Why didn't secondary emission tubes catch on? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: August 20, 2023, 11:45:41 am
content: Wouldn't they have longer lasting cathodes and filaments?

author: CaptDon
author_rank: Super Contributor
timestamp: August 20, 2023, 12:48:07 pm
content: Secondary emission is a good thing in photomultipliers which also find use in radiation detectors. In nearly every other vacuum tube secondary emission is a very bad thing. It usually leads to control grid or screen grid current and also runaway when the grid bias circuits don't control current flowing in the wrong direction. Download one of the RCA vacuum tube circuit / vacuum tube theory books and read.

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2023, 04:38:24 pm
content: I am very familiar with the secondary emission process in scanning-electron microscopes, photomultipliers, electron multipliers, and channel electron multiplier tubes, along with features and problems with secondary emission in tetrode vacuum tubes, but I have not seen secondary emission cathodes in practical amplifier tubes.Presumably, a normal cathode somewhere emits the electrons that induce secondaries from a cold cathode?There are other non-thermionic electron sources, such as field emission, and there is interest in single and multiple field-emission cathodes for x-ray tubes and illumination systems.My experience with field-emission sources for electron microscopes (decades ago) is that a high field is required, the surface must be kept clean in a very high vacuum, and the sharp points required for high field at a reasonable voltage are fragile (especially against arc discharges).One interesting modern development is the "Spindt array"https://en.wikipedia.org/wiki/Field_emitter_arraywith an addressable array of field emitters.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 20, 2023, 05:38:23 pm
content: As I recall, excessive plate supply / power dissipation. Since you need the dynode above the cathode, but you need the plate further above the dynode.I don't recall if I read about this here,https://www.dos4ever.com/EF50/EF50.htmlor somewhere else. There's a few good articles around the development and application of these types. And this was probably it (just skimming it at the moment).EE50 and EFP60 were the famous types, AFAIK.Datasheets:https://frank.pocnet.net/sheets/153/e/EE50.pdfhttps://frank.pocnet.net/sheets/030/e/EFP60.pdfSee also:http://www.r-type.org/exhib/aai0191.htmhttps://www.pye-story.org/history/world-war-2Basically, conventional types like EF50 (in the ballpark of 6AK5, 6AU6, 5702, and various other IF pentodes) were comparable, or quickly outperformed them. The dynode as a floating cathode (of significant current capacity) is interesting, and was probably particularly attractive for computer circuits, but not so much for general purpose radio amplification.Not so troublesome at these (SW) frequencies, but a significant impact going higher still: transit angle and bunching would've made these useless for upper-VHF / UHF work, at least without significant change to the design (hm, I suppose a radial-beam design is still possible, using a louvered rather than cupped dynode? hmm, I don't actually know what the dynodes were shaped like on these..). And by then (50s-60s), small radial-beam and even planar types were available.That is to say: beam physics begins to dominate at some point. Grid voltage modulating the beam current causes some change in its velocity, as well as "doing work" on the beam / electron cloud itself (hence there is some grid conductance, rising with frequency, which impacts maximum power gain of the device). The beam separates as it transits the interelectrode space, smearing out what would otherwise be amplified plate current, reducing gain severely. Which is, in part, why many types are rated for less power above some frequency range: they simply can't amplify that well, and plate current starts to look "class A" (the peaks and troughs get smeared together) even if the grid bias and drive is obviously "class C".Also, I don't know about dynode lifetime itself; if they were using low-work-function materials, evaporation or sputtering might be a problem. With 50-150 eV bombarding it, I wouldn't think too low [work function] would be required, and maybe it's just ordinary metal (nickel or what have you)?Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2023, 05:54:51 pm
content: Looking at the EA50 datasheet posted above, it has a "normal" heater for the thermionic cathode (6.3 V, 0.3 A), and the current gain from that cathode to the secondary cathode is modest, corresponding to normal secondary-electron yields (2 to 4 electrons/electron is common for metals).Of course, in an electron multiplier, that yield is raised to the power of the number of dynodes.