12ax7 amp with ignition coil output transformer issue

Discussion in 'The Projects Forum' started by 100Dave, May 7, 2010.

  1. 100Dave

    Thread Starter New Member

    May 4, 2010
    6
    0
    hello all, I am building the tube amp circuit in the tutorials section of this site, it is located here: ​


    I have everything assembled, but I have not yet been successful due to some problems with the output transformer ( the auto ignition coil ) I get 0 volts output from the coil, get 160 VDC into the coil from the power supply.

    I looked up how to test an automotive coil, and found that mine had acceptable resistance between plug wire connector and the 12V connections, but that between the 12V connections only, I had 0 resistance, which told me this coil is BAD.

    I promptly ran down to my autoparts store and got a brand new coil for $35 and hooked it up, preparing to be amazed at my tube amp....only to find it has the exact same ailment as the first coil. ( No resistance between "+" and "-" contacts )

    What are the chances I would have two bad coils like that? Could I have ruined both coils un-intentionally? The box says it handles up to 4800V, I hit it with 160V, and 12 V. Can anyone shed some light here?

    thanks
    Dave
     
  2. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,907
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    The DC resistance should be very low on the 12 volt side. Your coil is GOOD. Have fun but +160 vdc on the speaker wires looks dangerous. A nice audio transformer is much better.
    http://www.hammondmfg.com/pdf/5C_125SE.pdf
    http://www.mouser.com/ProductDetail...GAEpiMZZMv0IfuNuy2LUWsMe/iM%2b5SyIdID3sjBmtY=

    http://www.fas.harvard.edu/~scphys/courses/E1b/E1b_3b.pdf
    http://www.crystal-radio.eu/entrafotest.htm#bobine
     
    Last edited: May 7, 2010
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    I hadn't looked at that schematic before. I also feel that it's not safe.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    The ignition coil as an output transformer is an example.
    Better use a real output transformer, that will give a much better result.

    Also take a look here:
    http://www.geofex.com/tubeampfaq/taffram.htm

    It is an old text, but read the safety paragraph.

    Bertus
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
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    Having your speaker at 160V above ground is not good. Read about tubes, play with transistors & ICs. Or if can be found some battery operated tubes, 1.5 or ct 3V filaments, 45V plate. Nice schematic of 5 tube battery radio inside back cover of 1945 RCA tube manual.
     
  6. 100Dave

    Thread Starter New Member

    May 4, 2010
    6
    0
    hello all,
    I have gotten rid of the auto coil and purchased a hammond 125ASE output transformer, no more high voltage on the speaker wires..... but there is one thing......

    This thing BARELY amplifies the signal that I input... i believe the circuit is connected correctly, the tube glows, voltages all seem correct... but I hear only maybe a tiny bit more volume than hooking the signal output up to the speaker leads directly and bypassing the amp altogether !!

    So I am wondering, is this the normal output of a 12ax7 used like this? Is the purpose of this tutorial to show me the workings of transformers/tubes/rectifiers etc.? Because the author claims at the end of the tutorial " You may be pleasantly suprised at the depth and tone from this amp" Well I am NOT, in fact, Im not convinced that this even is an amplifier.

    It would be helpful if someone could explain what this exercise is for .... and if I actually achived anything in learning about tube circuits ...seems like a big waste of time and money at this point.


    My future plans are to take the components I have assembled here and add to them until I can build another circuit that adds a 6L6 tube ( Fender Princton "5 F E" Circuit )
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    What's the input signal level? You need at least a 2v pp volt input signal to drive those tubes to full power. If it's from a mp3 type player speaker output try this, feed your input signal into the 12 volt side of the auto coil and connect the spark wire side into the input of the amp. This should act as a step-up transformer to match the amps high Z input. ( a small 70 volt audio line transformer also works)

    Las Vegas was built on wasted time and money. At least you are gaining something:D

    Looking at the tube curve, I would add about a 250-500 ohm resistor from the cathodes to ground with a 100uf bypass cap to bias the grid at -1 to -2 volts.
    http://en.wikipedia.org/wiki/Cathode_bias
    [​IMG]
     
  9. Bychon

    Member

    Mar 12, 2010
    469
    41
    I don't think you said what you are amplifying, but the 12ax7 is famous for being in guitar amplifiers. A guitar will produce the voltage needed to drive the tube input properly.
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    You probably have it as a preamp. That would barely amplify the signal.

    In my studio, I used the ART tube preamps (single channel) and built a few with 12ax7's. They added a WHOLE LOTTA great brightness and pop.

    great little tube.

    But, I still had to amplify the signal after conditioning it with the preamp. (Or we went direct to tape.)
     
  11. 100Dave

    Thread Starter New Member

    May 4, 2010
    6
    0
    Hello all thanks for the replies, there was alot of information to work through..... I ended up adding a 70v transformer (per nsaspook suggestion)to the input as a step up transformer and this seemed to make it all come together..... there was obvious audio amplification occuring !!!

    One poster suggested that the output from a guitar pickup would be a suitable input and get this thing working, unfortunately, that is not the case, which is sad, cause ultimately I am trying to turn this into guitar amplifier, not keep it an mp3 player(line level) amplifier.

    I will start a new thread for the problem amplifying guitar signal, I dread having to explain why I have a circuit with one tube and 4 transformers, and all that. Im sure the problem lies withi the inputs having a 100k resistor and a .1uf cap on it...probably ok for line level but not good for guitar input.

    I have tried to copy fender champ designs with a 68k and 1 meg resistor on the input, removing the step-up 70 volt transformer, but still no guitar signal through the amp.

    Dave
     
    Last edited: May 29, 2010
  12. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    It should be easy to design a pre-amp stage for the guitar input. Here's a link to some old tube designs.

    http://www.drtube.com/guitamp.htm
    http://www.drtube.com/schematics/ampeg/v3pre-jp.gif
     
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