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There are several methods that are used to bias valve amplifiers, the following is one of
the easiest and ensures that the output valves do not draw too much current which can
result in thermal runaway.
To check or reset bias on currently installed valves:-
1. Ensure unit is correctly loaded (speaker is connected). Connect to mains, switch on
(STANDBY) and allow to warm up for at least one minute.
2. Ensure that the MASTER control is turned to zero and switch from STANDBY to ON.
3. Set voltmeter to 200mV DC range. Insert black probe into the 0V hole and red probe
into V1. There should be a reading of 33mV (+/-3mV) on the voltmeter. If necessary,
using a trimmer tool or small flat bladed screwdriver carefully rotate the appropriate
BIAS ADJUST control to give the correct reading on the voltmeter.
4. Now insert black probe into V2. The reading should be as close to 0mV as possible
(+/-1mV). If necessary, using a trimmer tool or small flat bladed screwdriver carefully
rotate the appropriate BIAS ADJUST control to give the correct reading on the
voltmeter.
5. Repeat steps 3 & 4 until no more adjustment is required.
To install new valves and bias amplifier:-
1. Ensure unit is disconnected from mains supply and that the valves have had time to
cool down.
2. Either remove front grill or complete amplifier assembly and carefully remove
old valves.
3. Install new valves making sure all are inserted correctly.
N.B. Only use matched sets of output valves.
4. Using trimmer tool turn BIAS ADJUST controls fully anti-clockwise.
5. Ensure unit is correctly loaded (speaker is connected). Connect to mains, switch to
STANDBY and allow to warm up for at least one minute.
6. Ensure that MASTER controls are turned to zero and switch from STANDBY to ON.
7. Set voltmeter to 200mV DC range. Insert black probe into the 0V hole and red probe
into V1 probe.
8. Using trimmer tool carefully rotate the appropriate BIAS ADJUST control to give the
correct reading on the voltmeter as shown below.
EL34
KT88
6L6
6550
5881
33mV (+/- 3mV)
37mV (+/- 3mV)
28mV (+/- 3mV)
37mV (+/- 3mV)
30mV (+/- 3mV)
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