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. Operation In the triode, electrons are released into the tube from the metal cathode by heating it, a process called thermionic emission. The cathode is heated red hot by a separate current flowing through a thin metal filament. In some tubes the filament itself is the cathode, while in most tubes there is a separate filament which heats the cathode but is electrically isolated from it. The interior of the tube is well evacuated so that electrons can travel between the cathode and the anode without losing energy in collisions with gas molecules. A positive DC voltage, which can be as low as 20V or up to thousands of volts in some transmitting tubes, is present on the anode. The negative electrons are attracted to the positively charged anode (or "plate"), and flow through the spaces between the grid wires to it, creating a flow of electrons through the tube from cathode to anode. The magnitude of this current can be controlled by a voltage applied on the grid (relative to the cathode). The grid acts like a gate for the electrons. A more negative voltage on the grid will repel more of the electrons, so fewer get through to the anode, reducing the anode current. A less negative voltage on the grid will allow more electrons from the cathode to reach the anode, increasing the anode current. Therefore, an input AC signal on the grid of a few volts (or less), even at a very high impedance (since essentially no current flows through the grid) can control