Tube Amp problem

Discussion in 'The Projects Forum' started by bagster, Oct 15, 2014.

  1. bagster

    Thread Starter New Member

    Oct 15, 2014
    20
    0
    I need some help with a tube amp ... so... i have built a weber inspired smokin joe 2 tube amp. i want to mention that i did not use a kit...i scavnaged the parts from here and there..it worked as a charm from the first volts he sucked in... i tested it.... for an hour or so..anyway...the next day...i plugged it in and noticed that it sounded really poor-- tone and volume loss without limits...and the day before i almost got kicked out of the building by my neighbours... i redesigned it...measured all the caps the resistors...they seemed to be fine...i used new one... just to make shure...still the same...the only thing that i did not change are the powercaps
     
  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    You should start your own thread for your question. This thread is for introducing yourself. Try not to hijack other peoples' threads.
     
  3. bagster

    Thread Starter New Member

    Oct 15, 2014
    20
    0
    I am sorry...i didn't lnow how...
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,776
    1,103
    Bad tubes? Bad power caps?
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the power supply caps going bad would cause a lot of hum, which you didnt mention. might check the tubes.
     
  6. bagster

    Thread Starter New Member

    Oct 15, 2014
    20
    0
    Ok, so i just took the turretboard out and measured the powercaps... well...instead of 47uf i get 100 on each...tubes are new...caps were 2... they must have shorted somehow because of a bad wiring...or a cold solder joint at the ground...i did some mistakes at the begining... could that be the problem...?
     
  7. bagster

    Thread Starter New Member

    Oct 15, 2014
    20
    0
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,776
    1,103
    That blue electrolytic half-way down the board looks ANCIENT. If so, try replacing it with a modern one.
     
  9. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the power supply caps are in paraless, the cap adds, so 100 isnt far off.
     
  10. bagster

    Thread Starter New Member

    Oct 15, 2014
    20
    0
    Yep...just measured them again...separately...i still think the tube sockets or solder joints... or God forbid...the OT..
     
  11. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    is there any distortion now that wasnt there before?
     
  12. bagster

    Thread Starter New Member

    Oct 15, 2014
    20
    0
    Yes it is...actually lots of it ...there is no clean...just with the volume down to 1...it sounds kinda funny...i mean i was really content with the sound it had the the first time...
     
  13. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    check the cap from the output tube cathode to ground. also, check the brid to cathode voltage on all tubes, it should NEVER go positive.
     
  14. bagster

    Thread Starter New Member

    Oct 15, 2014
    20
    0
    And if they were positive, what would you suggest doing?
     
  15. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    change the coupling caps. the plate is leaking through them to the grid of the next tube. if the cap across the cathode resistor for the output tube opens up, it changes the gain of the output stage, also the bias on the output tube\, usually causing distrotion. if the grid of a tube goes positive, it will draw current and nolonger control the current for the plate. any time a grid is positive to the cathode, except in very strange cases, not audio amps, the coupling from the previous stage is leaking.
     
    Last edited: Oct 16, 2014
  16. bagster

    Thread Starter New Member

    Oct 15, 2014
    20
    0
    Ok, so this is the layout, .....ppul
     
  17. flat5

    Active Member

    Nov 13, 2008
    403
    17
    If you ran the amp at high power with out a load on the output transformer you might have very well damaged it.
    Is there always a load on the output transformer? A 47 ohm 2 watt resistor might be enough.
     
  18. #12

    Expert

    Nov 30, 2010
    16,257
    6,758
    Many mistakes to be made, there are. A vacuum tube output without a load tries to arc itself to death, and often succeeds. A leaky coupling capacitor will often show itself as a tube that looks like it is going to melt. Turret boards are very pretty, but you need to kink at least one lead on each part, so they don't try to pull out of the components as temperature changes over many use cycles. Single point ground, use shielded cable while running the power line right next to the input jack, use an output jack with a switch on it that puts a load on the output transformer if you forget to plug the speaker in.

    Just a few tips from an old guy.
     
  19. flat5

    Active Member

    Nov 13, 2008
    403
    17
    I like to put a resistor on the output jack to keep a small load on the transformer.
    I did this in the 1970s and it saved OTs on Fender amps. Best I can remember is using 47 ohm 2 watt.

    Another trick was to put a neon bulb across the transformer primary.
    I never did it but saw it on some Fender amps I worked on.

    Leaky coupling caps were most often the problem that brought the amp to me.
     
  20. #12

    Expert

    Nov 30, 2010
    16,257
    6,758
    I don't see how that's going to fix anything. Even a 6V6 running 35 ma at 315 volts will smoke an NE-2 in a couple of seconds in a no-fault condition...let alone a fault condition!
    Let's just say I doubt this method. ;)
     
Loading...