Biasing
Do not even attempt to make any adjustments to this before reading and understanding
this section thoroughly. As with any power amplifier, incorrect biasing can result in the
complete electronic destruction of the output devices, in this case expensive valves. As
stated on the WARRANTY FORM, valves are not covered by the standard two year
warranty except under certain conditions at the discretion of TRACE ELLIOT.
If you are in any doubt as to what you are doing, refer the unit to TRACE ELLIOT or
other approved electronics service centre.
The holes in the chassis in the BIASING section allow the biasing of the output valves to
be checked or reset easily and safely.
On a new unit the biasing will be factory set for the particular set of KT88’s supplied with
the amplifier. Although this should not need to be adjusted unless a new set of output
valves is fitted, on all amplifiers as valves age their bias requirements may change.
There are several methods that are used to bias valve amplifiers, the following is one of
the easiest, safest, and ensures that the output valves do not draw too much current which
can result in thermal run away and eventual destruction.
To check or reset bias:-
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 MASTER control is turned to zero and switch from STANDBY to ON.
3) Set volt meter to 200mV range. Insert black probe (-) into 0V hole, red (+) into V1.
4) The reading on the volt meter should be ≈30mV (+/-3)
5) If necessary, using a trimmer tool or small flat bladed screwdriver carefully rotate the
left hand ADJUST control to give the correct reading as above.
6) Now insert red probe into V2 and, if necessary, carefully rotate the right hand ADJUST
control to give a reading of as close to 0mV (+/-1) as possible.
7) Repeat points 3 to 6 as appropriate. Amplifier is now biased correctly.
The above suggested value for bias should be OK for most brands of valves, although the
bias point for any amplifier is somewhat subjective. Increasing the values shown may
improve the tone but will make the valves run hotter, whereas lower values will increase
valve life and reliability at the expense of sound quality.
For more advice on this please contact the TRACE ELLIOT UK Service Dept. or
distributor in your territory.
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