mblemmy wrote:
...that's when I noticed no one ohm resisters on 1/8 to ground just wired straight to ground.... I was under the impression those are used sorta as a fuse to protect the rest of the board if the power tube for some reason got shorted out.
No, that's not at all the function of the 1 ohm resistors. They are installed as a means to safely and conveniently measure plate current when biasing an amp's output tubes. From the
Aiken Amplification "The Last Word on Biasing" page:
"
The plate current can be indirectly measured by determining the cathode current in the tube. Since no appreciable current flows in the control grid of the tube, all of the plate current must also flow in the cathode. The cathode current, however, will also have all of the screen grid current flowing in it as well, since the cathode current is the sum of the plate and screen currents. Typically, the screen current in most commonly used pentodes is around 5mA (this varies, of course, depending upon the class of operation, the bias point, and the type of tube used). The screen current can be accurately measured by determining the voltage drop across the screen resistor and dividing by it's value (for example, 5V across a 1K screen resistor indicates 5mA of screen current). If you install a 1 ohm resistor in the cathode lead of each output tube, you can measure the voltage drop across it to get the cathode current, as described above in the plate resistor measurement. The advantage of this method is that there are no high voltages involved, since there will only be a few millivolts difference between ground and the other side of the 1 ohm resistor. The disadvantage is that you must subtract the screen current in order to accurately determine the plate current. However, since the screen current is only a few mA, it can usually be ignored, and the error will be in the conservative direction, i.e., less plate current than expected, which is good for tube life. This method of biasing is the most highly recommended. There are a few bias meters on the market that use this method, including the SwAMProbe (it doesn't measure across a resistor, rather, it breaks the cathode circuit so you can insert a milliammeter for direct cathode current measurement), and the Bias King. These devices have socket adapters that go in between the tube and the socket to make it easy to measure the bias current on an amplifier without having to modify it."
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