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I am confused regarding the terminology 'diodes', 'triodes' etc and the number of pins in these elements. If a diode has a cathode, an anode, and a supplementary anode (the control grid or plate) and each electrode are assigned to a pin in order to receive or give electricity, shouldn't the diode being a triode really, since it must have three pins?

If the current enters from the cathode and exists from both anodes, I'd say this element should have three pins. Why is that it is called a 'diode' instead? -- unless one counts only the number of anodes, that is...

Thank you

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  • \$\begingroup\$ you are aware are you that vacuum tubes have extra pins for the heater, and then sometimes unused pins just because they use a larger standard base? \$\endgroup\$ – Neil_UK Jun 16 at 9:40
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    \$\begingroup\$ The number of pins is rather irrelevant in identifying the type of tube - tubes are usually designed to fit into a standard base, and there may be unused pins. A triode will have a 'control grid' that accepts an input signal. Other grids may be present to optimise performance, but they are not, then, 'triodes'. Current enters at the anode and leaves at the cathode, btw. \$\endgroup\$ – Chu Jun 16 at 10:17
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    \$\begingroup\$ A "supplementary anode" is not a grid. You may be thinking of a device with two identical anodes, one each side of the cathode. This is common : it's a full wave rectifier. Electrically only one anode is active at a time, so the device is functionally just a diode. \$\endgroup\$ – Brian Drummond Jun 16 at 11:34
  • \$\begingroup\$ Most tubes need HEATER/FILAMENT power. That is done as 6.3volt or as center-tapped 12.6 volts, for some tubes. Then your choices may be direct-heated cathode or indirect-heated cathode; the indirect-heated allows wiring the cathode to voltages other than the filament heating energy; for low-noise (reduced hum, e.g.), this separation of filament and cathode becomes crucial. Thus the simple "cathode" can be as few as 2 pins, and as many as 4. \$\endgroup\$ – analogsystemsrf Jun 16 at 12:27
  • \$\begingroup\$ Nope, I was not aware that they had extra pins for the heater (although I knew there are direct and indirect tubes); in fact, I have never seen a tube before. Anyway, I understood that pins and electrodes are not related. \$\endgroup\$ – Gigiux Jun 17 at 8:33
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"If a diode has a cathode, an anode, and a supplementary anode (the control grid or plate) "

No it doesn't. A diode has a cathode and an anode, so it has two (active) connections. (Additionally, it will probably have one (direct heating) or two (indirect heating) filament connections).

Add a control grid and you have 3 connections, and that thingy is called a triode.

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  • \$\begingroup\$ Thank you, but I am still confused. From Wikipedia, I gather that John Fleming built a tube that contained "only a heated electron-emitting cathode and an anode". But, in order to heat the filament, the electrons should flow into it; in order to do that don't they need a point of access (the cathode) and a point of exit (the anode)? Or 100% of the electrons is emitted in the vacuum thus nothing reaches the end of the filament hence no need for the terminal anode? \$\endgroup\$ – Gigiux Jun 17 at 9:17
  • \$\begingroup\$ I consider heating a non-functional aspect of a tube (you could heat the cathode by steam if you wanted to), so that tube you describe would in my terms be a diode (two functional connections). In addition, it would have either two more connections for the heater (indirect heating) or one more connection for a directly heated cathode. \$\endgroup\$ – Wouter van Ooijen Jun 18 at 17:50
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There are dual diodes which have two anodes sharing a single cathode. These are frequently used in full-wave rectifiers. They will have five active pins - one each for the two anodes, one for the cathode, and two for the heater. Some such tubes had a five pin base, but others had an octal (8 pin) base, with three unused pins.

12AX7 and related types are dual triodes - two independent triodes, each with cathode, anode and grid pins, and three pins for a common heaters. The heater is center-tapped so it can be wired for eithr 6.3 or 12.6 volt operation. These tubes have a 9 pin base.

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