/2 of the wave is cycling above the "X" axis, and 1/2 of the wave is cycling below the "X" axis [abcissa]. In order for an incoming AC signal to regulate the tube as a "valve", the grid must be held negative enough so the incoming sine wave can control the grid as it cycles from negative to positive. This is how the tube changes it's conduction throughout the input cycle. Classes are a whole other subject, and I recommend Randall Aiken's website for excellent and easy to follow tech info.
As for biasing your amp--I recommend always at least checking your bias every time you use a different set of tubes. You may not necessarily "reset" the bias every time, but you can at least verify that the tubes are operating at their optimum points within the circuit. I find that cathode bias is tolerant of differences in output tubes as long as the plate voltage is sane, screen voltage and current draw is sane, and the OT primary is about right for the tubes in use. In the 18 Watt, it is pretty easy to get this circuit right on the money IMO. Although, this circuit can be a bit tricky as well--especially in the areas of plate/screen voltage changes. I'd recommend building the stock circuit and then tweaking to taste. It's a very fun amp to build, tweak, play, and look at. Worth the effort.
Jason
Yes, cathode biasing can be thought of as being 'Self biasing" to a certain degree. In a tube, plate current and cathode current are almost the same amount when operating [for practical purposes!]. Since there is a resistance in the cathode circuit, according to Ohm's Law , and since there is current drawn through this resistance, a voltage drop develops across this resistor. It is a positive voltage. Now, in order for the tube to act as a "valve", the grid needs to be negative with respect to the cathode. This is what fixed bias does. It injects a negative voltage at the grid and thus it is negative with respect to the cathode. With cathode bias, a positive voltage is developed across the resistor, and in turn makes the cathode more positive with respect to the grid. The tube sees this as the grid being made more negative.
The reason the grid must be negative with respect to the cathode is because AC audio signals are sine waves. In a sine wave, the average amplitude is 0--because 1/2 of the wave is cycling above the "X" axis, and 1/2 of the wave is cycling below the "X" axis [abcissa]. In order for an incoming AC signal to regulate the tube as a "valve", the grid must be held negative enough so the incoming sine wave can control the grid as it cycles from negative to positive. This is how the tube changes it's conduction throughout the input cycle. Classes are a whole other subject, and I recommend Randall Aiken's website for excellent and easy to follow tech info.
As for biasing your amp--I recommend always at least checking your bias every time you use a different set of tubes. You may not necessarily "reset" the bias every time, but you can at least verify that the tubes are operating at their optimum points within the circuit. I find that cathode bias is tolerant of differences in output tubes as long as the plate voltage is sane, screen voltage and current draw is sane, and the OT primary is about right for the tubes in use. In the 18 Watt, it is pretty easy to get this circuit right on the money IMO. Although, this circuit can be a bit tricky as well--especially in the areas of plate/screen voltage changes. I'd recommend building the stock circuit and then tweaking to taste. It's a very fun amp to build, tweak, play, and look at. Worth the effort.
Jason
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Alex_Law