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Thread: Understanding the heating of a 12AX7

  1. #1
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    Understanding the heating of a 12AX7

    Ive an old EL34 based tube amp, and I noticed a consistent orange wire running from the grids(pin 4) and the heaters(pin 7) of the power tubes to all of the heaters(pin 9) of the six 12AX7's. I suppose this is how the filaments are heated up?

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    Also, another conistent wire I noticed following a similar pattern. A brown wire coming the heater(pin 2) of both power tubes is going to every heater(pin 4) of each 12AX7. Meant to point out that the orange wire previously mentioned is going to each heater center-tap(pins 9). Anybody know just what the wires are acheiving?

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    Senior Member Enzo's Avatar
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    I have to doubt that the same orange wire connects pin 4 of the power tubes and any heater connection.

    Pin 4 of an EL34 is its screen grid, and screens sit at B+ voltage - several hundred volts.

    Pins 2 and 7 are the heaters of the EL34 - there should be 6VAC between them. In some amps that 6VAC could also be elevated above ground be some DC voltage - but not several hundred.

    In the 12AX7, pin 9 is the center tap of the dual filament heater. Pins 4 and 5 are the ends of it. COnnect 12V from pin 4 to pin 5 and the tube heater runs on 12v. But connect pins 4 and 5 together, and now the two halves are in parallel, so the tube heats with 6V. SO the same 6VAC for the power tubes could also run the preamp tubes by running to pins 9 and 4+5.

    The heaters of most power tubes are run on AC 6V, but tubes can also run on DC if you want. In power tubes it doesn't matter, but in sensitive preamp tubes, making the heaters DC operated is one way to reduce hum. SO sometimes there will be separate wiring to some preamp tubes. But sounds like your amp is plain old 6VAC throughout.

    Your heaters have their own 6VAC winding on the power transformer. It should be totally independent of the +400vDC on that screen grid.

  4. #4
    Supporting Member Alex R's Avatar
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    - maybe you're counting the pin numbers the wrong way. Go clockwise from the lug (or the gap) from under the socket.

    ...sorry Enzo if you were being Socratic about it.

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    Senior Member Enzo's Avatar
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    Nope, no hidden agenda here. Counting either direction, pin 4 won't be a heater, and pins 2 and 7 wirk out to be heaters either way as well.

    But as anyone who ever traced out a circuit knows, when wires go into bundles or even just parallel paths, they can easily be confused.

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    Enzo you are correct. There actually IS an orange wire coming off of both power tube screen grids, but its not going to pins 9 of the preamp tubes. Its going to the cap can. My apologies.

    man thanks for that knowledge. Now looking at it I see 6VAC coming from the EL34 heaters going to the center taps of the 12AX7's, whereas there heaters are indeed connected and in parallel producing 6V instead of 12V. cool stuff.
    Now what is the brown wire doing that the orange wire isnt, or vica versa??

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    Supporting Member Alex R's Avatar
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    Brown and orange are two halves of a 6.3v AC supply. People like to connect the preamp tubes in phase, apparently it keeps them quieter though I've always thought it would make them noisier, but what do I know.

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    but man am I having a noise issue. Is there anything you could recommend me doing with those wires to reduce the noise?

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    Old Timer Amp Kat's Avatar
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    Noise or hum ? If it's hum keep the wires twisted in between the tube sockets. Twisting the heater wires cancels out hum. If it's noise try moving wires around with a non conductive probe and see if it stops.
    KB

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    its definitely noise. Its an old tube PA and it could be a number of things that i dont know. but i will mess with that

  11. #11
    Old Timer olddawg's Avatar
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    Quote Originally Posted by Enzo View Post
    In the 12AX7, pin 9 is the center tap of the dual filament heater. Pins 4 and 5 are the ends of it. COnnect 12V from pin 4 to pin 5 and the tube heater runs on 12v. But connect pins 4 and 5 together, and now the two halves are in parallel, so the tube heats with 6V. SO the same 6VAC for the power tubes could also run the preamp tubes by running to pins 9 and 4+5.
    Enzo, I have always wondered if you could convert an old chassis that uses 6SL7s to 12AX7s by wiring the filaments in this way. There would be a lot of other rewiring as well , but could you rebuild and old amp chassis that has only a 6v filament supply to us 12AX7 preamp tubes? I have an old mil spec rack mount 6L6 power amp that I wonder if I could add a preamp section to this way.

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    Old Timer Amp Kat's Avatar
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    Quote Originally Posted by olddawg View Post
    Enzo, I have always wondered if you could convert an old chassis that uses 6SL7s to 12AX7s by wiring the filaments in this way. There would be a lot of other rewiring as well , but could you rebuild and old amp chassis that has only a 6v filament supply to us 12AX7 preamp tubes? I have an old mil spec rack mount 6L6 power amp that I wonder if I could add a preamp section to this way.
    Very easily ! 6.3VAC is 6.3VAC. It doesn't matter where the filament connectors are located they can be switched. All tubes are the same as for as 6.3 heaters go. The only difference with DUO Triodes like the 12AT series is they have a Center tap to operate the tube for 6.3 or 12 volt operation. To run them in series at 12 volts you use pins 4 and 5. To run them at 6.3 you tie 4 & 5 together and use pin 9 as the other connection. Any tube that is a 6.3 volt tube can be wired to a 12AX7 very easily.
    KB

  13. #13
    Senior Member Enzo's Avatar
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    MOST guitar amps run their 12AX7s on 6v.

    ANd pretty much the only time I see them on 12v is when they are wired in series over a 36v supply or a 48v one.

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