SAM-A1 Owner’s Manual
FSL0132
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FSL0132 Version 5.1 © 2020 Fireworks Solutions Limited Page 19
8.3.
Routine Output bias adjustment
This is the simplest of the routine maintenance activities. If you are in any doubt of your
competence to perform this operation or you are not in the possession of/competent with the
appropriate test equipment then DO NOT UNDERTAKE THE TASK; please contact FSL for our
maintenance service.
WARNINGS:
As adjustments can only be made when the amplifier is on and the valves’ temperatures have
stabilised, this operation must only be performed by a competent person with the appropriate test
equipment.
Trim pot adjustment tool
The correct, insulated tool must be used for all trim pot adjustments.
The correct tool is one of the following:
Vishay ACCTRITOB308 RS stock number: 543-434 OR Bourns H-90 RS Stock number: 743-2479
The output valves’ bias should be checked at 6 month or 12 month intervals depending on the
level of use. The objective is to ensure that the output valves are still biased correctly for the
desired mode of operation of the output stage, to correct for any drift over time as the valves age
through use and ensure that they are driving their output transformers and so the loudspeakers
equally and linearly.
The bias current is measured by measuring the bias Volts on the 10R current sense resistors in
series with the cathodes for each output valve, R19, R20, R47 and R48. 50mA reads 500mV. There
are test points labelled Vb at the 3 o’clock position of each output valve (accessible with the top
plate on). These are J51, J52, J53 and J54.
The bias current is increased by reducing the negative bias at the grids of the output valves. This
is achieved by winding the trim pots at the 6 o’clock position of each output valve in an
anticlockwise direction (accessible with the top plate on). These are RV7, RV6, RV13 and RV12.
If the output valves have been replaced, the bias setting will have to be done from scratch; all four
bias adjusters. RV6, RV7, RV12 and RV13 should be set at their maximum bias voltage (minimum
current) turn them all fully clockwise (clicking heard or 25 full turns) before the mains supply is
applied.
If the bias is just being checked do not touch the adjusters and proceed with the check.
1.
Connect the DMM negative to 0V, e.g. speaker output negative socket; set to read up to 2Vdc.
2.
Connect and switch on the mains supply.
3.
Wait 5 minutes before adjusting the valve bias currents.
4.
Adjusting the bias:
5.
The bias currents for V2 and V3 should be adjusted together; use the DMM to measure the dc
bias Voltage for each valve as it is adjusted. In increments of no more than 100mV (10mA) at
a time, increase V2 then V3 bias current until they are both at the desired value e.g. 250mV.
6.
The bias currents for V5 and V6 should be adjusted together; use the DMM to measure the dc
bias Voltage for each valve as it is adjusted. In increments of no more than 100mV (10mA) at
a time, increase V5 then V6 bias current until they are both at the desired value e.g. 250mV.
7.
Wait 5 minutes before checking the valve bias currents again,
8.
Repeat steps 5 and 6 until all the output valve bias currents are within 5mV of each other and
stable.
