Who doesn’t make radio vacuum tubes anymore

Light an old vacuum tube

Yes, you can safely examine a pipe to find the heater. Everything else in a tube is just some metal with a vacuum between it and the other parts. The small voltages and currents of an ohmmeter do not damage anything, not even close by.

The "12" in the part number indicates that the heater should be operated with 12 V. This has a separate heater, which usually means that it can be operated directly from a 12 V tap on the main transformer. The heater is electrically isolated from the cathode so that the alternating current doesn't get into the signal (at least most of the time it doesn't).

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

This confirms that the heater is designed for AC power and also explicitly tells you the pinout. Remember that the tube pins are numbered clockwise when you remove the tube from look at below as this is how you would see it when soldering wires to the jack. The convention is that there is always a spot around the circle that is special, usually a gap in the pins, but it could also be a mark or a protruding nub or something else. You start with pin 1 clockwise from this specially marked location. In this tube, the particular location is marked by a larger gap between the pins than the other gaps. It is very common.

Just because the heater can be operated with alternating current doesn't have to be the case. You can safely run this heater on a 12V battery such as a car battery, or even a 12V car battery. The car voltage is closer to 13.6 when the engine is running, but that's still fine. But don't go higher.

The 12DT8 information refers to the 12AT7, for which there are many more details:

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

It might be fun to see the tube light up, but it's a lot more fun when it actually does something. Fortunately, this is a tube that you can use to do some interesting things to yourself and see some results. For example, if you can find someone to help you with a little electronics, you can hook this up into a small amplifier. You can use a small loudspeaker as a microphone and have headphones controlled via an audio transformer at the output of the amplifier. Be careful with the high voltage these things take. Let someone help you here too.

Scott Seidman

Load lines! Good stuff and a valuable tool that the boys rarely see.

Star blue

Is µµf an old notation for pF?

Olin Lathrop

@star: Yes. At least the double mic is clear enough, although it's a bit cluttered. Some old schematics are much worse when they abbreviate "micro" with "m". If it's earlier than 1990, "mF" likely means micro-farads, not milli-farads. Sometimes you even see "mmF". In this case, you know that m is not milli.