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Valve Radio and Audio Repair Handbook by CHAS MILLER

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Chapter 21

Faults on detector/AVC/AF amplifier stages

Chas E. Miller

When a receiver uses a double-diode-triode with top cap grid connection in this stage a very simple and convenient test for the AF amplifier section is to apply a finger to the cap, whereupon a loud hum should come from the loudspeaker (assuming, of course, that the output stage has been passed as satisfactory). In the case of single-ended DDTs such as the 6SQ7 try touching the blade of a small screwdriver on the centre contact of the volume control, with this fully advanced. If no hum results measure the anode voltage on the valve, which normally will be between about 40 V and 100 V. Very low or no voltage suggests that the anode load resistor has gone high or open, whilst a high ...

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