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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: MB3045 KT88 Bias Levels posted by geraldm121 on June 18, 2009 at 08:00:23:
Instead of using a guideline of 0.5v or 0.75v as bias measurements, you can understand the true tube dissipation pretty easily.
What you are measuring is the voltage drop across a fixed resistor. If that resistor is 10 ohms and the voltage drop is 0.5 volts you can use Ohms law (Volts/Resistance=Current) to calculate the current passing through the resistor. In this case 0.5v/10ohms = 0.05 amps, or 50mA.
Just for the sake of simplicity, let’s assume that the current is from the tube’s plate. If the tube has 400 volts on the plate and 50 mA of current, then the tube is passing 400 * 0.05 = 20 watts. A KT88 is rated for 42 plate watts. You can go up to 80% of max plate watts without too much trouble. I like 70% on average. 70% plate watts would be 42 * 0.7 or 29.4 watts.
With 400 volts and 29.4 watts, your current is 29.4/400 or 73.5ma. Using your 10 ohm resistor and Ohms law (reconfigured as V = R * C) then V = 10 * 0.0735 or 0.735 volts across your 10 ohm resistor.
Note: I do not know the value of your current test resistor, if it is for one tube or both tubes in a push pull pair, or the volts on the plate of your output tube. Measure these first. If you learn to calculate what you want, you will not be haunted with uncertainty.
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Follow Ups
- Bias Levels - Chris O 08:58:05 06/18/09 (0)
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