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Vacuum Electron Tubes Electronics Pentode Part 2

OddMix.com - Technology Note - TN1202A - Karl Nagy

Picture 2. 6AG5 RF Pentode Screens and Assembly [14 KB]
Picture 2. 6AG5 RF Pentode Screens and Assembly

As seen on the 6AG5 RF Sharp-Cutoff Pentode image in Picture 2, the entire electrode assembly is supported concentrically by two circular mica insulating assembly holders within the glass envelope. At the bottom of the electron tube the connecting pins provide all of the mechanical support to the tube assembly and the required electrical connection that is often done by spot welding.

Picture 2 provides close up on the cathode and the first two spiral grids - G1 and G2. Interestingly, the third grid is there but difficult to see. Unlike the spiral wires for G1 and G2 the G3 grid is in the form of a few small sheet-metal pieces. Regardless of their form, the small metal plates with their cathode potential are capable repelling the secondary electrons back to the anode surface.

A seldom noticed or appreciated, but most important ingredient of any vacuum tube is the getter. The getter is responsible to remove residual gases after assembly and during the life of the tube. When the getter is flashed, it further increases the already high vacuum. Getters are in the form of a dark, shiny, mirror like, reflective deposit in the inside of the glass envelope, usually on top of the tube, far away from the electrode assembly.

Usual getter materials are often magnesium, barium, beryllium, zirconium and others may be used alone or in a mixture. A proper getter is inert and actively binding to gases and it is usually is in a powder form inside the washer like getter holder visible on top of Picture 1. Often getters act as "keepers" to be continuously active in binding to gases not only just at the time of firing.

Once the tube is assembled it undergoes many hours on the vacuum pumping until it is sufficiently evacuated. Then it gets heated to a high temperature to force all gases to boil off then the electron tube is sealed. After sealing is done, the getter is fired by electrical induction method.

Pentodes and other electronic tubes are useful for the restoration of vintage collectible radios and tube type electronics instruments. Many times, electronic circuits are simpler to design and build and more reliable when made with vacuum tubes then with semiconductors.


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