Low voltage 2 valve audio amp

Discussion in 'General Electronics Chat' started by Bigbill, Sep 2, 2010.

  1. Bigbill

    Thread Starter New Member

    Aug 27, 2010
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    Hi all,

    I've just finished a regen radio project using a single 1625 valve instead of the usual 2N3819 FET. I have over the years built a few regen radios using transistors but never one that uses a power beam tetrode, however this design without doubt works extremely well considering it only needs 12 volts on the anode insead of the normal 600 or so.
    The audio output however is very weak, and I wondered if anyone has any idea where I might find a circuit which uses one or two low voltage valves that can be connected to the 1625 to increase the audio output.
    Someone mentioned that a pair of EF183's would work very well on 12 volts but I've no idea how to built such an amp.

    Thanks,

    Bigbill :)
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Welcome to AAC.

    Do you have a schematic or a link?
     
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
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    The circuit in this link has a low voltage tube amplifier that uses a combination triode/tetrode valve. You should be able to use it with two separate valves and get the same result.

    http://www.qsl.net/k5bcq/REGEN2/regen3.html
     
    Bigbill likes this.
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
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    Bigbill likes this.
  5. Bigbill

    Thread Starter New Member

    Aug 27, 2010
    3
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    Thank you for your help regarding advice on circuits for low voltage valve audio amps. I'm going to build the 12U7 / 12AL8 version and hook it up to the grid of the 1625.
    As soon as it's complete I'll post a picture and the completed circuit.
    Once again, thank you for your help.

    Regards,

    Bigbill.

    P S, Does anyone know if there's a supplier of the excellent Bogen T725 transformer here in the U K ? :)
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Doesn't look like the 12AL8 is connected correctly, input's on the screen grid and the control grid is connected to Vcc.
    Then again that's the way they did it in the first circuit.

    I might try to look around a bit this weekend, seems to me there were some other audio output tubes that operated in low Vcc situations.
     
  7. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    From the first web site I posted:

    Normally, tubes operate with hundred(s) of volts of plate potential or around a hundred volts for battery tube sets. To ask them to work at very low voltages, something has to change because the lower cathode to plate potential is just not enough to enable enough current flow. The other problem is that the plate curves appear to become very non linear at low voltages. One method of improving the electron flow is to change the spacing and physical design of elements inside the tube. Another is to "boil" more electrons off the cathode by increasing the wattage of the filaments through redesign. A third is to place the 1st grid at B+ potential, thereby providing a nearby "pull" on the electrons from the cathode. Since it is a "grid", many of the electrons reaching the 1st grid zip right on through. The second grid (screen) is used as the "control grid" and the B+ charged plate gathers up the electrons. There is no suppressor grid.


    I haven't messed with these things in 50 years.
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Makes perfectly logical sense though, however when I think of a 6 or 12V tube radio I expect a vibrator driven step up circuit in it to create a decent B+
     
  9. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
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    That was the old way. This space charged 12V stuff is new technology, like CK722 transistors.
    I do recall taking bad vibrators apart so I could use the foam sound blocker as a handy tool for extracting hot octal tubes from their sockets.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I had a few GC tube pullers that would have been similar but if it was a horizontal output or damper that "cherried" I'd usually excuse myself to go do something else for a while.
     
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