The General Tester

Radio Record, Volume II, Issue 32, 22 February 1929, Page 30

The General Tester

Additional Operating Hints A FEW weeks ago "Pentode" described the construction of a useful general-purpose tester for testing valves, batteries, etc., under operating conditions. A little doubt seems to exist regarding what the amateur is to expect when he places his valves in the tester and the adapter in the valve socket, The full description was overlooked at the time, but it is hoped the following will fill in the omission :- The purpose of the tester, when con-. sidering valves, is to check over the filament supply and see what is happening in the plate lead. A milliameter is a current-operated instrument, and as only voltage fluctuations occur, or should occur, in the grid ‘circuit of each valve, no advantage would be gained by the insertion of this instrument in the grid lead. Current. flows through the valve from filament to plate, and it is this current that we desire to test. Now, a general-purpose 201 A type of valve should draw no more than 5 m.a. when used in any but the final stage of a receiver, and if excess of this is being taken by a high-frequency or first audio valve, the receiver is putting fur too much of a drain on the B batteries with no better results. In fact, the results will be far worse in many cases when excessive current is flowing.’ The primary of most audio transformers is wound with extremely fine wire, not heavy enough to stand a constart load of more than 3 or 4 milliamperes. Therefore, if the detector or first audio "vequency valves are taking more current than that specified, grave risk is being run of the transformer primaries being burnt out. The natural question that will be asked is. what remedy is there if excessive current is found to be passing? The answer is: Adjust the grid vias of that particular valve. In general,

with 90 volts B battery a bias of 43 volts will be found correct. This decrease in current will also greatly prolong the life of the B batteries. When testing the last audio stage no general indication can be given as to the correct amount of current taken without first knowing the type and size of the power valve used. Small power valves draw 8 to 10 m.a., while larger ones may take as much 28 ~5 m.a. to operate them successfully. There is one test, however, that can be relied upon to indicate whether the last valve is overloading. When music is being received with the last valve under test, no appreciable movement should be noticed by the miiliameter needle, even on the loudest passages. If the needle jumps from side to side of the scale, distortion is taking place and adjustments should immediately be made to try to remedy this. Commence with the grid bias and gradually increase, noting the effect on the needle each time. If distortion appears, and it is quite easy to see it, to increase with an increase of grid bias, either lower the bias or raise the B voltage. This latter method will enable the valve to handle far more volume. If no distortion was present the needle would remain quite steady, and, neglecting the ear as a judge, make all adjustments according to the milliameter needle, If this shakes violently, adjust either the B or C voltage, or both. If this does not effect a cure, then a larger power valve is indicated. _ This same test should apply to all the valves in the receiver, and in no case should the inktrument needle quiver appreciably while music is being received.