HELP i'm stumped.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by brad347, Sep 9, 2006.

  1. brad347

    brad347 Member

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    Okay... i've been chasing this around for awhile and tonight it just got worse!!!

    In this pro junior that i've hacked on, everything was peachy after my mods for about 3 months or so. Beautiful sound, pretty quiet, it got used every day and I loved it!!!

    THEN, a few weeks ago, this intermittent staticy noise started. I replaced the plate load resistors and swapped every single tube (all 4 haha) to NO avail. I looked around the amp for dodgy connections, re-flowed any joints that looked suspect... nothing.

    Did the 'chopstick test' (actually a toothbrush for me heh) and NOTHING would change no matter what I tapped, poked, pushed around, or prodded. One cap seemed like it might have had some effect, so I changed it, but no diifference. The problem persisted no matter what the volume setting, but if I removed any one of the two preamp tubes, it would stop. Pulling either power tube made no difference but pulling both power tubes of course stopped it.

    Well, tonight things suddenly got worse. Plugged it in to de-bug again, and the static was worse, more constant... then it gradually quieted down as the amp warmed up. Then I realized EVERYTHING quieted down... I was getting no sound from the amp at all!

    I did some readings... I've got this amp modified for adjustable fixed bias with 1 ohm resistors between cathodes and ground so I can check bias. NO MATTER where I set the bias trimmer, I can't get over about 9.5 ma to either power tube! Even swapped in a new set of power tubes, to no avail. Plugging in a cord and turning it ALL THE WAY UP, it will pass a very small amount of signal. Like, you can hear it if you listen for it but it's not even practicing volume.

    WHAT THE HELL is going on here? Any ideas? I have no clue what's up. This amp is really useful for me, I bring it to rehearsals and practice with it constantly. School started this week and since i'm getting this master's jazz degree I need a "throw n' go" amp for all the classes/rehearsals and this was THE ONE. I need to fix this thing. Any ideas? Thanks!!! you guys rule.
     
  2. brad347

    brad347 Member

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    OTM.... the only thing un-original in terms of the circuit is the fixed bias adjustment and 1 ohm resistors between cathode and ground. I had some help from a forum member and got that up and running and like I said it worked flawlessly for a good while.

    I did replace several other components, OT, speaker, some caps, etc... but the values are all stock.
     
  3. brad347

    brad347 Member

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    I will get right on checking the voltages
     
  4. brad347

    brad347 Member

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    OTM: here is the schematic

    http://www.ampwares.com/ffg/schem/pro_jr_schem.jpg

    I read all the DC voltages. I don't have a signal generator so I didn't read any AC voltages. I found some (several) discrepancies but I'm not sure what to make of them.

    TP10 spec is 172VDC, I read 212
    TP12 spec is 172 VDC, I read 232
    TP17 spec = 316, reads 295
    TP19 spec = 316, reads 297
    TP20 spec = 319, reads 300
    tp23 spec = -10.4, reads -7.0
    TP24 spec = 319, reads 300

    It's not immediately clear from the schematic where TP15 is supposed to be. I read a few spots nearby. Spec for TP15 is 41 VDC. I read both sides of R15 and got 175 VDC. I read between C6 and pin 7 of V2 and got 90 VDC.

    Any thoughts? Any way to read AC voltages without a signal generator?

    Come to think of it, I do have a mini moog with an A440 generator but that's about all I've got.
     
  5. Fretts

    Fretts Member

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    Why don't you bypass the 1 ohm resistors with straight wire for awhile and see if it still crackles after heating up?
     
  6. brad347

    brad347 Member

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    will try that tomorrow, fretts. For safety's sake I think I better call it a night, i'm getting tired.
     
  7. trdlasvegas

    trdlasvegas Member

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    Brad,
    That 175V on either side of R15 can't be right. R15 is at the junction of the cathodes of V2A & B. There is supposed to be 172V at the plates, or in your case about 152V (Your DC measurements are all about 20 less than the schematic).

    If you are measuring 175V at R15, I'd start looking around there as it would be impossible for it to work with that voltage relationship between the plates, grids, and cathodes of V2.

    BTW TP 15 is the cathodes of V2.

    -Tony
     
  8. Blue Strat

    Blue Strat Member

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    The most obvious thing to check first based on the info you've given us, is the voltage to the grids of the power tubes, and whether it varies as you turn the bias pot. If it doesn't your bias circuit is screwed up.
     
  9. brad347

    brad347 Member

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    Fixed It!

    R13 was open circuit. Replaced it (spec is 33k but I only had a 30k lying around) and crackling is gone, and I get normal output. It must have just been on the blink.

    I have always hated the cheesy 1/4 watt resistors in this amp. I might just have to shotgun 'em all for bulletproof-ness' sake.

    Thanks VERY MUCH to everyone for all your help and suggestions.
     
  10. Blue Strat

    Blue Strat Member

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    Where is R13 in the circuit?
     
  11. Tuberattler

    Tuberattler Member

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    So what was the problem?? culprit?? Don't just leave us hangin' that's like a movie you get into then there's no ending.

    Velly bad karma my friend...:(
     
  12. brad347

    brad347 Member

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    Haha... ok sorry relax. I don't mean to leave anyone hangin'

    I'm not the expert you guys are, I just check values and follow recipes with something a little smarter occasionally so let me see if I can sort it out...
     
  13. brad347

    brad347 Member

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    OK so R15 is at the junction of the cathodes of V2a and V2b. R13 is a 33k resistor between the opposite side of R15 and ground. So in other words, one side of R15 connects to the cathodes of V2a and V2b, and the other siide of R15 connects to R13, which connects to ground. I'm guessing the path between the cathodes and ground was open which was causing the problem. I guess. YOU tell ME!! :D

    All I know is it's fixed!

    Have a look for yrself:

    http://www.ampwares.com/ffg/schem/pro_jr_schem.jpg
     
  14. brad347

    brad347 Member

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    OTM: Thanks! :AOK
     

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