Help me troubleshoot a tube amp

Discussion in 'Shock Brother's DIY Amps' started by SEK_Hakuna, Mar 17, 2009.

  1. SEK_Hakuna

    SEK_Hakuna Tele-Meister

    Age:
    63
    Posts:
    217
    Joined:
    Jul 29, 2003
    Location:
    Kansas
    Hi guys n gurus,

    I have a self built Princeton Reverb. The amp has been working fine for quite some time now. Last nite I turn it on and hear a loud buzzing/squaling noise. A guitar plugged in to either input makes no sound. The squaling noise is the same regardless of knob settings. The second power tube, (from the rectifier), gets extremely hot and has the blue color on both sides.

    I opened it up and checked all visible solder connections and all is good. I do have a tube tester and all tubes checked out fine.

    I would appreciate any and all suggestions.

    Randy
     
  2. Tim Swartz

    Tim Swartz Friend of Leo's

    Posts:
    2,009
    Joined:
    Mar 18, 2003
    Location:
    Michigan - Tweenst the Great Lakes
    Tubes are always the low hanging fruit. You can't trust a tube tester.
     
  3. Tidepoolbay

    Tidepoolbay Friend of Leo's

    Posts:
    2,824
    Joined:
    Jan 27, 2009
    Location:
    Franklin New Jersey
    I agree. I have a tube tester and it has not been calibrated in thirty years.
    So I do not really go by what it may say about a tube.

    Replacing the power, or all the tubes usually does the trick.

    Tidepoolbay
     
  4. SEK_Hakuna

    SEK_Hakuna Tele-Meister

    Age:
    63
    Posts:
    217
    Joined:
    Jul 29, 2003
    Location:
    Kansas
    Thx for the replies. I'll swap out everything tomorrow and update this thread.
     
  5. mojo2001

    mojo2001 Tele-Holic

    Posts:
    687
    Joined:
    Feb 5, 2008
    Location:
    DC
    Swap the two power tubes and see if the symptom follows the tube.

    If so, it is the tube. If not, see if there is excess positive voltage on the grid, which would indicate a leaky cap...somewhat unlikely with 150s or Orange Drops, but ya never know.

    Light blue illumination on the glass is not necessarily a bad sign, Purplish illumination down around the grid inside the plate is a symptom of a gassy tube.
     
  6. SEK_Hakuna

    SEK_Hakuna Tele-Meister

    Age:
    63
    Posts:
    217
    Joined:
    Jul 29, 2003
    Location:
    Kansas
    I had a bit of time before church to swap out the tubes.

    I swapped all tubes with known good ones. Problem still exists. I unplugged the reverb and pedal with same results.

    I'll check voltages next. It sounds like something is cooking. Lots of buzzing with crackling sounds thrown in.

    The 2nd power tube from the rectifier is still getting very hot. The other one seems normal.

    Randy
     
  7. SEK_Hakuna

    SEK_Hakuna Tele-Meister

    Age:
    63
    Posts:
    217
    Joined:
    Jul 29, 2003
    Location:
    Kansas
    Following up on previous post.

    I pulled all tubes except the recto and took readings.

    Pin 8 of recto is 464 v
    Pin 3 of both power tubes is 458 v

    All tubes in and I'm getting 274 on pin 3 first power tube and 268 on pin 3 of the second PT.

    I have installed a bias adjustment and have the resistors on pin 8. Bias reading is good.

    If I leave the amp on and don't touch anything it eventually gets quiet. The moment I touch something, anywhere, the noise starts up. Poking around the wiring with a stick I noticed a more pronounced noise coming from the pre amp end of the board while tapping on wires and such.

    I'm at a loss here. The voltages seem good on the power tubes but why is the one PT getting so hot. The extra noise produced at the pre amp end would indicate something is amiss there. I don't know where or what to look for.

    I would appreciate further suggestions.

    Randy
     
  8. ThermionicScott

    ThermionicScott Poster Extraordinaire

    Age:
    39
    Posts:
    5,913
    Joined:
    Dec 3, 2005
    Location:
    SW CR IA US NA PE
    Is one of the power tubes glowing orange/red, or does it just *feel* like it's throwing off more heat?

    Would you mind measuring pin #5 of both power tubes? If one is losing bias (due to runaway), then the voltages will be different. They ought to be around -30V.

    Sometimes, a crackling noise can be from dirty tube pins. Might be worth pulling the tubes one at a time, starting from the first preamp tube, and seeing if the absence of one makes the crackling go away.

    - Scott
     
  9. SEK_Hakuna

    SEK_Hakuna Tele-Meister

    Age:
    63
    Posts:
    217
    Joined:
    Jul 29, 2003
    Location:
    Kansas
    Pin 5 on 1st power tube, (next to recto), is -34 v
    2nd tube is -7 v

    The second power tube gets hotter than the first and glows redder.

    What is the fix for runaway bias?
     
  10. muchxs

    muchxs Doctor of Teleocity

    Posts:
    13,128
    Joined:
    Aug 14, 2004
    Location:
    New England
    Take a look at the voltage divider that provides bias to your power amp grids. It's a couple 1/2 watt 220k resistors way down at the end of the board. You did use the AA1164 layout, no? Make sure those 220ks are still 220k each, re-flow the solder joints while you're there.

    "Second power tube"... cathode should be grounded. Make sure this is so. Both cathodes should ground to the same point so if there's a ground fault on one there will be a ground fault with the other. Still...
     
  11. SEK_Hakuna

    SEK_Hakuna Tele-Meister

    Age:
    63
    Posts:
    217
    Joined:
    Jul 29, 2003
    Location:
    Kansas
    It is an AA1164 layout.

    The 220k resistors are within tolerance.

    The power tube ground is pin 8 I believe. Since I installed the bias adjustment there is a resistor from pin 8 to chassis on each power tube.

    Does anyone know why pin 5 on powertube 2 is -7 volts instead of - 30 v?

    Edit:

    After letting the amp run a bit I rechecked pin 5 on power tube 2 and found -2 v.

    Once again....If left untouched the amp becomes quiet. The slightest vibration, or tap, will start the static noise which then dies out to quiet again.

    I adjusted various wires to try and improve dressing with no positive results.
     
  12. muchxs

    muchxs Doctor of Teleocity

    Posts:
    13,128
    Joined:
    Aug 14, 2004
    Location:
    New England
    Yeah, 1 ohm. Is it really 1 ohm? Measure Pin 8 to ground.

    Which may be a bad solder joint related to your bias problem or a microphonic tube unrelated to your bias problem.
     
  13. SEK_Hakuna

    SEK_Hakuna Tele-Meister

    Age:
    63
    Posts:
    217
    Joined:
    Jul 29, 2003
    Location:
    Kansas
    The resistors measure 1.2 ohms

    I've swapped out all tubes with known good ones. No change.

    I plug in a guitar and get sound. High E string sounds about normal with a slight bit of static. Low E string causes more speaker vibration which causes more static noise. Low E also sounds very distorted.

    I am starting to believe, as suggested, there is a loose connection somewhere. Finding it is the difficult part. I've checked all visible connections 3 times now. I'm hoping it's not under the board.
     
  14. SEK_Hakuna

    SEK_Hakuna Tele-Meister

    Age:
    63
    Posts:
    217
    Joined:
    Jul 29, 2003
    Location:
    Kansas
    The amp is now fixed.

    I gave up and took the amp to my good friend Dave. He owns the local repair shop/TV store.

    I gotta get me one of those magnifiying hats!! You know, the headband with the double lens flip it up outta the way when needed kind. He looks inside the amp for about 15 seconds and finds two bad solder joints on the filament wires. I never would have dreamed those loose joints would cause so many problems. The power tube no longer overheats and the pin 5 voltage is back to -34 v.

    A big thankyou to all who gave suggestions and help.

    Randy
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.