Hot Rod Deville: Sounds like a bad power tube, but tubes are fine

Discussion in 'Amp Tech Center' started by sulaco, Apr 5, 2019.

  1. sulaco

    sulaco Tele-Meister

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    Diagnosing a friend's HRD.

    I'm handy with a soldering iron and have experience swapping components on other PCB fenders. But I've since moved on to vintage stuff.

    Anyway, friend of mine loves this hot rod but it sounds kinda buggy lately. Quieter than normal and really bad breakup on the clean channel. Having owned like 5 hot rods before I instantly recognized it as sounding like a dead power tube (or a loose power tube).

    The problem is the tubes are fine. I swapped in a known good set and it made no difference. However, when I yank the right power tube in the amp there's no change. When I pull the left power tube (and leave the right side in) the volume drops to almost nothing. I assume it's a resistor stuck open that feeds signal to/from that tube? Where would I begin?
     
  2. flyswatter

    flyswatter Friend of Leo's

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    Start by checking solder joints where the tube sockets attach to the PCB. Non-leaded solder under vibration makes cracked solder or broken pads a common problem with all the Hot Rod series amp. Inspect visually for broken solder, wiggle tubes gently to see if there is any movement on the PCB itself.

    The symptoms do often sound like bad tubes and tend to be intermittent.
     
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  3. sulaco

    sulaco Tele-Meister

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    Everything down there appears fine. I can't see any loose components or cracked solder joints on that whole board.
     
  4. D'tar

    D'tar Tele-Afflicted

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    Did you swap the phase inverter tube as well? V3
     
  5. sulaco

    sulaco Tele-Meister

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    Just tried it, no difference.
     
  6. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Step 1 in any amp troubleshooting is checking voltages. That'll tell you real quick if a tube is amplifying properly.
     
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  7. peteb

    peteb Friend of Leo's

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    you found the problem



    check the voltages on the right power tube, something is wrong there.

    check the voltages on the left power tube for reference and compare
     
  8. sulaco

    sulaco Tele-Meister

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    right, my issue is how do I trace it back to find why the right power tube isn't getting (or sending) voltage? I'm not an amp tech, but I have done plenty of "well that solder joint/resistor/cap is obviously bad" and replacing it myself. but I can't see any joints or anything that appear bad.
     
  9. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Well...you take measurements at the tube pins and work backwards. Say you get a bad measurement - what connects to that pin? A resistor, most likely. Well, then you'd measure voltage on both sides of the resistor - does that seem like a normal value? You can also measure resistance itself.

    A lot of problems won't be "obviously bad" parts as things can look fine but still be bad.
     
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  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Start with just measuring the DC voltage on all the tube socket pins except ac for the two heater pins. Let us know what you find.
     
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  11. Wally

    Wally Telefied Ad Free Member

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    Start by being careful....live voltages present.

    Duncan Tube Data Sheets for tube pinouts.
    Schematic Heaven or EL34 World for schematics.

    You could have a bad heater connection in that socket....or a bad plate connection. You could have a bad screen grid resistor. You could have a bad coupling cap from the PI to that tube. Voltages will likely tell the story.

    Again....live circuits are dangerous. One hand behind the back mentality, right?
     
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  12. clintj

    clintj Friend of Leo's

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    I've had more than one of these in where half the PI dropped out, usually from a bad plate load resistor on V3. The screen resistors are another common one, but it's usually pretty obvious from sight and smell when they've gone.

    +1 on tube pin voltages throughout the amp. Those are often very telling.

    Sent from my Pixel 2 using Tapatalk
     
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  13. Faceman

    Faceman TDPRI Member

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    The fromel mod takes care of a lot of the common issues on these amps. New load resistors, switching resistors, electrolytics and tone caps, input jacks, etc..

    Outside of that, like mentioned in other replies, the tube sockets solder joints crack very often as well as broken wires from the main board to the tube board.
     
  14. Andy B

    Andy B Tele-Afflicted Gold Supporter

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    Seen this many times also.
     
  15. schmee

    schmee Poster Extraordinaire

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    Have you visually looked at the board? Those and the related similar Hot Rod amps have some resistor burn out issues that are common and they are usually visually burned.
     
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