Nuvistors the tiny tubes, Tektronix Time Mark Generator Teardown - Page 1
Forum section: electronics / projects

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 06, 2022, 07:49:33 am
content: Nuvistors ? do you remember them ? know them ? have ever worked with them ?here is a teardown of a Tektronix Time Mark Generatorthat uses 6 nuvistors,2 of them run at 500MHzhttps://youtu.be/rnWniLNTdjQfor the people who are a bit in a hurry, ? of just want to share a link about nuvistorsfeel free to use this onehttps://youtu.be/5NJbeDd6sJ4it is actually the last 2 mins of the first video

author: Stray Electron
author_rank: Super Contributor
timestamp: May 06, 2022, 03:02:35 pm
content: I worked in a TV shop for a time in the late 1970s and I remember that many of the TVs (RCA brand IIRC) used Nuvistors in the tuners. I think that I had to replace ONE in the year or so that I worked there. The rotary dial tuner contacts were by far the biggest problem and we were constantly having to clean them. Somewhere I still have at least one Nuvistor that is NIB.

author: CaptDon
author_rank: Super Contributor
timestamp: May 06, 2022, 03:31:36 pm
content: I had a military rack mount 2-30MHZ distribution amplifier. It was designed to feed 8 radios from one receive antenna. Each of the 8 amplifiers had a pair of NuVisters in a cascode arrangement and the 8 amplifiers were fed from a single NuVister buffer. The unit actually worked very well with a decent noise figure for low band use.

author: TimFox
author_rank: Super Contributor
timestamp: May 06, 2022, 04:03:09 pm
content: Due to their heavy use in military hardware, surplus Nuvistors are still readily available.The hard things to find are the special sockets, and the pins are not solderable.My personal favorite is the 7586 medium-mu triode, which I believe was the first one released commercially by RCA.Besides their longevity, another feature is radiation resistance: see page 3 ofhttps://worldradiohistory.com/Archive-Catalogs/RCA/RCA-Nuvistor-Tubes-1963.pdfI like to run 7586s at 75 V plate-cathode voltage, which allows the use of reasonable-voltage capacitors and other components in audio applications.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 06, 2022, 04:08:11 pm
content: Sure, lots in my Tek plugins. I have a Nuvistor on my desk right now! Nice engineering.

author: TimFox
author_rank: Super Contributor
timestamp: May 06, 2022, 04:20:10 pm
content: "Normal" vacuum tubes, e.g. 6SN7GT, were assembled in a glass envelope that was then evacuated and sealed.The Nuvistors were actually assembledin vacuohttps://en.wikipedia.org/wiki/Nuvistor

author: Tom45
author_rank: Frequent Contributor
timestamp: May 06, 2022, 04:32:58 pm
content: In the mid 60s, transistors were the only active device we studied. The one exception was a single lab using Nuvistors. I don't remember the details of the lab, or of the corresponding lectures. But tubes, in the form of the Nuvistor were clearly an afterthought.That was early enough that integrated circuits weren't really a thing yet.

author: PaulAm
author_rank: Frequent Contributor
timestamp: May 06, 2022, 10:47:52 pm
content: I have a Tek 453 and 454 which used a few nuvistors. Later production replaced them with FETs.. Pretty cool actually. They last practically forever. I have a few spares but never had an occasion to use them.

author: SL4P
author_rank: Super Contributor
timestamp: May 07, 2022, 01:33:58 am
content: AMPEX broadcast video recorders (2” analog), used nuvistors in the ‘quadruplex’ head pre-amps.Not my area of expertise, but interesting nonetheless.

author: TimFox
author_rank: Super Contributor
timestamp: May 07, 2022, 01:37:57 am
content: The Ampex MR-70 audio studio tape deck used only Nuvistors: 6 x 7587 Nuvistors, 12 x 7895 Nuvistors (for each channel).The 7895 is a high-mu triode and the 7587 is a sharp-cutoff tetrode.https://www.historyofrecording.com/AMPEX_MR-70.html

author: David Hess
author_rank: Super Contributor
timestamp: May 07, 2022, 02:05:41 am
content: I have a 184 time mark generator that I refurbished also.Quote from: TimFox on May 06, 2022, 04:03:09 pmThey were preferred for a long time in RF input stages because of their resistance to damage from overload and better performance compared to delicate solid state devices.

author: CaptDon
author_rank: Super Contributor
timestamp: May 07, 2022, 02:44:37 pm
content: I am curious, with the age of these nuvistors and the tiny internal volume I would suspect they would be far more influenced by even a few molecules of atmosphere leaked in over age? Is there any proof that these devices get gassy over time even just sitting on a shelf? How were they gettered or weren't they? Maybe something on the filament or between the filament and cathode?

author: Gyro
author_rank: Super Contributor
timestamp: May 07, 2022, 03:06:29 pm
content: I'm sure they are gettered like other tubes (it would be even more essential with such a small internal volume). Their ceramic lead seals might be better than glass to metal seals too. All vacuum tubes are ultimately prey to very slow helium infiltration though - getters are ineffective against noble gasses.EDIT: On secont thoughts, I don't know how you would inductively activate a getter in a metal can - I know of a few tubes where the getter is resistively heated, but it depends on the presence of spare pins. As they were assembled under vacuum, it would have been possible to high temperature outgas all of the components, and activate the cathodes, before they were assembled. These are the main sources of initial gas evolution during manufacture of glass tubes. Large power tubes (eg industrial heating or transmitter types) have zirconium and similar coatings on their anodes as getters, but they require the anodes to be running at very high temperature to become active, something that would never happen in a Nuvistor.EDIT1: It appears that they may indeed had no getter...http://www.r-type.org/articles/art-150.htm

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 07, 2022, 03:37:57 pm
content: Great questions about Nuvistors, that I have absolutely no answer for. I assume that the metal for the envelope was carefully designed to be able to adsorb gas. The ceramic to metal seal must also pretty good and the material expansions carefully matched.Since none of my vintage stuff is used in any serious capacity to measure anything, I don't really know how well or poorly the circuitry is performing. I just use my scope to fool around my old computers, I don't get too upset if the trace is a few nanovolts noisier than in 1965....I wonder though if these concerns about aging and seals are addressed somewhere in RCA information.

author: PaulAm
author_rank: Frequent Contributor
timestamp: May 07, 2022, 04:11:25 pm
content: Here's a one page spec sheet for RCAhttps://worldradiohistory.com/Archive-Catalogs/RCA/RCA-Nuvistors-Reference-1965.pdfOf interest is their ability to function under extreme radiation conditions and lifetime up to 20,000 hours.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 07, 2022, 07:10:56 pm
content: Yeah, they were brazed under vacuum so the whole thing heats up nice and toasty, and all that gas diffuses out. Either it makes a metal-on-metal (and metal-on-ceramic and ceramic-on-ceramic) seal, or it's a reject. Think it would be hard to have e.g. barium getter inside there without it contaminating the whole damn thing, and nothing runs hot enough to use e.g. a zirconium anode.Tim

author: TimFox
author_rank: Super Contributor
timestamp: May 07, 2022, 07:21:22 pm
content: Vacuum brazing to ceramic normally involves a previous stage that metallizes the region of the ceramic to which the braze alloy will adhere.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 07, 2022, 10:39:04 pm
content: Yeah, they would be, either vacuum deposited or metal frit glazed, on the ends to "butter them up" first. Or, possibly electroless deposited? I suppose that might work too.Tim

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 09, 2022, 09:05:00 am
content: i am VERY impressed about the GLUE type, they used to fit the ceramics part into the bottom of the metal house,this needs to handle temperature changes and stay insanely air tight,how did they invent that ?

author: Gyro
author_rank: Super Contributor
timestamp: May 09, 2022, 09:13:35 am
content: It's not glue, it's solder or brazed. If you look at the exploded diagram in the link that I posted in Reply #12, there is an item labeled 'solder ring'. Good soldering counts as hermetic.

author: TimFox
author_rank: Super Contributor
timestamp: May 09, 2022, 01:36:07 pm
content: Yes, it's a braze joint (sometimes called "hard solder" to distinguish it from the "soft solder" used for circuit wiring).

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 09, 2022, 04:41:57 pm
content: braze joint ? i am not familiar with the term, yet it looks like soldered when i look very closely on mine,this mean the ceramic base plate must be threaded with metal on its edge for this to work,i assube they had a few challenges to solve when they designed this.

author: TimFox
author_rank: Super Contributor
timestamp: May 09, 2022, 05:11:25 pm
content: "Brazing" is when a different "filler" metal is used to join two pieces of metal, often called "silver soldering" or "hard soldering", where the filler metal has a lower melting point than the metals being joined.When brazing to a piece of ceramic, the ceramic must be metallized first.The English word is related to "brass", but the usual brazing alloys are made from copper, silver, and various other metals. Pure copper is sometimes used as a brazing filler metal.One can use brass (Cu-Zn) to braze metals, but zinc has a high vapor pressure at room temperature and is not used for good vacuum systems.The copper-silver eutectic (Ag 72, Cu 28), which melts at 779oC, is an example.When I was in grad school, we used ultra-high-vacuum techniques, which often required brazing between dissimilar metals or metal-to-ceramic joints. Luckily, the vacuum-tube industry wasn't dead yet and the materials and techniques were commonly available.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 09, 2022, 08:06:34 pm
content: now you say it.. inside this time mark generatoryou see ceramic silver plated standoffs for components and wire assembly several places,they even included a little reel of the special solder wire needed to perform correct rework

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 09, 2022, 08:08:28 pm
content: Quote from: oz2cpu on May 09, 2022, 08:06:34 pmhttps://w140.com/tekwiki/wiki/Ceramic_Strips

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 09, 2022, 08:10:07 pm
content: here you go, found the pictures from the video real fastso i assume you MUST use the correct solder, else you cause damage to them

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 09, 2022, 08:13:04 pm
content: thanks super cool link Alex, the close ups reveal the metal inserts into the ceramics ?so it is not a magic coating after all

author: TimFox
author_rank: Super Contributor
timestamp: May 09, 2022, 08:21:41 pm
content: The old Tektronix ceramic terminal strips were first metallized with silver (IIRC), and if you used a normal 63-37 or 60-40 Sn-Pb soft solder too many times, the silver would "leach" away from the ceramic and the bond would fail.Therefore, Tektronix used SN62, Pb36, Ag2 ternary eutectic, where the 2% silver would inhibit the terminal's silver from going into solution in the molten solder.That alloy is still available where ROHS does not prevail, and is a good general-purpose leaded solder alloy.This alloy is also useful with ceramic parts with Ag-Pd metallization (they usually have a nickel barrier above the metallization to inhibit leaching).Of course, the Nuvistors were brazed with much higher melting point alloys, since the manufacturer's spec for maximum envelope temperature was 150oC, and soft solders usually melt somewhere around 200oC.

author: David Hess
author_rank: Super Contributor
timestamp: May 09, 2022, 08:33:08 pm
content: Quote from: TimFox on May 09, 2022, 08:21:41 pmThe Tektronix solder was apparently 3%, but of course 2% works just fine. I thought it was 4% silver.Special silver-bearing solder is used to solder wires and components directly to thenotches in ceramic strips and has a composition of 60% tin, 37% lead, 3% silver.https://vintagetek.org/ceramic-strips/

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 09, 2022, 08:40:48 pm
content: Quote from: oz2cpu on May 09, 2022, 08:13:04 pmThey're not inserts, they are sintered right into the ceramic. If you don't use 3% silver-bearing solder you can risk damaging the silver in the notch and cause it to disappear somehow. I think tin-lead solder dissolves the silver and sucks it out of the notch and causes that notch to be bare ceramic, as the solder blob no longer wets the ceramic and instead forms a big blob on the components leads.Radio Shack used to sell 2% Ag solder which was good enough.

author: Gyro
author_rank: Super Contributor
timestamp: May 09, 2022, 08:48:38 pm
content: Coincidentally, this is exactly the same process as the sintered metalisation on the ceramic base of the Nuvistor to allow the leads to be brazed (hard soldered) into it, and itself to be brazed into the metal can.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 10, 2022, 12:11:47 am
content: Here's a deeper view into Nuvistorshttp://www.r-type.org/articles/art-150.htmAnd since we're talking about ceramics and small tubes, we can't forget this handsome fellowhttp://www.ase-museoedelpro.org/Museo_Edelpro/Catalogo/tubes/docs_pdf/GE_Cermet_notes.pdf

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 10, 2022, 02:30:04 am
content: For the curious, a close-up of a ceramic strip and a fun variation; a notched disc that goes under a tube socket to connect the tube's circuitry!

author: tggzzz
author_rank: Super Contributor
timestamp: May 10, 2022, 09:42:32 am
content: 7587 nuvistors up for sale athttps://www.ebay.co.uk/itm/165474558179No affiliation

author: David Hess
author_rank: Super Contributor
timestamp: May 10, 2022, 12:27:08 pm
content: Quote from: Alex Eisenhut on May 09, 2022, 08:40:48 pmIt is the tin in the solder which dissolves the silver, or gold, so solders with high tin content are worse. Using a lower temperature for soldering also helps preserve the silver or gold. Note that adding silver to tin/lead solder will not stop it from dissolving gold.When Lavoie copied the Tektronix 545, they used tin plated steel inserts in ceramic strips.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 11, 2022, 09:16:28 pm
content: amazing how nice those solderings looks,a pro did that..

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 29, 2022, 05:01:59 am
content: I didn't know there was such a thing as a dual triode Nuvistor.https://www.ebay.ca/itm/154641184003Quite goofy looking. I like it.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 30, 2022, 08:46:34 am
content: WOW alex, thanks for the linkthat surely added wisdom to my skill set within this field :-)darn now i need one more "thing" to my collection ..so they are stacked ? that is why it is so tall ?any teardown pictures exist for this one ?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 30, 2022, 12:31:25 pm
content: LOL I have no clue! I did all my Nuvistor learning on my Tektronix stuff which only uses the 7586 triode. I've seen Nuvistor tetrodes and I thought those were the only two types.Guess I was wrong. I have no idea how this dual-triode thing is built. It looks like it needs a different socket too.But hey it's inexpensive so I bought one. Totally useless but it will look nice next to my 1965 SONY consumer video camera, I guess.

author: mawyatt
author_rank: Super Contributor
timestamp: May 30, 2022, 12:41:06 pm
content: Another small tube, the Acorn, was used in some equipment way back in 50~60s.https://en.wikipedia.org/wiki/Acorn_tubeBest,

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 03, 2022, 11:10:42 pm
content: Well, the reason I've never heard of a dual triode Nuvistor is that there's no such thing. This tube is nothing more than a subminiature dual triode in a aluminum sleeve.https://www.effectrode.com/news/7550-dual-triode-powerful-subminiature-tube-world/Well played, Marconi. To be fair, it doesn't say Nuvistor anywhere on the box.