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Recently resurrected, the darling of the SE set, the amplifier without a coupling capacitor, the Loftin-White amplifier often blows up. By removing the input tube while the amplifier is in use, or by a failure of the input tube, or by even jiggling the input tube in its socket, without the safeguard of a coupling capacitor, the output tube can easily lose its negative bias voltage. While the result can be damage to the tube, the usual victim is its cathode resistor, which sees a large increase in voltage across its leads. |
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In AC terms, the inductive load makes for high gain and low noise as it approximates a current source in many ways. But why not replace the inductor with a current source?
Below is a current source loaded Loftin-White amplifier. The grid of the output tube is still directly coupled to the previous stage, but the amplifier remains protected from tube removal,
A coupling capacitor can be a very cheap insurance policy. Using a coupling capacitor in a Loftin-White amplifier is heretical, but safe. Although we have lost the direct coupling feature, the output tube still serves as a voltage regulator of sorts for the input tube. How do we retain the direct coupling feature without the danger to the amplifier? The following schematic doubly uses an inductive load: on the driver tube and on the output tube.
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pg. 17 |
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