Re: what is the difference between a tetrode and a triode

Hi Lorenzo,

That's a good question. Put simply, a triode is a vacuum tube with three elements (tri for three): a cathode, a grid, and an anode. The tetrode is an improvement of the triode, it contains four elements (tetr for four): a cathode, a control grid, a screen grid, and an anode. The screen grid helps to reduce capacitance between the anode and grid in a triode. This stray capacitance led to unwanted oscillations in amplifiers designed with triodes. Oscillation was even more problematic at higher frequencies.

Further refinements to the tetrode led to the pentode (five elements), which contained one more grid called the suppressor grid. The purpose of this grid was to collect secondary electrons that could be dislodged in high current applications. Vacuum tubes were commonly used in radios and television sets up to about 1970 when they were completely replaced by transistors.

Although semiconductors have largely displaced tubes, they are still used in some high voltage and/or high power applications. Microwave ovens use a type of vacuum tube called a magnetron. A local UHF television station not far my home uses six water cooled klystron tubes in their transmitter. Total radiated output is roughly 7 megawatts.

There is a very good explanation of vacuume tubes at this site: http://www.wikipedia.org/wiki/Tetrode

I hope that answers your question. If not, you can reach me at madhu@madhu.com