This is trickier than is sounds because of the feedback relationship between the cathode resistor and the current draw, because the cathode resistor sets the grid voltage in tandem with the current.Īs the cathode resistor is reduced in value, the tube will draw more current in order to reach an appropriate grid bias voltage. Of course if you are biasing a single power tube amp like a 5F1 Champ, omit the "divide by 2" in step 5, as you will already have the power dissipation of the single 6V6 You may have to repeat this process several times to get it dialed in, but it is worth it, and your ears will thank you. After installing the new cathode resistor, do ALL of the steps again to see what you now have. If it's 10.5 watts or less, you need to install a lower value cathode resistor.Ħ. For 6V6s, if it is over 12 watts, then you need to install a higher value cathode resistor. This is the power dissipation (in watts) of each tube. Take the figure from step 4 and divide by 2. This will give you the dissipated power (in watts) of both power tubes. Take this value, and multiply it by the current (milliamps) from step 2. Now, subtract the voltage from the cathode resistor in step 1 from the voltage measured on the plates. Measure the voltage on the plates of the power tubes to ground. This gives you the amount of current being drawn by both power tubes in milliamps. Divide this voltage by the value of the cathode resistor. Measure the voltage drop on the cathode resistor to ground. The voltmeter only method requires a few more simple math steps to get you there.ġ.
If you have a bias probe, it is much simpler, as the bias probe is already reading the plate voltage in relation to the cathode, so all you need to do is multiply the plate voltage reading by the current reading to figure power dissipation. The following steps are a way to figure the bias without a bias probe, and only a voltmeter. It may be of help to some of you.Īs with fixed bias amps, cathode biased amps sound their very best when biased correctly. This is taken from a post I posted in another thread.
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