Editing Triode (section)

=== Lighthouse tubes ===
{{More citations needed section|date=April 2022}}[[Image:Scheib3.jpg|thumb|Soviet lighthouse tube '''6С5Д''' (6S5D)]]
A type of low power triode for use at [[Ultrahigh frequency|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&nbsp;GHz. They feature greatly reduced grid-cathode spacings of the order of 0.1&nbsp;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.