Does anybody know how to light up a vacuum tube? I have about 9 of them and only 1 I can see the name. It is a 12DT8. Is it possible to find the heater without the datasheet by using a multimeter etc... And if so, is it possible to power it with a DC battery?

I am not trying to operate it for complex electronics, I just want to light them up for decorational purposes. I got these from a 1950's radio, if that's any helpful info.

  • \$\begingroup\$ I've never worked with tubes, so I'll leave this as a comment rather than an answer. There's a tutorial on ehow about this that might be useful: ehow.com/how_4914157_make-vacuum-tube-light-up.html \$\endgroup\$ – JYelton Sep 17 '12 at 3:02
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    \$\begingroup\$ When I was 12ish (in the days of tubes) I would bring home dead TVs from the pile behind the repair shop to cannibalize for parts. It was much easier then since there were no printed circuit boards, and you could easily clip loose individual components. If you are lucky, your radio had a power transformer which still works and would provide 12 volts for the tube filaments. Since you now know which pins on the tube, you could trace the wires to determine the filaments of the others. BE CAREFUL! There would also be a high voltage winding, typically 250 volts. Continued in next comment.. \$\endgroup\$ – UnconditionallyReinstateMonica Sep 17 '12 at 3:59
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    \$\begingroup\$ continued... It is more likely that this is an 'AC/DC' transformerless radio, in which, for example, 10 tube filaments of 12 volts each were connected in series across the 120 volt power line. This is potentially even more dangerous since the power line voltage is present in the circuit. If you are interested in this stuff you can read up first and do it safely. It fascinating stuff, but please do read up and understand what's going on before mucking about in there. \$\endgroup\$ – UnconditionallyReinstateMonica Sep 17 '12 at 4:05

You can use AC or DC voltage to power the filament of a vacuum tube. The filament will glow when powered up which is what you are referring to as "lighting up". So a battery is fine. The voltage required by a particular tube is given by the initial digits in the part number. Thus your 12dt8 requires 12 volts to light up the filament. Most tubes that were used in radio and television sets used either 6 or 12 volts although many other voltages were used. You will need to determine which pins are connected to the filament. A tube manual or a web search should provide this information. Since you don't intend to actual use the tube in a circuit, you could just try different combinations of the pins until the filament glows.


Don't fear the datasheet. Tube datasheets are a fun read. Google for "12DT8" and the datasheet is the first or second link.

Here are the relevant parts for your question:

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The heater connections are labeled "H". I believe you will find many tubes have the same pinout. If you look at the radio itself there is usually a label listing the tube complement.

For the heater locations of the unmarked tubes, look to the radio they came from. The heaters are all wired together, either in parallel (for transformer radios) or series (for "AC/DC" sets).

Anyway, lighting up the tubes is kinda boring, just using them as light bulbs. Why not get the old radio working again? There is a lot of old radio service information on the web these days.

  • \$\begingroup\$ Note the pin numbering is from the bottom. It's also the numbering on the socket, if you're underneath the chassis soldering wires to the socket. Now note that if you look at and I.C. from the bottom, the numbering goes in the same direction! Many early IC's were in a round can, even more similar to the tube layout. \$\endgroup\$ – gbarry Sep 17 '12 at 5:19

Yes, you can safely probe a tube to find the heater. Everything else in a tube is just some metal with vaccuum between it and the other parts. The small voltages and currents of a ohmmeter won't damage anything, not even close.

The "12" in the part number indicates that the heater is intended to be powered from 12 V. This one has a separate heater, which usually means it was designed to be run directly from a 12 V tap of the main power transformer. The heater is electrically isolated from the cathode so that the AC doesn't get into the signal (at least mostly not).

Here is the section on this tube from the 1960 RCA Receiving Tube Manual:

This confirms the heater is intended for AC and also tells you the pinout explicitly. Keep in mind that tube pins are numbered clockwise when looking at the tube from the bottom, because that's how you'd be looking at it when soldering wires to the socket. The convention is that there is always one place around the circle that is special, usually a gap in the pins, but it could also be a mark or a nub sticking out or something else. You start with pin 1 going clockwise from that specially marked place. In this tube, the special place is marked by a larger gap between the pins than the other gaps. That is very common.

Just because the heater can be run from AC, doesn't mean it needs to be. You can safely run this heater from a 12 V battery like a car battery or even off of 12 V car power. Car voltage is more like 13.6 when the engine is running, but that's still OK. Don't go higher though.

The 12DT8 information refers you to the 12AT7, for which there is considerably more detail:

This extra stuff doesn't matter if you just want to make the tube light up. Just connect 12 V between pins 4 and 5 and it should glow nicely after a few seconds.

It may be fun to see the tube light up, but it's a lot more fun to have it actually do something. Fortunately, this is a tube you can do some interesting things with yourself and see some results. For example, if you can find someone to help you with electronics a bit, you could wire this up to make a small amplifier. You can use a small loudspeaker as a microphone, and have the output of the amplifier drive a pair of headphones thru a audio transformer. Just be careful with the high voltage these things need. Again, have someone help you with this.

  • \$\begingroup\$ Load Lines! Good stuff, and a valuable tool that the young ones rarely see. \$\endgroup\$ – Scott Seidman Sep 17 '12 at 14:35
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    \$\begingroup\$ Is µµf an old notation for pF? \$\endgroup\$ – starblue Sep 17 '12 at 17:58
  • \$\begingroup\$ @star: Yes. At least the double-micro is clear enough even though a bit cluttered. Some old schematics are much worse where they abbreviated "micro" with "m". If it's older than 1990, "mF" probably means micro-Farads, not milli-Farads. Sometimes you will even see "mmF". In that case you know m isn't milli. \$\endgroup\$ – Olin Lathrop Sep 17 '12 at 18:13

Stupid people don't ask questions and live in ignorance. Smart people ask questions and learn, so no worries about your question. Keep asking them!

The part of the vacuum tube which "lights up" is called the heater. For US tubes (which you have here) you can tell the voltage of the heater based on the tube's number. Standard US tube numbering has numbers, letter(s) and numbers (and maybe letters). The leading numbers indicate the heater voltage in volts. The following letters and numbers specify the specific tube. So, your tube: 12DT8 uses a 12V heater voltage. It has the same electrical performance as a 6DT8 tube.

While you can figure out which pins it is connected to by using an ohmmeter, you can also look at its data sheet which shows the heater is connected to pins 4 & 5.

Note that you don't need to use the full heater voltage to get the tube to "light up", so you might try just using a 9V battery. Note also, that the tube will draw down the battery reasonably quickly, so you may want to look into getting a small wall power supply. Any wall adapter with 12V (ac or dc) would work just fine.

If you actually want to use the tube as a tube, lower voltages will reduce the performance of the tube since it produces fewer electrons; however, it will still work. Sometimes, designers would operate tubes with reduced heater voltages to extend their life; however, their circuits would take into account the tube's characteristics with this operating voltage.

  • \$\begingroup\$ Stupid people also ask stupid questions. of course there are stupid question, although this is certainly not one of them. \$\endgroup\$ – Olin Lathrop Sep 17 '12 at 15:35

You can use your ohmmeter to find the filament. Because all the other elements of the tube are really just pieces of metal hanging out in a vacuum, there are only two pins where you will get a low ohms reading. Those are the filament connections.

Without a datasheet, you can try bringing up the voltage until the filament glows. If you don't have the luxury of a variable power supply, you can rely on the fact that most tubes made were either 6- or 12-volts. So, try 6, then try 12. And by the way, the first number--the 12 in 12DT8--is the filament voltage! How nice is that?

There was a 1-volt tube used in TV's, and there were some 3-volt tubes used in portable (!) tube radios. Those didn't look like much when lit up. There are also two common tubes in table-top radios, a 35 and a 50 volt tube. You'll know you've got one if it doesn't light up and you didn't burn it out trying.

The filaments are heaters, and use a moderate amount of power. They'll work fine with a battery, 12V DC is the same as 12V AC in this case. But the batteries won't last long.


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