Vacuum tube heater

Discussion in 'The Projects Forum' started by sportsfan27, Jan 12, 2011.

  1. sportsfan27

    Thread Starter Active Member

    Jan 29, 2009
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    I've been trying to experiment with vacuum tubes and am having issues with the heater. I'm just using a simple triode gain stage using a 12AX7, with a 6.3V DC heater. According to the datasheet, the heater should draw about 300mA at that voltage. However, using a multimeter, I found that there was little to no current (less than 1mA) flowing. Also, I measured the resistance of the heater and found that it was infinite (Ohm's law led me to expect about 20 ohms). At first, I though the tube might have been burned out, so I tried several other tubes but got the same result. Any ideas what's up? Thanks in advance.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Have a look at this page: http://www.nj7p.org/Tube4.php?tube=12AX7

    Note that the "ends" of the filaments are on pins 4 and 5, and pin 9 is the center; this way you can use either dual 6.3v or one 12.6v supply.

    Make sure you're on the right pins. You might be on pins 5 & 6 instead of 4 and 5.

    Note also that the filaments are fragile, and are more sensitive to shock/vibration when they're warm.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    You learn something every day - normally the "12" at the beginning of the tube designator states the filament voltage.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I'll bet that, if you only needed a single triode, you could run only one of the heaters at 6.3V, drawing 150mA.
     
  5. sportsfan27

    Thread Starter Active Member

    Jan 29, 2009
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    I double checked the pins, tried 3 different 12AX7 tubes and a 12AU7, used three different power supplies, and tried 6.3V (parallel) and 12.6V (series). Still nothing.
     
  6. Ron H

    AAC Fanatic!

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    Hopefully you are aware that the pin diagram is bottom view.
    We don't know your experience level. Do you have the ammeter in series with the heater? Do you have it set on DC amps? Are you sure the ammeter fuse is intact?
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    In my country we don't so often see 12AX7 tubes, so we usually make do with ECC83 valves!:D

    I second that point about the pin view - it's the opposite of what is done with through-hole ICs. I would guess that this results from tubes being used with chassis wired from underneath rather than PCBs, at least early on.

    Perhaps you really have three tubes with both sections of their heaters open? Could they have all been taken out of equipment as being faulty?

    Another possibility might be a defective socket, although it would need to have at least two faults to give the problems you describe. You could try checking the filament resistances again, using another socket for this purpose if you can get hold of one. Testing directly at the tube pins might bend them, which can cause cracking at the base seals, leading to premature failure.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    Vacuum Tubes are definitely fragile as compared to solid state, which makes the 'Proximity Fuse', .... (one of the United States most closely guarded secrets during WW2), seem like a <snip> near an impossible undertaking. For certain, they're not so fragile that one has to worry about breaking the vac seal or connection from the base pins merely by the pressure of test probes, as stated elsewhere in this thread.... Unless you're King Kong! :D
     
    Last edited by a moderator: Jan 13, 2011
  9. k7elp60

    Senior Member

    Nov 4, 2008
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    To add to the SGT's post. For 6.3V pins 4&5 need to tied together and connected to one side of the AC source. Pin 9 needs to tied to the side of the AC source. For 12.6 volts pin 5 is to one side of the source and 4 4 to the other side of the source. Looking at the 9A figure of the link by SGT. the pins are counted counter clock wise from the opening. Pin 1 is at the left side of the opening if the opening is down. The current of 0.3A is for AC rms current.

    To CDdrive, some of us older members can remember some tubes that are not so fragle....like the 12S series that were metal shells...like 12SA7, 12SK7 just to name a few.
     
    Last edited: Jan 12, 2011
  10. sportsfan27

    Thread Starter Active Member

    Jan 29, 2009
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    That must be it, I guess I just got used to reading DIP pinouts and just assumed that it was the same with tubes. I'll test it out tomorrow just to make sure, it seems like the most logical explanation. Thanks!
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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    It was and still is SOP to draw VT basing from the bottom view with Pin 1 always oriented at ~7 0-clock (left side of gap - or 'key' for octals). The count from there is clockwise to the right side of the gap or key.

    FYI: A split filament like this 6V/12V, as stated, should have both 6V legs wired in parallel to produce the same wattage that a 12V hookup would produce. That said, if only one 6V leg of the filament is used the tube will still work. It will be working at =< 50% efficiency though, and will not meet its rated specs.

    There are few tubes that can claim the fame of the 12AX7. It was and is a very hi gain tube and was ubiquitous in its day.
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Why is that? Looking at a photo of the tube, each cathode has an entire 6V heater dedicated to it. Why should the efficiency or the specs suffer?
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
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    Yup, I believe my old SX42 has one or both of those tubes under the bonnet. The last tube project that I built was about 1969. It was a two stage cascode amp with a common grid front end for the 21MHz amateur band. The 6CW4 (Nuvistor) was a metal can tube about twice the size of a TO5 transistor. Back in the old days this tube was found in the front end of damn near every UHF TV tuner in the US.

    One last note before I'm guilty of hijacking this thread.. The military had Mil Spec versions of most common consumer tubes but they were a different animal. The 5R4 Diode was the Mil version of the 5U4. It had a glass envelope that appeared to be many times thicker than the 5U4 and like most Mil Spec, had over ratings far above consumer grade. The military (especially air craft radios) used many tubes that civilians never new existed. We called them 'Pencil Tubes', as they were only about 3/8" in dia by 1" tall (ARC-34's) went into F4 era air craft and plate voltage was supplied by Dynamotors!

    Signed.. Older Than Dirt ;)
     
  14. CDRIVE

    Senior Member

    Jul 1, 2008
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    The 12AX7 is a Dual Triode with a center tapped filament. They're not separate. Hey, even some non dual tubes were made with 6V/12V split fils.
    For argument sake, I've always assumed they were split filaments because that's the way they were always drawn (as I remember it). When you posted this I thought that my old brain simply remembered wrongly but then I verified.
    http://www.alldatasheet.com/datasheet-pdf/pdf/121279/GEC/12AX7.html
     
  15. Ron H

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    Apr 14, 2005
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    I'm trying to envision how you would physically heat two cathodes with one center-tapped heater, unless the center tap was between the cathodes, in which case, you could heat one cathode and leave the other one cold. Can you describe that, or better yet, draw a picture? :confused:
     
  16. CDRIVE

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    Ron, I've never really examined the internals of a tube but I've been reading schematics since the 1950's and the link I posted shows the filament the way I've always remembered it being drawn. On the chance you are correct, I'm sure after viewing that schematic you can see why I've always believed them to be center tapped, because it certainly isn't drawn as two separate filaments.
     
  17. Ron H

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    I'm not sure where AAC got this diagram, but scroll about 2/3 of the way down this page.
    Also, search Google images for 12AX7, and many of the brands clearly show two separate structures. A cathode has to be in the center of each structure, and that in turn has to be heated by an independent filament.
    I didn't mean to start a row here. I had speculated in post #4 that the heaters were independent, and I am just fighting that out to the bitter end.:D
    No hard feelings, I hope.:)
     
  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    Ron, by the time I finish writing this and posting it there may be additional posts, either supporting my contention or supporting yours. Either way, ... this will be posted even if it parrots the contention of another post.

    My last post (last night - actually early Thursday morning) was close to 0130Hrs. You had quite an advantage over me because it wasn't nearly that late in Idaho. This debate caused me undue stress, manifested by tossing and turning till nearly 0300Hrs, when I took a sleeping pill and stressed for another 45 minutes before nodding off. The prospect of discovering that I have been wrong about this for over 54 years was more than I could digest at that hour of the night. I hate you for that! Ha, just kidding!! :D

    Anyway, I had been considering cleaning off my book shelves and donating some of my older manuals and text books to the library. I'm a pack rat, so I doubt that's ever going to happen any time soon. Besides, this spirited debate has convinced me that they hold more value than just my sentimental attachment to them.

    Ron, this scanned page from my 1966 RCA Receiving Tube Manual is not going to make your day, but it certainly is going to make mine, as I will sleep like a baby tonight! And absolutely... no hard feelings. After all, we are here to educate, which includes debate over fact or misconception.

    Re; your link:
    That is possibly the scariest circuit and breadboard setup I've ever seen, and I also have issues with the way an automotive spark coil is schematically drawn there, but that's for another thread and only after I've verified what I believe. If I'm wrong, then that would be also 54 years of having it wrong; but I don't think so this time either.

    By the way... In the course of my entire life, I've never been wrong. I thought I was once, but I later discovered I was wrong about that! :rolleyes:

    Chris
     
  19. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    So - you're another one of those people who wrong once, just like me.;)
    That scanned datasheet doesn't do much for me. The text says the sections have a common heater, but to me that just means you can run the filaments off 12.6V.
    Regarding the drawing, it's symbolic. The cathode, grid, and plate certainly are not flat and parallel as shown. They are concentric (possibly elliptical). Each cathode has to have a heater inside it.
    If I had a 12AX7, I would break it open so we could settle this.:mad::D
    And I'm sorry you lost sleep over this. I slept like a baby, except for getting up to pee 4 or 5 times.:D
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    It's been a LONG time since I've fooled around with 12AX7 tubes, but I remember the heaters going up inside the cathode in a rather rectangular loop. You could see the middle of the loop of each of the two heaters glowing red-orange near the top of the tube.

    I busted one apart some 40 years ago to have a look at the "guts".
     
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