SAN FRANCISCO (AP) _ You could call it the great-granddaddy of high-tech: A homely, microwave- squi rting vacuum tube called the klystron, built for $100 in a Stanford University lab, is 50 years old this month and still going strong.

''Fifty years later we still depend on the $100 gamble,'' said Joel Shurkin, spokesman for the university in Palo Alto, about 50 miles south of San Francisco. ''The klystron probably represents one of the last artifacts of vacuum-tube technology still in wide use in the world.

''Modern semiconductor technology cannot match the kind of power a klystron produces,'' he said Wednesday. ''It is a million times more powerful and it is likely to remain so for the foreseeable future.''

Klystrons, which can be several inches to several feet long, are found in weather, air traffic control and aircraft radar systems. Many long-distance telephone calls still use klystron-driven microwave relays.

Every major UHF television station broadcasts its signal with a kylstron. The tubes also are used in cable television to relay signals to remote communities.

Satellite communications depend on klystrons, as does the National Aeronautics and Space Administration's Deep Space Network to keep touch with far-flung spacecraft in the solar system and beyond.

Particle accelerators, especially those employing electrons for high energy physics research, generally use klystrons to kick the particles to near light speed.

Cancer patients often are treated with radiation produced by small atomic accelerators run by a klystron.

The tube remains one of the mainstays of America's defense system, forming integral parts of anti-missile systems. Even the so-called ''Star Wars'' space-based weapons defense system will use them.

The klystron was the brainchild of two California brothers, Sigurd and Russell Varian, respectively a barnstorming World War II-era pilot and a dyslexic research assistant.

The Varians came up with 36 inventions of ''varying impracticality,'' said Edward Ginzton, who worked with them and later became chairman of their company. They took their 37th invention, the klystron, to their friend William Hansen, a physics professor at Stanford.

At Hansen's urging, Stanford President Ray Lyman Wilbur agreed to pay the $100 for supplies, and lend them use of a small laboratory where, on Aug. 19, 1937, the klystron beamed its first microwave.

Using the klystron tube, the British improved their radar system to the point they could spot German bombers on their way to England, said Shurkin.

So valuable was the secret that the British decided not to put the klystron-radar in planes that flew over occupied Europe, in case one of them crashed and the klystron be discovered.

The klystron works like this:

An electric current heats a cathode, which gives off electrons. The electrons then combine into bunches and speed up as they pass through several cavities of the tube.

Microwaves interacting with the electrons are amplified and accelerated before they leave the tube.

After working on war-related projects, the Varians returned to California and founded Varian Associates in San Carlos. Russell died in 1959, Sigurd two years later.

The klystron industry last year sold more than $150 million worth of the vacuum tubes worldwide. Varian Associates account for more than 40 percent of worldwide production.