4 watt power building

Discussion in 'The Workbench' started by zebrarock, Feb 24, 2020.

  1. zebrarock

    zebrarock Member

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    Hi to all,
    i just built a low watt amplifier using 2x miniwatt tube NOS Philips EL91 in push pull for a total output power of 4 watt.
    I used a Hammond 125B OT for 8 ohm secondary and 22.5K primary impedance.
    the ampli sounds great, as i aspected, but i canno get 4 watt out of my secondary OT, but only 2.3.
    I configured tubes as datasheet suggests for class AB push-pull amplifier, i.e. 250V on the plates, 560 ohm as shared cathode resistor for a grid-to-cathode bias voltage of -13.5.

    Power stage is driven by cathodyne PI with a 6C4 tube which is not overdriving for the moment.

    Now for power test, i followd this procedure:
    Inject 1 KHz signal into ampli and turn up volume before the clipping point, where the wave form is a clear sine.
    Then measure voltage at the leads of 8 ohm dummy load and apply formula P = (V*V) / R.

    I am puzzling myself to know why but i am not understanding,
    could you please give me any suggestion o what may i check ?
     

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  2. South Park

    South Park Well-Known Member

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    The watt rating is for heat dissipation not power to speaker .
     
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  3. zebrarock

    zebrarock Member

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    I knew that max heat dissipation was measured at idle state, the document shows that with no signal applied Watt are 0, and with 12 Veff of input signal is 4W...am I interpreting correctly ? They seem to be power rating values, not so ?
     
    Last edited: Feb 24, 2020
  4. South Park

    South Park Well-Known Member

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    Tubes are valves the power to ground all the time . The voltage drop that drives the speaker does not increase the wattage of the tubes. Unlike solid state that goes from o to full power . It is a wired one how tubes work. It takes some time. Hope that helps . Read as much as you can about tubes it will makes sence
     
  5. zebrarock

    zebrarock Member

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    Thank you South for you explanation, but I don't think I understand... :wallbash:
    what did i wrong on my interpretation ?

    Hence, getting 2.3 watts RMS from ot secondary could be right ? Don't i have to expect 4 watts RMS instead ?
     
  6. South Park

    South Park Well-Known Member

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    the more time you study this stuff the better you get. None of this stuff is easy . You are on the fight track
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Peak to peak sine wave / 2 = peak sine wave.
    Peak sine wave X .707 = RMS sine wave voltage.
    RMS voltage / 8 ohms = Amps (= I)
    I X RMS voltage = (what?)
    Is this how you are measuring ? because I don't understand if you are turning the scope wave form into RMS voltage.

    How many volts peak to peak are you measuring before clipping?
     
    Last edited: Feb 24, 2020
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  8. zebrarock

    zebrarock Member

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    I am measurnig referring to my voltmeter which gives me rms values, right ? ... honestly i am not referring to oscilloscope wave, and i could try...
    what you wrote made me understand one thing about peak voltages that i did not understand before, thank you a lot !!!!
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    What is the RMS voltage at the load, before clipping?
     
  10. thetragichero

    thetragichero Well-Known Member

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    cathode bias generally produces lower output power than the same setup were it using fixed bias, no?
     
  11. johan.b

    johan.b Well-Known Member

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    250x250/22500=2.77...you need higher voltage or lower p to p impedance
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    Well we don't know if the measurement is right yet...he might have been doing it wrong.
     
  13. mickeydg5

    mickeydg5 Well-Known Member

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    The specifications indicate 12V RMS signal to the power tubes. So all you have to do is make sure your signal is that, with a multimeter.

    You are measuring at 1kHz for 2.3W output. Is the grid input signal 12V RMS?

    The specifications do not specifiy but most likely the output measured is an average at frequency/fequencies for speech. Try something below 300Hz.
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes because cathode bias is class A which runs the tube at 100% full time.
    And so the power has to be de-rated because there is too much heat.
     
  15. mickeydg5

    mickeydg5 Well-Known Member

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  16. ampmadscientist

    ampmadscientist Well-Known Member

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    That's true the meter may not measure RMS accurately at 1KHz.
     
  17. mickeydg5

    mickeydg5 Well-Known Member

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    If you look at the Philips specification data @zebrarock linked in the first post it indicates cathode biased with an output power of 4 watts.

    Everything he has mentioned is in relation to the Philips specification for two tubes push-pull, except for the frequencies of speech.

    And uh, nothing is perfect so allow at least +/- 20%.
     
  18. mickeydg5

    mickeydg5 Well-Known Member

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    Most lower cost multimeters should handle 45-400Hz bandwith when measuring AC voltages. 1000Hz is high for the average multimeter.
     
  19. zebrarock

    zebrarock Member

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    From my multimeter is 4.2 v ac
     
  20. zebrarock

    zebrarock Member

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    Input voltage to power tube before clipping is 8.9 v ac rms... i though the 12 v from datasheet was peak to peak voltages, not so? It is written 12 Veff...
     

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