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help understanding amplifier schematic

Discussion in 'General Electronics Chat' started by whiz115, Nov 9, 2008.

  1. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    That sounds something like.

    Go for it.

    Changing the cathode resistor is essentially changing the negative bias voltage on the grid - for really high power amps you normally supply a negative bias, which will usually be adjustable, in order to get maximum power from it. For a small amp cathode biasing is beter, and it's self adjusting.

    The PSU voltage probably won't change a great deal, particularly with using a semiconductor bridge rectifier, and a much larger electrolytic - original valve amps would be a half wave valve rectifier, and a 16uF or so. Guitarists still like to use that configuration, as the supply sagging excessively adds to the distortion.

    I never like to see valves with a blue glow.
     
  2. flat5

    flat5 Member

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    Don't measure the voltage from cathode to anode.
    Measure the voltage from cathode to ground.
    This tells you two things...in simple circuits.

    Since the control grid is at ground potential (because there is no leakage from the coupling cap connected to the triode plate, and the grid resistor to ground...done very poorly in this circuit) the cathode voltage tells you the grid bias.
    Cathode voltage wrt ground also tells you the current going through the tube by Ohm's law. I=E/R (cathode resistor).

    That is why I asked you much earlier to tell us the cathode voltages.
     
  3. whiz115

    whiz115 Member

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    wow!!! :eek::eek:
    with just a simple capacitor...the sound changed so dramatically!

    The results are very satisfying! although there's still noise which is audiable if you litteraly get your ear into the speakers cone...

    nevermind! it's perfect! :D

    now about the tubes...what am i going to do?? the cathode resistors are currently 2x150R/2W but EL84 is still shy... :mad:

    i think i might somehow damaged them.
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It's an absolutely essential component - otherwise you're feeding mains hum straight in the amplifier.

    What do you mean by 'shy'?.
     
  6. flat5

    flat5 Member

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    What part of "measure the voltage from cathode to ground" do you not understand?
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I've just been back and looked at the original circuit, and the ones I posted.

    Your original used a 135 ohm cathode resistor, I suggested increasing it - moving to 150 ohms is hardly much of an increase at all. Notice the circuits I posted, the first used 560 ohm, and the second 300 ohm.

    I would suggest moving to at least 300 ohm, and see what happens then (and as suggested, measure the cathode voltage to check the current through the valve).
     
  8. whiz115

    whiz115 Member

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    one of the EL84s still gets a bit red... the other one doesn't... but both
    have that blue glow...



    i don't have much time now...i'll answer you later.



    here's one more updated schematic of what i did (excluding the nfb which i'm about to add to the circuit maybe to day).

    Nigel your schematics proved very helpful! :D
     

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    Last edited: Nov 28, 2008
  9. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Well you haven't reduced the anode current worth mentioning - an obvious method based on the circuit you posted is to simply remove one of the parallel 270 ohms and half the current. Notice that's still about double the current used in the Mullard 3-3 amplifier (from the people who designed the EL84).
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The quality of the output transformer determines how much (if any) negative feedback can be added before the amplifier oscillates.

    Solid state amplifiers have a lot of negative feedback because they don't have the phase-shift problem caused by an output transformer.
     
  11. flat5

    flat5 Member

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  12. whiz115

    whiz115 Member

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    one more small update...

    i also did some changes to the circuit for the nfb but i haven't yet applied it to my amp...

    @flat5 you said something about a capacitor to the screen...you mean the one the datasheet shows 8uF? is it important?
     

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  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Phase shift in the output transformer will cause the amplifier to oscillate at a high frequency and destroy your tweeter when too much negative feedback is added. The 220pF capacitor and parallel resistor have values selected to match the transformer to reduce negative feedback above a certain frequency.

    EDIT: There is no negative feedback with the feedback parts feeding a huge capacitor to ground.
     
    Last edited: Dec 5, 2008
  14. flat5

    flat5 Member

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    I don't know what data sheet you are refering to.
    On the EL84 pin9 you could try an electrolytic cap to ground.
    See if the hum is even less. I don't know what value. Use what you
    have for now or try an 8uf. Just make sure it's voltage rating is high enough.
    Much higher than the max power supply voltage before the tubes heat up. (450 vdc?)
     
  15. whiz115

    whiz115 Member

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    you mean the 10uF/25V ? i thought it would be better like this...
    the previous design is more correct?


    that datasheet, the mullard one... :p

    http://www.electro-tech-online.com/custompdfs/2008/12/el84.pdf

    btw what does it mean "working the output tube as triode or as pentode" ?
    what's the difference? what are we trying to do by that??

    [Edit]

    @Audioguru i have 1/2 of the ECC83 currently spare! i could try to fix the phase shifting and see how the amp sounds... don't you think?
     
    Last edited: Dec 5, 2008
  16. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Like it says, running it as either a pentode (actually more like a tetrode) or a triode, by strapping G2 to the anode.

    No, it's not a fixed degree of phase shift, it will vary with frequency - basically due to the transformer. It's NOT an opamp, you can't just add feedback as you like - check the earlier designs I posted.
     
  17. whiz115

    whiz115 Member

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    ok i can understand that..but what are we trying to succeed with that?
     
  18. flat5

    flat5 Member

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    If you have a suitable electrolytic lying around try it and see if bypassing the screen lowers noise.
    Still would like to know the cathode voltage.
     
  19. whiz115

    whiz115 Member

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    i don't have...

    as far as i remember it's 270V
     
  20. flat5

    flat5 Member

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    You are still confusing cathode and anode (plate).
     
  21. whiz115

    whiz115 Member

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    i don't know what exactly you want me to measure...
     

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