Variations Of Vacuum Tube

Thursday, February 22nd, 2018 - Basic Electronics

Variations Of Vacuum Tube

Frequency Conversion can be accomplished by many different methods in superheterodyne receivers. Tubes with 5 grids, called pentagrid converters, were generally used although alternative such as using a combination of a triode with a hexode were also used, even octodes have been used for frequency conversion The additional grids are either control grids, with different signals applied to each one, or screen grids. In many designs a special grid acted as a second ‘leaky’ plate to provide a built-in oscillator, which then coupled this signal with the incoming radio signal. These signals create a single, combined effect on the plate current (and thus the signal output) of the tube circuit. The heptode, or pentagrid converter, was the most common of these. 6BE6 is an example of a heptode (note that the first number in the tube ID indicates the filament voltage).

It was common practice almost everywhere in the world to combine more than one function, or more than one set of elements in the bulb of a single tube. The only constraint was where patents, and other licencing considerations required the use of multiple tubes. See British Valve Association.

The RCA Type 55 for example was a double diode triode used as a detector, AVC rectifier and audio preamp in early AC powered radios. The same set of tubes often included the 53 Dual Triode Audio Output. A German firm actually built a multi-section tube with the coupling components inside the envelope. In that case the cost of individually sealing the parts in separate glass tubing to protect them from exposure to the vacuum ended up increasing the final cost.

Other Variations Of Vacuum Tube

Another early type of multi-section tube, the 6SN7, is a “dual triode” which, for most purposes, can perform the functions of two triode tubes, while taking up half as much space and costing less.

RCA 12AX7 dual-triode tubeAn RCA 12AX7 dual-triode tube (1947)

Currently the world’s most popular vacuum tube is the 12AX7, with estimated annual worldwide sales of greater than 2 million units. The 12AX7 is a dual high-gain triode widely used in guitar amplifiers, audio preamps, and instruments.

The invention of the 9 pin miniature tube base, besides allowing the 12AX7 Family also allowed many other multi section tubes, such as the 6GH8 triode pentode which along with a host of simalar tubes was quite popular in television receivers. Some color TV sets even used exotic types like the 6JH8 which had two plates and beam deflection electrodes (known as ‘sheet beam’ tube). Vacuum tubes used like this were designed for demodulation of synchronous signals, an example of which is color demodulation for television receivers.

The desire to include many functions in one envelope resulted in the General Electric Compactron A typical unit, the 6AG11 Compactron tube contained two triodes and two diodes, but many in the series had triple triodes.

An early example of multiple devices in one envelope was the Loewe 3NF. this device had 3 triodes in a single glass envelope together with all the fixed capacitors and resistors required to make a complete radio receiver. As the Loewe set had only one tubeholder, it was able to substantially undercut the competition since, in Germany, state tax was levied by the number of tubeholders.

Loewe were to also offer the 2NF (two tetrodes plus passive components) and the WG38 (two pentodes, a triode and the passive components).

Vacuum tubes on a Philco model 20 tabletop radio setVacuum tubes on a Philco model 20 tabletop radio set.

The beam power tube is usually a tetrode with the addition of beam-forming electrodes, which take the place of the suppressor grid. These angled plates focus the electron stream onto certain spots on the anode which can withstand the heat generated by the impact of massive numbers of electrons, while also providing pentode behavior. The positioning of the elements in a beam power tube uses a design called “critical-distance geometry”, which minimizes the “tetrode kink”, plate-grid capacitance, screen-grid current, and secondary emission effects from the anode, thus increasing power conversion efficiency. The control grid and screen grid are also wound with the same pitch, or number of wires per inch. Aligning the grid wires also helps to reduce screen current, which represents wasted energy. This design helps to overcome some of the practical barriers to designing high power, high efficiency power tubes. 6L6 was the first popular beam power tube, introduced by RCA in 1936. Corresponding tubes in Europe were the KT66, KT77 and KT88 by GEC (the KT standing for “Kinkless Tetrode”).

Electro-Harmonix 12Ax7EH Russian tubeAn Electro-Harmonix 12Ax7EH Russian tube.

Variations of the 6L6 design are still widely used in guitar amplifiers, making it one of the longest lived electronic device families in history. Similar design strategies are used in the construction of large ceramic power tetrodes used in radio transmitters.

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