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Amp/PT diagnosis

Discussion in 'Amp Tech Center' started by jsnwhite619, Oct 22, 2020.

  1. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I've got a 5f11 kit here that someone asked me to look at. It's about 8-9 months old, so it wasn't immediate failure. He said it was humming, and had been off/on lately until it warmed up. Apparently it was humming a lot, he leaned it forward to look at the tubes, and the fuse blew. He tried another fuse and it blew. Pulled all the tubes and another fuse...it blew as well.

    I've never tried to diagnose a bad PT. Here's what I can tell you so far:
    • 60W bulb glows brightly in the light bulb limiter with NO tubes installed and with all the secondaries detached and taped off
    • both tubes - JJ's - have a cloudy/smoky spot at the base between pins 4&5 (pics attached)
    • the center tap is still attached and grounded, but one of the high voltage secondaries has continuity to ground, one does not
    • all the E-caps look fine and all have ground continuity
    • no resistors look damaged or burned
    I'm assuming the PT is done, but I would like to know what may have caused it or what to look for and not risk another PT and probably pair of tubes.

    Any troubleshooting help & advice is appreciated.

    Thanks


    IMG_20201022_165129880_HDR.jpg IMG_20201022_165145231_HDR.jpg
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The most likely cause would be a shorted tube.
    It could however be a bias problem which would bother both tubes as indicated in your post. That would also be consistent with the hum. The tubes would redplate from too much current. The PT HT secondaries would get hot (too much current). It appears one *side* of the HT secondary couldn't handle the heat and opened. And... the PT primary insulation melted causing the short as indicated by the light bulb limiter. Then the fuse blew.

    Check the bias circuit. Check the screen resistors. The tubes may or may not function properly depending on what truly happened and how hot they got. Any time an event like this happens I check the filter caps/resistors and tube sockets.
     
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  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    A look at the 5f11 schematic shows no screen resistors.

    It also shows bias voltage of negative 31V serviced with a 25V cap.;)
     
  4. schmee

    schmee Doctor of Teleocity Silver Supporter

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    I've not seen that on JJ's. Maybe it is the tubes,... but ... what caused it?.. and...both?
    You are using slo blo fuses right?
    The use of a Diode in the bias circuit can be confusing on the 5F11 for me, Is it in the right direction?
     
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Rob has *protection mods* on his 5F6A mods page. (Fuses on the HT secondaries.)

    A lot of builders these days install hefty power dropping resistors so they won't burn up when a tube fails. This 5F11 had a failure mode where a lower wattage 470R resistor could possibly have saved the PT.
     
  6. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I have a slo-blo in there now, but I haven't turned it on without the limiter. But it glows full power and bright.

    With the limiter, is it ok to leave it on long enough to check the voltages on all the secondaries? It is a 60W Edison style bulb, but if it is glowing that brightly, I didn't know if it would possibly further damage the PT - assuming it isn't shot already.
     
  7. D'tar

    D'tar Friend of Leo's

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    That's how I use the bulb limiter! Long enough to get the numbers and as often as needed. If your tranny is smoked..... well..... it can't get much worse right.
     
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  8. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Yeah, I jumped on that bandwagon for a while even using 5 watt ceramics! Then that thought came to light with people. I wonder if those Bassman's I bought cheap with bad screens would have blown the PT with big resistors?
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yes you can use the limiter and check the secondaries.
     
  10. jsnwhite619

    jsnwhite619 Friend of Leo's

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    So, with the bulb burning bright, I've got a whopping 5.8v showing where the power cord meets the base of the fuse holder. I'm guessing that's bad...
     
  11. NTC

    NTC Tele-Meister

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    Could there be a shorted capacitor from the pi to the 6v6's?
     
  12. zook

    zook Friend of Leo's

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    If the fuse blows with no tubes in the amp it indicates a shorted (and no good) power transformer.
     
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  13. radiocaster

    radiocaster Poster Extraordinaire

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    Could be a rectifier problem.
     
  14. jsnwhite619

    jsnwhite619 Friend of Leo's

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    So, my amp mentor around here has an old 60's tube tester. We checked out the 5y3 & both 6v6's and all tested GOOD and no shorts. He loaned me a newer Orange tester to check the 12ax7's because I forgot them - haven't tried them yet.

    Since it's looking like the primary is what smoked - post #10 above - I asked the owner if his home was prone to power surges or anything like that. He said they have a whole house backup generator that kicks in whenever there is a power loss. Apparently, he lives in a rural area that is affected more with storms & downed lines. And he always leaves the amp plugged into the wall even when not in use.

    So, if all the tubes are OK & the caps inside are looking good, nothing burned or smoked looking, and the primary is seems to be damaged, what would be the odds of that backup generator kicking in and surging the PT during a power failure?
     
  15. Wally

    Wally Telefied Ad Free Member

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    Subscribed....with observations. Anytime an amp changes it’s sound, even for the better, one should inspect the tubes for redplating. Anytime an amp is humming, it needs immediate attention. Anytime an amp is off/on/off/on, it needs immediate attention. I pass this on to all of amp owners I know. This owner likely could have saved some dollars by not trying to ignore problems. I am subscribed in order to see who can diagnose and explain what electrons did to take this PT out. ??? And....would I trust those 6V6s? I don’t think so. Why not? Because the owner continued to use the amp when it obviously was not in proper operational cndition, and those tubes have been stressed. I might even be suspicious of the 5Y3. Ommv.
     
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Agree with Wally.
    From your post #1, I think it is possible a surge could have started the ball rolling. As mentioned before I would take a hard look at the bias circuit.

    You will need to replace the PT to get started.
     
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  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Good call. Yeah the 5Y3 is rated for 125mA. Might be able to handle twice that for a short while.

    The amp made the 6V6s extremely hot and fried the PT. The current had to be way past the 5Y3 rating. Best to have a replacement 5Y3 for the next startup.
     
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  18. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Well, just for an update, I had a chance to test the preamp tubes on his Orange VT1000 tester, and everything tested GOOD on it. I did the 6v6's again on this one, and they both tested GOOD. So, all the tubes are testing good on one if not two testers.

    Before I go down the rabbit hole wondering about it - the amp is wired up cord -> fuse -> ON/OFF -> PT primary. Would it be possible for a power surge/spike to do any damage if the amp was plugged into the wall, but the switch was off?
     
  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Nope
     
  20. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Well, with the PT disconnected and the heater center tap disconnected, I can't find anything in there with a ground short using my multimeter. Everything that is supposed to be grounded checks out, and everything that isn't seems to check out as well.

    Obviously, it needs a new PT, but this is new territory for me with dealing with a bad PT and finding the cause. When the new one comes in, I assume the bulb limiter will let me go through and either find the problem or chalk it up to a faulty original PT? Or one that may have been damaged during the original build process? Also, is my 60W enough for one that could possibly still have a serious problem when the new PT arrives?
     
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