page_title: Triode
section_title: Lighthouse tubes
hierarchical_section_title: Triode / Construction / Lighthouse tubes
caption_attribution_description: Deutsch: Scheibentriode als Oszillator oder Verstärker bis ca. 2GHz, sog. „Leuchtturmröhre“ English: A so-called "lighthouse tube", a high frequency triode vacuum tube used as an amplifier or oscillator at UHF frequencies up to 2 GHz. The purpose of the unusual design is to reduce the inductance of the leads which connect to the electrodes, which limits the frequency of ordinary triodes. Lead inductance can cause the conductors in the tube to act like resonant circuits at high frequencies, causing instability and parasitic oscillations. The grid terminal is the flat metal ring around the center of the tube while the plate terminal is the button at the left. Soviet lighthouse tube 6С5Д (6S5D).
context_page_description: A triode is an electronic amplifying vacuum tube consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate. Developed from Lee De Forest's 1906 Audion, a partial vacuum tube that added a grid electrode to the thermionic diode, the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention founded the electronics age, making possible amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use is in high-power RF amplifiers in radio transmitters and industrial RF heating devices. In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-type audio systems by audiophiles who prefer the sound of tube-based electronics.
  The name "triode" was coined by British physicist William Eccles sometime around 1920, derived from the Greek τρίοδος, tríodos, from tri- and hodós, originally meaning the place where three roads meet.
context_section_description: A type of low power triode for use at ultrahigh frequencies (UHF), the "lighthouse" tube, has a planar construction to reduce interelectrode capacitance and lead inductance, which gives it the appearance of a "lighthouse".  The disk-shaped cathode, grid and plate form planes up the center of the tube - a little like a sandwich with spaces between the layers.  The cathode at the bottom is attached to the tube's pins, but the grid and plate are brought out to low inductance terminals on the upper level of the tube: the grid to a metal ring halfway up, and the plate to a metal button at the top. These are one example of "disk seal" design. Smaller examples dispense with the octal pin base shown in the illustration and rely on contact rings for all connections, including heater and D.C. cathode.
  As well, high-frequency performance is limited by transit time: the time required for electrons to travel from cathode to anode. Transit time effects are complicated, but one simple effect is input conductance, also known as grid loading. At extreme high frequencies, electrons arriving at the grid may become out of phase with those departing towards the anode. This imbalance of charge causes the grid to exhibit a reactance that is much less than its low-frequency "open circuit" characteristic.
  Transit time effects are reduced by reduced spacings in the tube. Tubes such as the 416B (a Lighthouse design) and the 7768 (an all-ceramic miniaturised design) are specified for operation to 4 GHz. They feature greatly reduced grid-cathode spacings in the order of 0.1 mm.
  These greatly reduced grid spacings also give a much higher amplification factor than conventional axial designs. The 7768 has an amplification factor of 225, compared with 100 for the 6AV6 used in domestic radios and about the maximum possible for an axial design.
  Anode-grid capacitance is not especially low in these designs. The 6AV6 anode-grid capacitance is 2 picofarads (pF), the 7768 has a value of 1.7 pF. The close electrode spacing used in microwave tubes increases capacitances, but this increase is offset by their overall reduced dimensions compared to lower-frequency tubes.
caption: Soviet lighthouse tube 6С5Д (6S5D)
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