Biasing of the Power Tubes

Bias voltage applied to the grids of the power tubes in the output stage should be checked from time to time (once a month) to keep these tubes operating at the optimum operating point. A properly biased power tube will have a longer, happier life.

To check the bias of the 6550C/KT88 tubes, place the amplifier in mute with the power on and the amplifier in operate mode (STANDBY LED Green; Power LED Red-for MUTE). Looking at the tube top, take note of the power tube location and the bias LED below the surface of the chassis next to each tube. Also note the printed line that leads to each power tube. At the opposite end of this line is the bias adjustment for each power tube, a small slotted metal shaft, again located below the chassis surface. See reference dia- grams for further clarification.

If a bias LED is glowing red the tube is overbiased; if a bias LED is glowing green the tube is underbiased, both conditions cause undue wear and/or are not optimal for the tube. The LED should not be glowing at all for the proper operating bias.

To properly bias the tubes is a simple procedure. For the power tubes on the left (LV5 to LV8) insert the biasing driver into the slot head of the bias adjustment control and turn the driver to adjust the control counterclockwise so the LED is green. Turn the adjustment back clockwise until the LED begins to dim. At this point; through careful adjustment, continue slowly turning the bias control, stopping the moment the LED goes out. No light from the LED indicates a properly biased tube. For the most accurate biasing, the biasing controls should always be rotated from the LED emitting gre e n to the LED emitting no light, stopping immediately, as soon as the LED emits no light. Repeat procedure on the next tube until all tubes are properly biased.

For tubes on the right side of the amplifier (V5 to V8) the biasing controls are opposite. Turn the biasing controls clockwise initially to decrease bias so the LED is green and counterclockwise until the LED goes out. Refer to the Tube Top diagram for direction.

A cutaway of the tube top showing the location of the biasing controls below the tube top surface and the biasing driver.

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