Vacuum tube grid spacing and filament alumina coating. - Page 1
Forum section: electronics / projects

author: Drirr
author_rank: Regular Contributor
timestamp: August 24, 2022, 06:46:14 pm
content: Hi,Today I finally made my first vacuum tube grid. Molybdenium wire spot welded to copper support... nothing much interesting. I know it looks funny... rods are bit oversized.First question is - what about grid wire spacing. I searched in "RCA Electron Tube Design 1962" but without any success. Or I overlooked semothing.Second question is filament alumina coating. I need somehow isolate tungsten filament from cathode. There are some informations "how to do it" in RCA book but is there any way how to do it at home? I found only some cement/binder.By the way I succesfully replicated RCA-C130 cathode coatingThanks a lotDavid

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: August 24, 2022, 07:31:12 pm
content: Hi Drirr,I remember you posted some time ago about your cathode process. Do you have any more up to date info on that and what worked for you?Thanks

author: TimFox
author_rank: Super Contributor
timestamp: August 24, 2022, 07:36:46 pm
content: Along with other geometric parameters, the wire pitch/spacing affects the mu of the tube directly.The grid-cathode spacing affects the perveance, along with the plate-cathode spacing.I’m away from my personal library of forgotten lore today, but I think that “Reference Data for Radio Engineers” has formulae for this, in the space charge limited case (normal for vacuum tube operation).

author: Drirr
author_rank: Regular Contributor
timestamp: August 24, 2022, 08:09:50 pm
content: @pardo-bssoI was partially successfull. Main problem was that oxides somehow did not want to stick to tungsten. Best way was to nickel plate tungsten before covering wire with nitrocelulose and (BaSrCa)CO mixtures.Slow heating is necessary. When cathode reaches high temperature, nitrocelulose is burned out and cathode is activated. Then cathode needs some time to create crystalisation.. or something like that.. magical. However cathode does not like air. When exposed to air almost all oxide coating falls apart.Cathode current was about 6mA at "low" (red/yellow) temperature.I will share all info and know how as soon as there are any interesting results. I added a few images of my cathode testing system.TimFoxThanks a lot for studying materials! . I expect some experimentation with grid spacing.

author: Drirr
author_rank: Regular Contributor
timestamp: July 04, 2024, 09:15:23 am
content: Hi, just an update I successfully tested my first indirectly heated diode, today I made my first indirectly heated triode. Time for testing... Added a few photos.Emissive material is RCA C-130 Ba Sr Ca

author: Wolfram
author_rank: Frequent Contributor
timestamp: July 04, 2024, 03:07:49 pm
content: Charles Alexanian is the resident expert on topics like this, not sure if he is active on the forum any more? It might make sense to reach out to him to see if he has any input.

author: quadtech
author_rank: Regular Contributor
timestamp: July 09, 2024, 02:35:08 pm
content: Also check out Ron Soyland (glasslinger on youtube). He does reply to comments / queries on his channel.For ex, in the video above, see the comment on cathode coating and activationby nanoindus (Dr Nabhiraj Yalagoud) and Glasslinger's reply

author: LaserSteve
author_rank: Super Contributor
timestamp: July 09, 2024, 09:34:49 pm
content: Your looking for a very expensive book, jealously guardedfrom becoming a PDF. Rosebury Handbook of Electron Tube and Vacuum Techniques, from the very skilled Technician who ran MITs tube lab. Its a practicum, not theory. Worth reading.Looks like its on the "pay per view" portion of the Internet Archive.The wet chemistry of production tube processing is actually very well detailed in the book. A whole lot of degreasing, cleaning, plating, activation, etching, and passivation going on. There is a formulary. Alumina electrodisposition for indirect cathodes is covered as well. Book is golden for tube making.Cleanliness snd surface state on tube metals is important.

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2024, 10:59:52 pm
content: Another reference, concerning materials but not so much construction, is available as .pdfhttp://www.tubebooks.org/Books/Atwood/Kohl%201960%20Materials%20and%20Techniques%20for%20Electron%20Tubes.pdf

author: LaserSteve
author_rank: Super Contributor
timestamp: July 10, 2024, 01:14:49 am
content: So your supposed to wind a double helix of heater on a piece of molybdenum wire. Etch away the Moly selectively. Electro-deposit the nano-alumina. Slide the outer tube over the coated wire. I'm sure there are additional steps to get an emissive layer and make the famous "dark" or low temperature emitter layer.This video is the lamp version. The plant was an hour to the North of My home. Getting on the once a year GE tours was impossible.Mom and Dad got a tour of Revenna Lamp while I was at Scout Camp. That was a conspiracy!https://youtu.be/ZuhapGSexyg?si=d3Tgam1Y0XqkgrxWSteve