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In Reply to: RE: Total NOOB bias Question posted by pictureguy on October 30, 2021 at 11:15:12
In most setups you measure a voltage across a resistor and then use ohms law to calculate the current.Setting the "bias" on output tubes means that you are adjust a negative voltage, connected to the grid of the tube, to control the idle current through the tube. Measuring the voltage across a small resistor connected from the cathode of the tube to ground is an easy way to determine how much idle current is flowing through the tube.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/30/21Follow Ups:
I need to see a 'sample' sketch-matic so I can LOOK at what you are talkng about.....
than I'll expect no issue understanding and hopefully LEARN something.......
Too much is never enough
On the power supply schematic there are two 10K pots. These are the bias pots and control how much negative voltage there is at the grids of the output tubes. One is for the left channel output tubes and one is for the right channel output tubes. The more negative voltage there is at the grids of the output tubes, the less idle current will flow.On the circuit schematic the red arrows point to where the negative voltage from the bias pots go. The negative bias voltage goes to the bottom of the grid resistors. Because there is no current flowing through the grid resistors (or the grid) the negative bias voltage makes it's way through the grid resistors and the grid stop resistors (the 1K resistors) to the grid of the output tubes (pin 5) without being reduced in value.
The 15.6 ohm resistors underlined in blue are the cathode resistors for the output tubes. There is one cathode resistor for each pair of output tubes. When you setup the bias current on a stock dyna st70 power amplifier you use a volt meter and set the bias pots until you have 1.56 volts across the 15.6 ohm cathode resistor. The only reason that there is a voltage across those resistors is because there is current flowing through those resistors.
1.56 volts across 15.6 ohms is 100ma. of current. That would be 50ma. per tube of idle current.
Ohms law, voltage divided by resistance equals current.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/31/21 10/31/21 10/31/21
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