andreus A-22 Manuale operativo - Pagina 10

Sfoglia online o scarica il pdf Manuale operativo per Amplificatore andreus A-22. andreus A-22 13. Head and combo

FOR TECHNICAL TYPES

We have provided this information for those who have a technical interest,

proper equipment and proper training in dealing with high voltage circuits.

GUITAR AMPLIFIERS CONTAIN LETHAL VOLTAGES. If you have any doubt

about your ability to safely perform this procedure, refer the job to a qualified

technician!

Bias Adjustment Procedure:

Precautions:

• Lethal voltages are present inside the chassis and at the tube pins. Do not

open the chassis or insert any objects into any openings other than where

directed by these instructions.

• Vacuum tubes get very hot. Always allow the tubes to cool completely before

touching them.

• When replacing output tubes, use only the very highest quality tubes or the

amplifier may be damaged. For more details, see the section below regarding

tube replacement.

1) Unplug the amplifier and allow the tubes to cool. The output tubes are the two tall

tubes on the left side of the amp when viewed from behind.

2) Unsnap the retaining clips that hold the output tubes in place and remove the

tubes. Install a new matched pair of EL84 or 6BQ5 tubes taking care not to bend

any tube pins. Snap the retaining clips back into position.

3) Using a digital multi-meter, set for DC Volts measurement and select a range that

will show 0-100 mV. Insert the black meter lead into the black test point terminal.

Insert the red meter lead into the left red test point terminal.

4) Insert a small flat blade screwdriver into the adjustment control hole and gently

turn the control to its mid position. The adjustment control is located on the

underside of the chassis near the bias test points.

5) Plug in the AC cord and turn on the amplifier's main power switch. After waiting at

least 30 seconds, turn on the standby switch. The meter should display a few

millivolts (mV).

6) While taking care not to touch any hot tubes, insert the screwdriver into the

adjustment control and very slowly turn the adjustment until the meter reads

20mV. Turning clockwise will increase the voltage, counterclockwise will reduce

the voltage.

7) Now remove the red meter lead from the test point and insert it into the right red

test point terminal and take a reading. The two terminals are reading a voltage

which represents the idle current of the two output tubes. If you get readings

within 5mV of each other at each test point, the tubes are considered to be

matched. Be sure to adjust the bias so that neither tube idles at more than 20mV.

We have found that the optimum bias point is reached when the tubes test points

read 20mV each but it is ok to set to any voltage from 15mV to 20mV without

causing any damage. Allow the tubes to warm up for at least 5 minutes and

re-check bias. Repeat the adjustment procedure if necessary.

8) It is a good idea to re-check the bias periodically (every 500 hours or so) as the

idle current can drift over time as the tubes age.