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Amp feeds uncontrollable

Discussion in 'Amp Central Station' started by DucDone, Jun 12, 2018.

  1. DucDone

    DucDone TDPRI Member

    64
    Apr 9, 2011
    Bergen
    Hi. I have an 18w amp that has a very nasty feedback that gets louder and louder till the speaker jumps out, can't let the amp stay on, the speaker will blow.

    This happens without the preamp tubes inserted, and also with only power tube inserted. (Two el84s)

    I don't understand what this could possibly be, the amp has worked, but now no sound, but for the feedback

    Anyone has a tip?
     
  2. charlie chitlin

    charlie chitlin Doctor of Teleocity

    Age:
    56
    Mar 17, 2003
    Spring City, Pa
  3. DucDone

    DucDone TDPRI Member

    64
    Apr 9, 2011
    Bergen
    No, very simple.
     
  4. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    if The noise goes away with both power tubes removed, then your problems are in the output section. You may have a bad power tube. If a good pair of EL-84’s does not eliminate the problem, then there is a problem other than the tubes in the output....resistor, cap, output transformer. If the noise continues with the power tubes removed, one suspects a filter cap in the power supply.
     
  5. VintageSG

    VintageSG Friend of Leo's Ad Free Member

    Mar 31, 2016
    Yorkshire
    What amp is it?
    What changed?, have you or anyone else altered the wiring to the output transformer?
    Does it do it with the phase inverter valve removed.
    Negative feedback?
    If it's a home build, check for very fine solder/wire bridges.
    Dry joints/dodgy capacitor or resistor somewhere?
    Does it do it with a different set of EL84's in?
     
  6. DucDone

    DucDone TDPRI Member

    64
    Apr 9, 2011
    Bergen
    I have changed all the parts in the amp, also the power tubes. The noise is there with only one power tube in either socket, but goes away when the tubes are out. I have checked the connections from the OT, both goes to seven on each tube.

    One thing that give a possible hint, is that the filament doesn't light the pilot lamp. I have tried several.
    What puzzles me is that there can't be many routes to make the amp feed back in an uncontrollable way, is there?
     
  7. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    IMHO, changing “all of the parts in the amp” without knowing why a part needed to be changed is not the best approach. Have you made up a voltage chart? Do you have a known good pair of EL-84’s?
    +1 on the question of what amp this is???? Is this a new build? +1 on when did the problem begin?
    That dead pilot light???. What are the voltages at the tubes’ heaters?
     
  8. DucDone

    DucDone TDPRI Member

    64
    Apr 9, 2011
    Bergen
    Hi. I have build this amp myself, it's a Hoffman Stout build. The problem started as a working amp with a quite loud hum. I tried to find the reason for this, but it wasn't quite obvious, all voltage measuring seemed to be within acceptable limits. So I decided, after a long period, to change all the parts. And to make a long story short, I am now with an amp that just feeds back, no sound. Right now I don't have a volt reading at the cathode resistor, so maybe the problem is in that area, the resistor itself is 130 ohm /10W and still reads 130 ohms. I have tried to change the choke, but still the same. I was hoping this feed backing would trigger someones mind about what reasons there could be for this?.
     
  9. Jakeboy

    Jakeboy Tele-Afflicted

    Jul 26, 2008
    Sedalia, MO
    Maybe I am misunderstanding...does it do it with all tubes in the amp?
     
  10. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Sep 13, 2008
    Lansing, MI
    It sounds like you need to check the wiring around the output tubes, especially since the issue only happens when there is even one tube installed.
     
  11. DucDone

    DucDone TDPRI Member

    64
    Apr 9, 2011
    Bergen
    Yes, only stops with all the tubes out, but with all the tubes in, the volume pot is acting like some kind of trebly parametric eq with narrow Q factor.
     
  12. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    That pic is an old rev, I think. The current one has a global negative feedback loop.

    If this is built to the latest revision, you may just have something like the OT primaries reversed, along with a dodgy connection in the feedback loop that wasn't making contact. Got a few pics?
     
  13. DucDone

    DucDone TDPRI Member

    64
    Apr 9, 2011
    Bergen
  14. DucDone

    DucDone TDPRI Member

    64
    Apr 9, 2011
    Bergen
    The OT is a Allen Amps OT20 with just 8 ohm to speaker.

    The red goes to the B+ supply, the blue and brown to the power tube places. Black is the common or grounded speaker lead and the green is the speaker output lead.
     
  15. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Sep 13, 2008
    Lansing, MI
    Right - try switching the blue and brown to each others tube socket.
     
  16. DucDone

    DucDone TDPRI Member

    64
    Apr 9, 2011
    Bergen
    I tried, no change. What voltage should there be on the cathode resistor I wonder, I get zero volt now?
     
  17. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Sep 13, 2008
    Lansing, MI
    Is the other end of your cathode resistor connected to ground? Can you confirm that with a continuity test with your meter?
     
  18. DucDone

    DucDone TDPRI Member

    64
    Apr 9, 2011
    Bergen
    So, the mystery partly solved, my grounding wasn't grounding after all, I changed the place for it, and Wham: Sound. So, now I am back to square one, the amp hums...

    Thank you folks...
     
  19. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Does it hum with all controls at zero? You have a connection problem, a bad cap, or a bad tube....my guesses.
    So, with the cathode voltage restored on the power tubes, have you made a voltage chart for the amp? Just because a circuit makes sound does not mean that it is working properly....and your amp obviously is not working properly. AS you found with that cathode situation, voltages need to be known. IF voltages are not correct, that is an indication of a problem. Imho, this schematic is incomplete....for a beginner. IF a person could take the AC voltage on the secondary of the PT that feeds the rectifier and calculate expected voltages for the circuit, that person would not need a schematic to build an amp. IMho, a kit should come with a schematic and a layout with expected voltages indicated so that a kit builder could have some sort of reference. Since you don't have those, you need to inform yourself on your own????
    What do they say about this over on the Hoffman boards???? MY guess is that you have the circuit miswired somewhere. I would suggest taking a break and coming back to the amp with a fresh mind. At that point, review all of the work with extreme skepticism...accept that there is an error....just as you found an error that you made in that biasing circuit, right?

    upload_2018-6-12_10-36-31.gif
     
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