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Tube amp acting very strange-- tubes switching on and off

Discussion in 'General Electronics Discussion' started by dietermoreno, Apr 28, 2013.

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  1. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    Tube amp acting very strange-- tubes switching on and off.

    The tube amp when I connect an audio source to it, the sound comming out of the speaker that the tube amp is connected to increases in volume on and off at an oscillation rate of about 1/4 cycle per second.

    Yes this is the same tube amp from my other thread.

    The tube amp was working normally yesterday, but when I left my friends to use it while I went shopping, when I returned I found that the feedback of them having two mics right next to each other was terrible sounding and, and then I went up there and I smelled the magical smoke and ordered everything turned off imediately. Then after the tubes cooled down and then I turned the amp back on, when I ordered the vocalists to move farther apart, the feedback was gone, but the "tubes switching on and off" effect that I described above began occuring.

    I try to make things idiot proof for my friends while I go to the grocery store 3 blocks away from my house and am only gone for half an hour, but it appears that the idiots just build a better idiot.

    Well also its kind of my fault because I set up a tube amp that is a guitar amp to use to amplify mics and music since my PA is broken, and maybee that wasn't a good idea to use something for something its not meant to be used for.

    Any hints what could have gone wrong and how to fix it?

    New power tubes maybee? At least since its a tube amp, changing a tube is as easy as changing a light bulb with no sodering required (but the power tubes are much more expensive than power transistors).
     
    Last edited: Apr 28, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    O, you must be an absolute joy to be around.

    Pray tell, was this the same amplifier that you tried to destroy a couple of days ago?

    If so, I wouldn't be blaming others.

    Coming from you, that's a mighty bold statement.

    Is this a case of "Do as I say, not as I do?"

    We need to know if this is the amp you tortured earlier.

    I can't guarantee it will be that simple. The transformer is likely to be pretty damn expensive too.
     
    Last edited: Apr 28, 2013
  3. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    Yes this is the same amp that I accidently tried to destroy by having the tube amp cranked to 11 with no load connected in my audio loop experiement to transmit audio frequency with an inductive antenna to be received by an inductive antenna.

    Yes I realize that I might not be in a position to be an authority on audio and electronics to others, as this thread and my other threads clearly indicate.

    Well at least I know more than they know.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    I'm no expert on repairs of valve electronics.

    My guess is that you've damaged the output valve(s), but that is based on nothing but knowing the cause of some valve failures.

    It is possible that you may have damaged the output transformer or other (cheaper) components.

    As such, I can't offer you any real help.

    We do have some people who seem to have significant valve experience, and maybe one will be able to help you. Knowing the history of the unit (operating without a load until smoke came out) followed by stress from a high output may point them in the direction of the most probable fault(s).

    Good luck.
     
  5. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    Mostly vacuum tubes does not have intermittent problem. It can only be weak or totally defective. Probably you have defective power supply. Let experience technician to troubleshoot. High voltage exist on power supply.+B.
     
  6. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    You will need to understand such circuits and to work gradually through the amp to find the problem. Since it has smoked, you may be able to see a faulty component.

    You are a novice and should not play with high voltages. Remember that even when the amp is turned off and unplugged, there could be high voltages on the capacitors.

    Instabilty as you describe (motor boating) is usually due to faulty power supply (B+) capacitors.
     
  7. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    Well that's a relief to know that "motor boating" is usually caused by capacitors, rather than a messed up output transformer.

    Output transformers don't run cheap. Capacitors do run cheap.

    I don't know where the power supply capacitors are. If I take photos of the chassis will you help to point me in the right direction?

    And if I find the power supply capacitors, what is a safe way to discharge them?
     
  8. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    Use the ac plug of soldering iron to discharge the capacitor. It will act a 30W bleeding resistor and well insulated. Touch ac plug both terminal to capacitor terminals and wait 30 second before removing it. You can safely discharge even 400V DC.:)
     

    Attached Files:

    Last edited: Apr 29, 2013
  9. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    Oh okay, well then that would require me to first buy a sodering iron.

    Well I'm not going to buy a sodering iron until I find out what is wrong with my other amp in my other amp repair thread, so I will focus on looking at that other amp first before looking at this amp.

    When I get around to it, I'll post pictures of the chassis of this amp.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    OK, but a soldering iron is probably a good purchase.

    (Is there any other country beside the US that has a "silent L"?)
     
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