Gassy tubes cannot always be detected by observation, but if a tube develops during operation a blue haze, both inside and outside of the plate, it is usually an indication of gas. Many good tubes will show a haze, which clears up in a few minutes, when first put into operation. If the gas condition is bad, arcing may occur between the grid and the plate when operating at high volume. This can be observed by looking down through the top of the tube. The effect of gas in a tube is to cause distortion and lessen the volume, and in bad cases noisy reproduction will be noticed.

Burned out filaments are easily detected by their failure to light. The effect of a burned out filament in a stage using two tubes, such as a push-pull stage, is to reduce the volume. If the tube is in a voltage amplifier stage it will only be necessary to increase the volume control setting to continue the operation, until such time as it is convenient to replace the tube. A burned out tube used in a stage employing only one tube will result in "no sound" and it must be replaced before the amplifier can be used.

Loose elements in vacuum tubes cause a grating or stuttering noise in the loud speakers. Although such defects may develop after the tube has been used for a considerable length of time, it usually shows up when the tube is first used or after a short time of operation. Tubes with defects of this kind can be readily located by tapping the top of the tube lightly, as the disturbance will be aggravated when the defective tube is tapped. Dirty socket contacts or dirty tube prongs will give the same indication, so that the condition of the socket and tube prongs should be checked before replacing a tube for this defect. The tube prongs may be cleaned with a fine sand paper. After sanding the prongs, any loose grains of sand should be removed by blowing, or by wiping the contacts with a clean piece of cloth.

The socket contacts may be cleaned by inserting a No. 6 machine screw into the socket holes. Care must be taken in scraping the contacts to avoid bending or breaking them. Caution-Do not clean the socket contacts with the voltage turned on.

Disarrangement of elements will not usually develop with the use of tubes, but may readily result from rough handling. Tubes should be handled carefully, especially when hot. A sharp blow may cause the electrodes to "short" (make electrical contact with each other). Disarrangement of elements may cause "no sound," low volume, noisy or distorted reproduction, depending on the nature of the disarrangement.

144. General Precautions in Replacing Tubes.-Do not replace tubes with the voltage on, because the removal of one tube may put excessive voltage on the other tubes of the amplifier, or the tubes may be damaged if the prongs were started into the wrong socket holes when replacing.

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