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Amp/PT diagnosis

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

  1. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    So you do have continuity on the both sides of the HT and CT???
     
  2. jsnwhite619

    jsnwhite619 Friend of Leo's

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    No, it has two voltage options, and both have one side with continuity to CT. The 5v and 6.3v neither have continuity to ground, the hot nor neutral has continuity to ground.
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The 6.3V heater wires should have continuity or a low resistance with the meter probes on each of the green wires.
    Likewise for the yellow 5V heater.
    They are not connected to ground. Each is just a coil of wire inside the PT.
     
  4. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Yeah, I meant that I was checking for a ground short in the heater wiring. Both HT secondaries have continuity end-to-end, as well has the 6.3v & 5v wiring, independently. Green to green, yellow to yellow, red to red.
     
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  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    This failure sure has me thinking about the 2A fuse choice. I have a silverface deluxe rev that specs a 1A fuse. The deluxe AB763 and the AA1164 spec a 1A fuse. Heck the 5F10 sports a 3/4A fuse. Most of the other amplifiers with a similar tube compliment suggest 2A.

    There is no guarantee a lower rated fuse would have kept this particular failure from happening but it very well could have.

    In another thread elpico had this truism:
     
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  6. King Fan

    King Fan Poster Extraordinaire

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    Jason, I *think* your 60W bulb is safer than a bigger one, since it limits current more, and should be very safe for short-finding.

    You'll know this, but for those who don't, our smart friend @jhundt once posted steps from Gerald Weber on using a light bulb limiter to localize a short (Weber apparently suggests up to a 100W bulb -- I wouldn't use Uncle Doug's 250W -- for all I know it might not glow or it might not limit current 'enough'. But I could easily be wrong.)
    ------------------------------------------------------------
    Plug the amp in to the light-bulb limiter, turn the amp on in 'standby' mode (if available).

    Switch standby to 'play'. If the lamp lights brightly, this indicates a short somewhere in the amp.

    Remove power tubes, one at a time. If the light stays bright, that tube can be replaced in the amp. If the light dims, the tube you just removed is shorted. If none of the tubes appears to be shorted, go to the next step.

    Remove all power tubes together. If the light dims, the problem is probably in the bias supply. If the light remains bright, the problem is (probably) a bad rectifier, shorted power transformer, or shorted filter cap - see next step.

    Now switch the amp to standby mode. If the light goes out, the problem is in the filter caps. If the light keeps shining bright, the problem is is the transformer or the rectifier - see next step.

    Remove the rectifier (or disconnect the diodes if it's a hard-wired solid-state rectifier). If the light dims, the rectifier is bad. If the light stays bright, the problem is probably in the power transformer.
    -------------------------------------------------------------

    On a side note, we've talked here about how hard it is to find an incandescent bulb in some places. Your incandescent 'Edison' bulb is perfect of course, but I wonder how long it'll be before someone gets fooled into using one of those trendy LED 'filament' bulbs. Although this is an actual 4W bulb, the website lists it as a 40W 'Edison' bulb.

    upload_2020-10-24_8-22-35.png
     
  7. jsnwhite619

    jsnwhite619 Friend of Leo's

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    2A seems to be the norm on the Mojo kits, and that's what this was built from, parts kit at least.

    Just for information and to make sure I'm not thinking backwards - happens a lot - can you or someone list what connections on a PT should have continuity?

    So, if someone would list out, please, expected continuity for
    • HT secondaries together & center tap
    • 6.3v together & CT
    • 5v together & CT
    • main primaries
    Meaning, which leads should have continuity and which should not. Just so I know for sure that I'm thinking correctly.
    Thanks
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    mojo756pt.png
    I have to preface the word *continuity*. A meter may not show *continuity* if the resistance is more than a few ohms. So you may have to meter for ohms.

    The layout shows what would have continuity.
    Secondary side:
    The red HT wires are connected along with the CT. It is just a coil of wire with a tap in the middle.
    In your case the HT has a couple other taps to provide a lower voltage HT option. All of the wires of the HT coil are connected. And... if there were a bias voltage tap it would be included on this winding as well.
    The green wires are connected. No CT in this case but some PT's have a tap in the green.
    The yellow wires are connected.
    The Primary side:
    In this example all of them are connected. It is one coil with several taps.
    There are designs with one coil, no taps. There are also designs with more than one coil.

    You will note there are no coils that *ground* to the frame of the PT.
    When the PT is installed in an amplifier chassis, usually a CT is attached to ground. When a CT is attached to ground the voltage can be metered between the wire and ground.

    (Some, usually older PT's, may connect a CT to the frame of the PT in which case there would be a connection with the frame and subsequently the chassis. This must be acknowledged, otherwise an artificial CT might be added, which would make for noise.)
     
    Last edited: Oct 24, 2020
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  9. jhundt

    jhundt Doctor of Teleocity

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    I suggest that it is time to stop reading all the well-intentioned advice offered here.

    From your original post, it is clear that you suspect a failure of the power transformer. You appear to have reason to believe this is the case. so replace the transformer. Follow and believe in your convictions!

    Use the limiter, it will save you. The 60-watt bulb will be fine. If the amp shorts completely, the bulb will just glow happily because it is receiving all of the 120VAC.

    Interesting that the owner (and presumably the builder) made it blow up by leaning the amp forward.
     
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  10. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Got a Hammond 290AX ordered today. I think it wasn't registering with me that using the limiter will save a new PT. I was trying to figure out what happened to hopefully avoid killing another PT, but it finally clicked that I'm going to have to start fresh to figure out bad circuit, tubes, or transformer.
     
  11. zook

    zook Friend of Leo's

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

    That means no rectifier tube also.
     
  12. radiocaster

    radiocaster Poster Extraordinaire

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    I guess I could easily have missed something in the thread, but I didn't see where it was posted that the amp didn't have a solid-state rectifier.
     
  13. zook

    zook Friend of Leo's

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    It's a 5F11 Vibrolux, that amp has a tube rectifier.
     
  14. tubegeek

    tubegeek Friend of Leo's

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    The ONLY reason to be leery of a low wattage limiter bulb, it will seriously affect (lower) your voltage readings while you use it, and if you don't remember/know that, it'll freak you out.

    Lower wattage=more limiting=safer to test with.

    Now that you know, don't freak out if your voltages measure low while the limiter is in use.

    When my wife was pregnant with our daughter, I asked a more experienced dad if he had any tips. His reply was, the first time you feed them beets, and their poop comes out bright red, don't freak out.

    It was excellent advice!
     
  15. jsnwhite619

    jsnwhite619 Friend of Leo's

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    The new PT was delivered today, so I should have a progress report later today or tonight.
     
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  16. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Well, it was the PT. Swapped it out, installed a 1A fuse, hooked it up the bulb the first time, no tubes, and nothing....no light anywhere. I didn't wire the heaters yet just because I wanted to go ahead and check the PT, so the pilot didn't light up, but the limiter didn't either. Voltages were there on the socket, but crazy low with the limiter. Tried the original 5y3, turned it back on and the limiter turned on and faded to a barely glowing orange & gray filament. Success! Added the 6v6's, still good, 12ax7's, still good. Checked everything out in the bias, and it was on the low side of OK. Finally tried it on it's own with the 1A fuse and still good.

    I added the backup diodes to the rectifier socket beforehand, and he said it was too clean, so I raised the first dropping resistor from stock 470R to 1000R, and the NFB from 56k to 100K. I tried 68k, and I guess the trem circuit maybe eats up some gain that I didn't realize compared to Harvards? The Harvard I did with a 100k NFB was hell on wheels ZZ Top sound with anything and super bright. This wasn't. 100k NFB, bias is set around 68% and it's a rocker with humbuckers, which is what he said he normally plays, that and P90's.

    I was proud of myself. The bias pot resistor had it where the hottest I could get it was 60%. I swapped it out for a lower value, eyeballed the 10k pot & turned it, and let lightning strike me now if I wasn't within 1v on the B+ when I turned it on with the new bias resistor!

    Anyway, I told him I wanted to keep it for another couple days to run it and let it actually get hot and be sure, but I think it's safe to say that the PT was shorted. What caused it - will never know. But, this is my first dead PT. When I get out there one night and bored, I'm grabbing a screwdriver and a hammer, and we'll all see what the inside of this thing looks like and if I can find where it shorted!
     
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  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    New 6V6? New 5Y3?

    Barring nuisance failures, will the owner try to keep it running with a 1A fuse?
     
  18. BobbyZ

    BobbyZ Doctor of Teleocity

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    Just for future reference, if a LBT glows full brite there's a short. If all the secondary wires are disconnected, not touching anything and the short isn't in the primary wiring, the PT is junk.
    It's really that simple.
     
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  19. tubegeek

    tubegeek Friend of Leo's

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    You don't have a door that needs to be kept open?

    Here's another way to do it, from Geofex:

    xform_test.gif

    http://www.geofex.com/FX_images/xform_test.gif
     
  20. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Same tubes. Everything checked out on 2 different testers, voltages are good, both are biasing within 1-2 mA of each other.
     
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