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Need Help with Ampeg J12

Discussion in 'Glowing Bottle Tube Amp Forum' started by bryankloos, Aug 11, 2018.

  1. bryankloos

    bryankloos TDPRI Member

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    Hello All,

    My buddy is complaining that his Ampeg J12 is running hot.
    He found the amp a few years ago at the local dump and handed it off to me. I replaced some leaky coupling caps and a few resistors that had failed and the amp had been working wonderfully. Unfortunately I lost the notes of the voltages prior to my repairs and don't have a reference.

    After a couple years of use he has recently noted it is hotter to the touch after gigging, and has asked me to poke around and see what I can find.

    Schematic here:

    schematic_1280.jpg

    Sure enough the output tubes are very warm and the chassis is getting hot, exactly what he is complaining of.

    I've poked around with my DVM and have the following voltages recorded:

    6V6's Measure the same on each tube at:

    P3 - 344
    G4(1) - 343
    G5(2) - 0.13
    K - 20.2

    Phase Inv 6SL7 measures:

    S1 - 0
    P2 - 185.6
    K3 - 21.79
    S4 - 2.179
    P5 - 135.2
    K6 - 2.043

    Trem/Pre 6SL7 Measures:

    S1 - 0
    P2 110.9
    K3 - 0.85
    P5 - 168.1
    K6 - 1.91

    Power Supply is reading:

    C - 351
    B - 345
    A - 278


    It seems to me the plate voltage is too high of the phase inverter at P2, reading 185.6 which should be 140 per the schematic. Could this be the cause of the hot output tubes or should I be looking elsewhere. I also not a high plate voltage on the pre-amp input tube at 168, which should be closer to 140 per the schematic.

    Any and all help is appreciated.

    Thanks,

    Bryan
     
  2. JD0x0

    JD0x0 Poster Extraordinaire

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    Probably needs new filter caps. You didn't mention changing those.

    What are you calculating as far as plate dissipation for the output tubes? Plugging things in I'm getting 103% dissipation with a 12W 6V6 if what you reported is correct. Even for cathode biased this is hot. With a 14W JJ you're still above 88.5%


    Also, add screen resistors to the 6V6's, for safety, IMO. Should also help them run a touch cooler.
     
  3. bryankloos

    bryankloos TDPRI Member

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    Ya know, I think I swapped out the multi-section filter can when I worked on it last. It’s pretty shiny and new looking. I’ll confirm once I get the kids to bed and can take another peek.

    What still has me stumped is the very high plate voltage on the phase inverter. Could this condition lead to heat on the output tubes?
     
  4. slider313

    slider313 Tele-Afflicted Silver Supporter

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    Replace the 250 ohm bias resistor with a 470 ohm. I've done it on my '59 Mercury M12 and 1961 Noble branded Ampeg Rocket. The tubes run cooler and they both sound much better.
     
  5. Wally

    Wally Telefied Ad Free Member

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    http://tdsl.duncanamps.com/show.php?des=6V6GT

    IMHO, that plate dissipation of around 100%is correct for cathode biased....and the amp should be expected to run to the hot side of things. US produced 6V6’s have a max plate dissipation of 14.ere was a revision from the 12 sometime in the ‘50’s, iirc.
     
  6. bryankloos

    bryankloos TDPRI Member

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    Ok. Assuming the 6v6 are running near abouts the correct range (hotter may be preferred) what should I do about the high voltage on the PI? Seems pretty far above spec. No?
     
  7. slider313

    slider313 Tele-Afflicted Silver Supporter

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    Have you checked the value of the resistors in the phase inverter part of the circuit?
     
  8. bryankloos

    bryankloos TDPRI Member

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    OK, some more poking around and things are still off.

    First off, I have a 1K negative feedback resistor instead of the schematics 6-10K. Wonder why this is???
    Secondly, relative to the plate voltage on the top section of the PI, my cathode resistor is measuring 247 Ohm, not the 220 called for. I"m wondering if I get this down to 220 if it will drop my plate voltage? Otherwise everything seems to be okay around the PI.

    I did notice that the schematic in the back of my amp is slightly different than the circuit. My schematic on the amp doesn't show the 500pF cap at the PI plate to ground, but my amp does have it as shown in the schematic above. Strange...

    Thoughts?
     
  9. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Resistors always drift high, and that's not out of the tolerance range anyway. You could change it or leave it.

    Can't explain the negative feedback value.

    How "hot" are we talking? Can you hold your fingers on it for a few seconds or is it insta-painful?
     
  10. bryankloos

    bryankloos TDPRI Member

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    How hot... well, with the back plate off the amp it’s hard to rest my finger on the metal chassis while I’m taking test voltages but only adjacent to the power tubes. Input and PI chassis is cool. With the back plate on the top of the amp is getting uncomfortable to touch after 15 minutes. My buddy tells me this is new and not the norm for the amp. It is possible that he is being paranoid. He did mention it was red plating (not even sure if he knows what that is) but I can’t see anything wrong internally with the power tubes when running.
     
  11. Inglese

    Inglese Tele-Meister

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    Keep it in for 5 minutes and the check touching carefully around... power trafo, output trafo, filtering caps.
    Who's getting hotter than normal?
    Btw 100% plate dissipation at idle is too much, no matter fixed or cathode bias.
     
  12. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    This is not true, and depends greatly on the amp in question.
     
  13. bryankloos

    bryankloos TDPRI Member

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    Here are some temps and observations.

    Power Transformer is cool to the touch.

    Output transformer is cool to the touch.

    PI and Input/Tremelo rube bases are slightly warm to the touch. Measures about 75-80F with laser thermometer.

    Output tube bases are painfully hot to the touch. Maybe about 150F.

    Rectifier tube base is hot to the touch. Near 130F

    Multi-section filter cap is cool to the touch. 70F with laser.

    All temps are with the amp out of the cabinet so tube heat is dissipating up and not collecting under the chassis as if the amp was installed in the cab.

    I’ll check some other amps that I have that run 6v6 for comparison of heat, but I’m starting to think it’s due to hot bias of the output tubes.

    It may be time to try lowering the bias and seeing if I can get to tubes to run a little cooler for longevity.
     
  14. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    I've never taken actual temperatures of an amp, but all that sounds about right. My Deluxe chassis' never get super hot, but I have an old Pro where the chassis gets pretty hot, even up by the switch.

    Easy test would be to just drop the bias by some proportionally-large amount. What that is, doesn't really matter - just lowering it to see if that actually changes the temps you're seeing. If so, then you'll know it's related.
     
  15. bryankloos

    bryankloos TDPRI Member

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    That’s the plan. I’ll drop the bias and see if it cools off a bit. New resistors are in the mail.

    Now if only I could figure out the voltages on the PI plates.
     
  16. radiocaster

    radiocaster Poster Extraordinaire

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    I think a 6SL7 can handle 250V on the plate, well depends on the make. But shouldn't you make sure the two sides are matched?

    What about the plate voltages on the 6V6s?
     
  17. bryankloos

    bryankloos TDPRI Member

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    Circuit calls for 140 and 135 on the PI plates. I have ~185 and 138. My concern is the disparity.

    Plates and grids on the 6v6 are about 347 and the cathode is 20.4.
     
  18. radiocaster

    radiocaster Poster Extraordinaire

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    You are correct, but I wasn't asking if the voltages are matched, I was asking if the triodes were matched.

    Keep in mind as well that I am not a real amp tech.
     
  19. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    The necessity of having matched sides of phase inverter tubes is a myth. Cool to check if you want, but not necessary at all.
     
  20. bryankloos

    bryankloos TDPRI Member

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    I cant say the sections of the triode are matched, but I do know the problem persists when I swap in a new 6SL7, so its not the tube....
     
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