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I got this tubes OT100 and I want to know their value, and what are they used for. They are NOS, unused and in working state.

OT 100 tubes

Mark Morgan Lloyd
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Mihai Gabriel
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  • As a matter of interest, how much are you selling them for? Strangely, eBay also uses the term "NOS" for their page selling OT100 tubes. – Andy aka Jul 01 '23 at 18:12
  • NOS = new old stock – Barry Jul 01 '23 at 21:45
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    @Andyaka That's very common in tubes and other vintage electronics; it stands for "New, Old Stock" and refers to things that were manufactured decades ago, but which are still technically new in that they've never been used. Usually they come in the original packaging and everything. – Hearth Jul 01 '23 at 21:52
  • I think you are missing my point. – Andy aka Jul 02 '23 at 00:37
  • I found them in an old factory in a cardboard with their sleeves and boxes still on, so I am pretty sure they are unused, given the fact that I found in each box a certificate of guaranty dated 1974. I received an offer for 70$ each in Romania, as I saw they are selling for at least 100$ each on Ebay. They are used in large amplifiers like MCintoshMI200 so they are pretty valuable. I have other more and I like to find someone to build an amplifier for me and to give him the rest of them – Mihai Gabriel Jul 02 '23 at 11:05

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With a quick search, I was able to find a reference by searching just "OT100 tube". In the interest of not making this a link-only answer, here's the information I've found:

The logo on them is the Tungsram brand, which was a Hungarian company making vacuum tubes and lightbulbs that apparently only went out of business last year (2022).

They're triodes, with an unusual four-pin base and a top contact for the anode, indicating that they're probably high-voltage/high-power tubes. The site I linked above claims that they're radio transmitter tubes, which would make sense given the top contact; transmitters are often fairly high power.

The filament is a 10 V, 32 W(!) directly-heated cathode between pins 1 and 4, the grid is pin 3, and the anode is the cap on top. Pin 2 is not connected. I'm afraid I can't tell you how to identify pin 1 (unless it's labelled with a clear molded "1", as these sometimes were), because I've never seen that particular base before. Several sources claim that the OT100 is equivalent to the 8005, which was produced by a number of American companies including RCA.

enter image description here
(image source; page is about the 8005, which has the same pinout as the OT100. Their image for the OT100 is hard to read.)

I wasn't able to find one for the OT100 itself, but here's a datasheet for the equivalent 8005. When reading the datasheet, be aware that μμF is the old-fashioned term for what we call a pF today; the datasheet predates the modern SI prefixes.

Hearth
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    Much to my surprise, indeed it's only in 1958/1960 that prefixes for units, like pF, where adopted (resolution 3 adopted by the International Committee of Weights and Measures, 1958; and 11th General Conference on Weights and Measures, 1960). – fgrieu Jul 02 '23 at 05:54
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    @fgrieu-onstrike And after that it took a few years for the conventions to take hold. Learning electronics in the 60's, I was used to seeing pF written as µµF or MMF, along with other oddities (kMc for GHz, ...). – John Doty Jul 02 '23 at 15:36
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    @JohnDoty I'm surprised it wasn't kMcps; the 1930s book about vacuum tubes that I have uses cps (cycles per second, if that's not obvious), I suppose it must have gotten abbreviated by the 60s. – Hearth Jul 02 '23 at 16:31
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    Stromberg-Carlson's engineering data for its 1935 58-T multiband receiver uses kc for radio frequencies, "cycles" for mains. It also uses ω for ohms, and Ω for megohms. The Radiotron Designer's handbook, (Langford-Smith, 1952) uses Mc/s and Kc/s (!). The RCA Receiving Tube Handbook (RC-19, 1959) uses Mc. The language and notation weren't stable until the late 60's. I remember being confused by the statement that clock ticks from the time signal station WWV were "5 cycles of a 1000 cycle tone". – John Doty Jul 02 '23 at 17:17
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    Note that the symbols for pins 1 and 4 are larger than pins 2 and 3. The physical pins on the actual tube follow a similar pattern -- pins 1 and 4 are larger diameter. – Dave Tweed Jul 02 '23 at 19:41
  • That's right. Also, you can indentifly them by the 5th pin placed on the ceramic socket, as seen in the botton tube from the picture. – Mihai Gabriel Jul 02 '23 at 20:30