Anthem Audio Amp 1 manual Bias Adjustment

BIAS ADJUSTMENT

WARNING: Hazardous and potentially lethal voltages! The Bias adjustment procedure described below involves working with equipment covers off and power applied, exposing the operator to potentially hazardous or lethal volt- ages. These tests and adjustments should be performed only by qualified personnel with the proper testing equipment and training.

The Anthem AMP 1 has been designed to give many hours of listening pleasure and should only require biasing at tube replacement time.*

Should one of the EL34 output tubes fail or otherwise require replacement, it will be necessary to adjust the out- put stage bias current.

The factory set bias current of 40 ma per tube is a good compromise between long tube life and low distortion. For the highest performance the current may be increased to 50 ma per tube, of course at the expense of output tube longevity.

Bias current is measured by sampling the voltage drop across a 10 ohm resistor connected in the cathode circuit of each output tube. Using Ohms law E=lxR we can see that 40 ma current results in a 0.4 volt drop across the

10 ohm cathode resistor and 50 ma is 0.5 volts. For setup below, 40 ma or a measurement of 0.4 volts is assumed. A suitable analog or digital multimeter (20,000 Ohms/Volt) capable of accurately measuring 0-2 Volts DC. eg. Fluke 77 DMM or a Simpson 260, is required.

1.Disconnect the power cord and allow the amplifier to cool 15 minutes before removing the top cover.

2.After removing the top cover, with the front end of the chassis facing you and referring to Fig. 6, observe the location of four test points TP-1, TP-2, TP-3 and TP-4. These are located left to right along the the top edge of the main printed circuit board assembly. Also note the position of the four potentiometer adjustment shafts protruding from the rear of the PCB mounting bracket. The poten- tiometers from left to right are VR1, VR2, VR3 and VR4.

3.Connect the multimeter’s negative test lead to the AMP 1’s left channel output COM or black binding post on the rear panel.

4.Carefully apply line voltage to the amplifier and observe that all the tube filaments light. As the amplifier is warming up touch each test point, TP-1, through TP-4 in succession with the positive meter test probe, noting the voltage at each point is less than or equal to 0.4 volts. Adjust the corresponding potentiometers VR1 through VR4 to main- tain voltage at 0.4 volts at each test point over a twenty minute period.

5.After 20 minutes if no further movement in the test point voltage is observed, then disconnect the amplifier from the power source and replace the top cover.

*Optional periodic checks of the bias may be made every 500 hours or so. Dramatic changes in bias over a 500 hour period generally indicates that the output tubes are becoming weak and should be replaced.

TP-4

TP-3

	TP-2
	TP-1
VR4*
(for V7)	VR3*
	(for V6)
	VR2*
	(for V5)
	VR1*
	(for V4)
	PCB Mounting
	Bracket

Amp Rear

* To increase bias turn knobs counterclockwise.

FIGURE 6

Adjustment of the output tube bias.

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