Quote:
Originally Posted by cdm1christopher
Lawrence do you mean R36 as this is connected to C75 and C109 on output valve UBL21?
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No, insofar as R36 does not develop the bias voltage, R75 does due to the current flowing through it.
No current flows through R36, its job is to connect the bias voltage that's developed across R75 to the control grid of the sound output valve in order to bias the grid -ve with respect to the cathode, that resistor is a relatively high value in order to "isolate" the signal on the control grid from the bias source, that's because the grid bias source is bypassed by C75 which would otherwise present a very low impedance to ground for the signal if the grid was connected directly to the bias source.
Remember that normally the control grid of a valve that's suitably biased will not pass current unless the peak value of the signal voltage during its +ve half cycle on the grid exceeds the grid's bias voltage.
Lawrence.