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I am building a prop for a film that needs to have a bunch of glowing vacuum tubes on it. Unfortunately, beyond basic "valve theory", I'm not finding a lot of information about how to interpret the datasheets.

Very simply, what I need to figure out is how to safely make a bunch of tubes glow. I don't have the tubes I've not designed the circuit because I'm mystified by the datasheets. Whether I make a circuit and fit tubes to IT, or whether I buy tubes and design a circuit around THEM is irrelevant at this time. Until I can interpret the datasheets, either option is impossible.

Consider: this ECC81 tube datasheet. Heating voltage is 6.3V, current is 300mA and under the "Typical Operating Characteristics", the anode voltage is 100V at 3mA. Looking at the pin configuration and notes (on this particular datasheet at least), I can't make heads or tails of how to read it. It looks to me, for instance, like pins 4 & 5 get the heater current (f), but then what the heck is pin 9 (fc) ? Further, it looks like pin 6 is the anode input (a), but what's pin 1, designated by a'?

With my limited knowledge, I'd send 6.3V to pins 4 & 5 and watch the healthy glow. MMy gut-level reaction though, is that's only half a circuit. So what's the other half? I would think I'd need to somehow draw off of the cathode and dissipate that power somehow...but when all I want is the glow, where does the back half of the circuit lead to? Am I making any sense here? Part of this is that I'm not quite sure I'm asking the right questions. Help!

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  • \$\begingroup\$ I'd be awfully tempted to just buy a tube and try only powering the heater as you suggest. If that didn't work I'd be rather tempted to just glue a PWM'd throbbing LED behind the tube(s). I'm all for an easy life. \$\endgroup\$ – RedGrittyBrick Jan 22 '14 at 14:31
  • \$\begingroup\$ The second I go for easy, the director wants a closeup of the tube warming up. Doing it the hard way guarantees that the piece stays in the background and likely doesn't even read on film. :) But yeah, I hear ya. I actually had some LED mockups that I didn't like before I decided to research real tubes. \$\endgroup\$ – dwwilson66 Jan 22 '14 at 14:41
  • \$\begingroup\$ Heater current is 300A!!!! \$\endgroup\$ – Leon Heller Jan 22 '14 at 14:41
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    \$\begingroup\$ @LeonHeller Thank you for pointing out (TWICE!) that the datasheet current is actually in mA instead of A. \$\endgroup\$ – W5VO Jan 22 '14 at 15:23
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    \$\begingroup\$ Yeah but if you want that eerie blue glow you need a nice fat beam tetrode (KT66 or EL34) with about 400V on the anode as well as the heater current... (keep the anode current down to 50-100ma). If the film has a particular time period or country, add that to the question; valves come in all shapes and sizes and you might want to be period accurate. \$\endgroup\$ – Brian Drummond Jan 22 '14 at 15:38
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As Dave said, you don't have to actually run the tube in a real circuit to get the visual effect. All you have to do is power up the heaters.

Your particular tube actually is two tubes in one package and has two heaters. Each heater is 6.3 V. You can put 12.6 V accross pins 4 and 5, or 6.3 V accross both pins 9-4 and 9-5. In the latter case, you connect one side of the 6.3 V supply to pin 9, and both pins 4 and 5 to the other side.

Don't worry about the exact voltage that much. These things were intended to be run directly from a 6.3 V output winding of a power transformer. It will be OK with variations of this 6.3 V due to input line voltage variations. Get a "6.3 V" transformer intended to run from whatever power you have in your location and the tube will be fine.

You do have to make sure the transformer is rated for the total current of all the heaters together. For example, if you are powering 5 heaters that all take 6.3 V at 300 mA, then the tranformer output must be rated for at least 1.5 A.

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If all you're after is the visual effect, the only thing you need to do is apply power to the heaters. Ignore everything else. Get a hefty 6.3VAC transformer and wire all of the heaters in parallel, making sure the total current doesn't exceed the transformer's rating.

All of the other terminals of the tube don't need to be connected to anything at all.

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  • \$\begingroup\$ Ahhh. Sweet. That's good to know. Now all I need to do is find something that takes a reasonable current. \$\endgroup\$ – dwwilson66 Jan 22 '14 at 14:42
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Rather than using a transformer, which will be expensive and heavy, you could as easily use DC from a 12V (2 filaments in series) or 19V laptop adapter (3 filaments in series). An impressive manly vacuum tube such as an 807 beam tetrode will have about a 1A filament, so an ordinary laptop adapter should be able to power at least 9 tubes (check the nameplate rating and the tube datasheets and add maybe 30% margin). Your diminutive tube requires only 300mA, so many more could be powered.

Most tube datasheets have been scanned and are available on the net. Only connect the filaments of tubes of the exact same type in series with this arrangement (you can have different groups in parallel, but each type in series should match its neighbor(s)).

enter image description here

If you do use something like a beam tetrode with a cap on top, please attach a fake connector with a fake wire to it, nothing blows the suspended disbelief like seeing obvious misteaks(sic).

American tube types usually have the filament voltage as the first digit(s) of the part number, so a 12AX7 requires 12.6V (or 6.3 if you use the center tap) on the heater, whereas a 6L6G (shown below) requires 6.3V RMS.

enter image description here

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    \$\begingroup\$ The 12AX7, and many other 12.6 volt tubes, have heaters like the OP's ECC81 that can be wired for either 12.6 or 6.3 volts. \$\endgroup\$ – Peter Bennett Jan 22 '14 at 17:03
  • \$\begingroup\$ Good point, I'd forgotten the center-tapped heater in those. \$\endgroup\$ – Spehro Pefhany Jan 22 '14 at 18:56

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