compactness, and light weight are the primary considerations, and a tube life of only a few hours is required. "(1)
     Anyone who has looked at transmitting tube manuals has come across ICAS and CCS ratings. The difference between these is explained in the RCA Transmitting Tube Manual:

Continuous Commercial Service (CCS) covers applications involving continuous tube operation in which maximum dependability and long tube life are the primary considerations.
Intermittent Commercial and Amateur Service (ICAS) covers applications in which high tube output is a more important consideration than long tube life. The term "Intermittent Commercial" in this title applies to types of service in which the operating or "on" periods do not exceed 5 minutes each, and are followed by "off" or stand-by periods of the same or greater duration. The term "Amateur Service" covers other applications where operation is infrequent or highly intermittent in nature, as well as the use of tubes in "amateur" transmitters. ICAS ratings generally are considerably higher than CCS ratings. Although the ability of a tube to produce greater output power is usually accompanied by a reduction in tube life, the equipment designer may decide that a small tube operated at its ICAS ratings meets his requirements better than a larger tube operated within CCS ratings. (2)

     Radio amateurs are notorious for "pushing" their tubes, often sacrificing lifetime. They sometimes counteract this by using forced air cooling or even running tubes like the 6L6 upside down in oil! Since most hams only use their equipment a few hours a day at the most, their tubes still will have a reasonable lifetime. On the other hand, the tubes running in a radio or TV station's transmitter are on essentially all the time, and down-time is to be avoided, hence the more conservative CCS ratings.

   Of the most important group of specifications of an electronic device is its "maximum ratings". The meaning of these maximum ratings range from recommended operating conditions that promote the most reliability to conditions, above which, that will result in catastrophic failure. Anyone designing or upgrading electronic equipment should be familiar with these ratings and what they mean.
   The maximum ratings for tubes and semiconductors generally have different implications. Generally, the failure modes in tubes are gradual wear-out or catastrophic failures caused by wear-out or extended abuse. A momentary overload or over-voltage seldom cause a failure. Semiconductors, on the other hand, are quite reliable as long certain voltages or currents are not exceeded, but can fail if these limits are exceeded, even momentarily. On semiconductor data sheets, these maximum ratings are typically called "Absolute Maximum Ratings" and must be met for all possible conditions - no matter what. Not accounting for all the possible things that could cause the ratings to be exceeded is one of the main reason high-power solid state equipment can be unreliable.
     The maximum ratings of tubes is a softer specification than for semiconductors. Generally tube maximum ratings are chosen to give the tube a reasonable lifetime under all expected "normal" operating conditions. The intended use of the tube can influence the maximum ratings. The RCA "Special Red" tubes (5690, 5691, 5692, 5693) were designed for 10,000 hours of use, and have somewhat lower maximum specs than their construction would suggest. The 6026 subminiature tube has fairly high ratings, since "Ratings for this tube have been established on the basis of its intended use in radiosonde and similar applications where power output,