Name: Terry
Email: musicmanamp@yahoo.com
Subject: BIAS CALIBRATION
Thread: 196
Time: Sat, 16 Mar 2002 18:53:14 UTC
1338 State College Parkway
P.O.Box 4589
Anaheim, California 92803
Telex 692-397
SERVICE BULLETIN #1
Section I DRIVER TRANSISTOR BIAS CALIBRATION PROCEDURE
A. This applies to all models containing the following circuit boards:
DB-2, DB-3, DB-4, GP-1, GP-2, GP-3, GP-3A,GD-1,GD-2 AND GD-2A.
B. Adjustment is as follows:
1. Turn the amplifier to "ON" with the HI / LO Standby switch in the HI
position. No Signal.
2. Using a voltmeter measure the voltage from emitter to ground on each of
of the two driver transistors. Across the 3.9 OHM emitter resistors is a
convenient measuring point.
3. Adjust the bias trimpot (TR-1) until you read 25mv DC across the 3.9
OHM emitter resistors. If there is a difference in voltage between the
emitters of the two driver transistors, set the lower of the two to 25mv.
The higher of the two should not exceed 55mv DC.
NOTE: Use only a 1458 op amp to drive the output section.
This applies to all models "not using 12AX7 driver tube".
______________________________________________________
Section II GRID BIAS CALIBRATION
This applies to all models "using 12AX7 DRIVER TUBE".
A. This applies to all models using a grid drive. (12AX7 driver tube).
B. Adjustment is as follows:
1. Turn the amplifier to "on" with the HI/LO Switch in the HI position.
No signal.
2. Using a volt meter, measure the voltage from cathode (pin 8) to ground
on the output tubes. Across the 10 OHM cathode resistor is a
convenient measuring point.
3. Adjust the bias trimpot located farthest to the right (looking from the
front) on the GB-1 and BB-1 circuit boards until you read .5 volts DC
across the 10 OHM cathode resistor.
NOTE: Be sure tubes are warmed up before adjusting the bias voltage.