Huh? Tubes Running with 12V on the Plates?

Yup. That, or even less. You don't have to risk your skin building 250V power supplies just to build a simple radio receiver with tubes. Note well that this applies  almost exclusively to receivers. Getting any significant amount of power out of a tube—more than about fifty milliwatts—requires voltages that edge up into lethal territory. But if you've got or can acquire a pair of headphones, getting started can be done on 12V or less.

There are two tricks that make this possible:

Let's talk about the tubes designed specifically for 12V operation first.

"Space Charge" Tubes for  Car Radios

Way back in the late 1950s, engineers at Tung-Sol decided to do away with vibrator power supplies in car radios. Vibrators were plug-in modules roughly the size and shape of octal tubes, which created an interrupted DC current using a solenoid coil and reed-style contacts. The interrupted DC current allowed a step-up transformer to accept a 12V input and produce the 180V output required by conventional vacuum tubes. Vibrators were noisy (both at audio and at RF) and because of arcing across their contacts had a finite life and had to be replaced periodically. The transistors of the time weren't as good (nor as cheap) as they needed to be, especially at RF. So instead of using late 50's not-quite-ready-for-prime-time transistors, Tung-Sol created a line of tubes that could work with a car-battery plate voltage of 12V, which by 1958 had become ubiquitous in new American cars.

These tubes worked on something called the "space charge" principle, which basically used the first grid after the cathode to accelerate electrons toward the weakly-charged plate, which was now working at 12V instead of 180V. Between 1958 and 1962 Tung-Sol, GE, RCA and other manufacturers released quite a few different types, all in 7- and 9-pin miniature packages. Virtually all of these tubes were designed for RF work up to reasonable HF frequencies. Note that there are no true power amplifiers in the lineup. Getting a tube to output significant power with 12V on the plate is almost impossible. Nonetheless, if you're content with headphones or QRP operation, the space-charge 12K5 driver tube can source as much as 35 milliwatts at either audio or RF.

Note also that conventional high-voltage tetrode tubes can be used in space-charge mode at 12V, but the high capacitance between the second grid and the plate make them function poorly compared to tubes designed specifically for space charge operation. Nonetheless, reasonably effective circuits have been published using classic tubes like the Type 49. (I'll provide pointers to numerous circuits later on this page.)

There are a few tubes that blur the difference. I've heard a couple of people say that the 12U7 dual triode is basically the same tube as a 12AU7 dual triode with a little more gain.

The main reason to use space-charge tubes is that they do not require dangerous voltages to operate. Both the filaments and the plates run at 12V, so your risk of shock while testing and using your projects is virtually nil. In the past 40 years or so we've gotten very used to poking our solid-state projects with our fingers, forgetting the "one hand in the pocket" rule and general caution around high voltages that kept us all alive when tubes had 180V, 280V, or even 450V on the plates. The idea is to present a technology that you can work on with your kids (or encourage them to explore on their own) without fretting about shock hazards.

Space-Charge Tubes and Tube Data

Below is a list of all the space-charge tubes that I have discovered. Others may exist, especially from non-North American sources. Left-margin links to tube data are to the wonderful NJ7P Tube Data Site. The data PDFs are from wherever I can find them.) If you know of any other tubes specifically designed for 12V or less on the plate, please let me know!

Tech Tips for Space-Charge Tubes

The best way to get a sense for space-charge tubes is to study some real circuits. Look at the online circuits listed below, and see if you (or one of your friends) have any of the older print articles listed after the online articles. That done, choose a simple circuit for a "learning project" and just have at it! That said, here are some tips:

Low-Voltage Operation With Ordinary Tubes

Keep in mind that a lot of "ordinary" high-voltage tubes can be used very effectively with DC voltages as low as 3V on the plates. Some of these tubes can be operated in "space-charge" mode with the first grid used as an electron accelerator, but that's not entirely necessary. I cite some circuits below with ordinary triodes operating at very low voltages.

The problem with tubes generally is that taking power out of a tube circuit requires operating at a high voltage. If you don't need significant output power (say, for a loudspeaker) and can settle for very low values of plate current, you can use plate voltage values under 20V, and often less than 10V. When building radio receivers, this generally requires using high-impedence headphones for audio output, where "high-impedence" means 600 ohms or more at audio, the higher the better. Such headsets are getting increasingly hard to find; Antique Electronic Supply used to sell them, but do not show them in the 2007-2008 catalog or later. I do see them regularly on eBay. You can also try military surplus outlets like Fair Radio Sales.

Crystal earphones are also high impedence and can be used, with one gotcha: They do not pass DC. In many older circuits, the plate voltage passes through the headphones, which requires a dynamic headset (that is, one with wire-wound transducers) rather than a crystal transducer. For an example, scroll down to the schematic for KB7NRN's 18V radio. A crystal earphone will not work in this circuit, nor any circuit that passes B+ to the plate of the tube (or anywhere else) through the headset.

Many of the circuits you'll find in books and magazines specify 45V, 67.5V, or 90V batteries, which can still be had but are expensive. A lot of those circuits will work just fine on  much lower voltage. For example, I built the 3V4 BCB receiver from Harry Zarchy's kid-hobby book Using Electronics, and the circuit has a 100K resistor in series with the regen control pot. The resistor was necessary because the specified 45V battery supply was too high. Without the resistor, the receiver oscillated at 45V irrespective of the setting of the regen pot—and worked perfectly at 9V, using an ordinary (and cheap) transistor radio battery.

I use a beat-up "hamfest special" adjustable lab supply for simple tube radio work, and I can spin a knob to vary the voltage between 0 and 50V. When I have a circuit on the bench spec-ed for 45V B+, I set the supply at 45V, make sure the circuit works, and then start dialing down the voltage to see what happens. Most of the time, increasing the amount of regeneration on receiver's throttle control will compensate for the reduction in voltage. Sometimes (as with Zarchy's circuit) you have to tweak component values a litttle to get down under 10V B+, but with decent headphones and a good antenna you will get signals. The art lies in tweaking things to get the most signal at the least plate voltage.

Drawing Your Own Tube Schematics with Visio

If you modify published tube circuits before you build them, it's always a good idea to redraw them so you don't forget how the circuit on the bench differs from the one in that old article in QST. There's a very nice drawing program called Visio (now owned by Microsoft) that I've been using to draw schematics for over ten years now. Older versions can be had on eBay for as little as $20, and in fact the version I use—Visio 2000—is now seven years old and still perfectly useful. (The latest versions require product activation, which I cannot abide, but in truth there's little in the newest Visio versions that isn't in Visio 2000. Just don't use Visio 1.0)

I created a stencil file full of tube pinouts, including all the common tube types (dual diode, triode, tetrode, pentode, and so on) and all you need to do is drag them off the stencil and start connecting the leads. (Visio comes with stencils for all the common components like resistors, capacitors, and inductors.)

You can download the stencil file here. It has stencils for the several earlier versions of Visio, but the Visio 5 stencil works fine with Visio 2000 and later versions. Here's an example of a schematic I drew with the stencil. (285K GIF.)

Low-B+ Tube Articles and Circuits on the Web

Print Articles on Low-B+ Tube Technology

The articles shown below are listed in chronological order, with the earliest first. If anyone has a circuit not listed here that uses any 12V space charge tube or conventional tube operating with 20V or less on the plate, please contact me, so I can get a copy and add it to the listing. Many thanks to Michael Covington N4TMI for locating some of these ancient articles for me in the University of Georgia stacks. Also thanks to John Bauman KB7NRN for providing some of the scans out of his personal library.

You can find some of these publications on eBay, or on the used book services like ABE Books.