Low voltage dual Pentode audio amp ?

Discussion in 'General Electronics Chat' started by Bigbill, Oct 9, 2010.

  1. Bigbill

    Thread Starter New Member

    Aug 27, 2010
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    0
    Hi all,

    Have a couple of EF183 Pentode amp valves and so far have managed to hook one up to the output of my regen radio, but the volume is very low due to only running 12volts on the anode. I wondered if anyone has a sugestion on how I can connect the other EF183 and therefore end up with a 2 pentode audio amp operating in space charge mode that might give enough otput to drive a pair of headphones with good volume via the output transformer .
    I have a diagram of what I've done so far but not sure how to upload it to the forum.

    Thank you,

    Big Bill
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You can use the "manage attachments" button to upload images:


    Bertus
     
  3. timrobbins

    Active Member

    Aug 29, 2009
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    Umm - what HT supply voltage do you have available for the anodes?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    If you only have 12v available, you need to build something like a flyback converter to give the anode a voltage within it's normal operating range.

    Ronald Dekker made a great DIY'er page on boost and flyback converters, here:
    http://www.dos4ever.com/flyback/flyback.html

    The circuits shown certainly aren't the most efficient, but they're great as learning tools.
     
  5. bertus

    Administrator

    Apr 5, 2008
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  6. timrobbins

    Active Member

    Aug 29, 2009
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    However the EF183 is not of that type, it's just a 'normal' type.
     
  7. daviddeakin

    Active Member

    Aug 6, 2009
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    When operating pentodes at low voltages you need to abandon some of the traditional design methods.

    You will need a large anode resistor (traditional starved designs used anything up to 5Meg!).

    You also need to be aware of grid current, which will bias the valve very cold if you have a conventional grid leak resistor. A neat way to get around this is to connect you grid leak to the 12V supply, rather than to ground/cathode! Playing with the value, you should be able to get some useful gain.

    Also, play with the screen voltage, as the gm of pentodes varies wildly at low screen voltages there will usually be a 'sweet spot'.
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    On the page I posted there is this for using ordinary tubes on low voltages:

    Bertus
     
  9. daviddeakin

    Active Member

    Aug 6, 2009
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