Voltages way off on 5f11 build

Discussion in 'Shock Brother's DIY Amps' started by unezrider, Apr 30, 2020.

  1. unezrider

    unezrider Tele-Meister

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    Hello friend,
    A couple of days ago I finished my 5f11 Vibrolux build. plugged it in, a crackle, then it became quiet, like it should be. Started playing & heard some crackles, kinda like a bad solder joint sound. But the volume, tone, speed & depth pots were all working like they should. Not noticing anything apparent I switched out a 12ax7 (the socket is a little wobbly). That seemed to help, but as I continued to play I noticed the occasional crackle still. & something just seemed off. Tapped around the board with a chop stick, but pretty much everything made a noise. Took the multimeter to it last night & found my voltages were considerably off.
    For note, my wall was reading 120v, & the PT I have in the amp is a 305v.
    Both my filament leads were 3.4vac, pin 2 on the 5y3 was 7.6, pin 8 was 6.8, & pins 4 & 6 were 290 & 291.
    On the 6v6's pins 3 were 269, & pins 4 were 274.
    This morning I took another reading & pins 2 & 8 on the 5y3 were 9.9 & 10.4. & pins 3 & 4 on the 6v6 were 207 & 235. (the filaments were 3.4 still at each lug off the lamp.) (also at one point pin 5 on the 6v6 was -18. from last night's notes. Believe pin 4 at that reading was 230) (The wall was still at 120)

    I have built one amp before, but when it comes to getting into the weeds with this stuff I'm a complete novice. I thought I had it narrowed it to the plate of the 6v6 & the OT, but then the numbers started changing. So now I don't want to keep turning the amp on to try to figure what the issue is in fear of damaging the trannys. Nor since I can't seem to pinpoint the issue.

    Is there a good place to begin with this? What exactly should I be looking for? Or is this a take to the shop (once they open back up) situation?

    woefully, yours
     
  2. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Pictures would help notice anything obvious in the wiring or setup.

    Sent from my moto g(6) using Tapatalk
     
  3. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Waiting for pictures.:cool:
     
  4. unezrider

    unezrider Tele-Meister

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    I updated the AC wiring per suggestion on here. & slightly modified the grounding on the depth & speed pots. But for my own ease I had the green ground run along behind the board as in this layout. Also there is a connection between the 25uf & the 16uf, its just behind the board. same with the n4007 & lower terminal of the 25uf.

    oldmojo5f11.jpg IMG_4136.jpg IMG_4130.jpg IMG_4135.jpg IMG_4131.jpg IMG_4132.jpg IMG_4133.jpg IMG_4134.jpg
     
  5. unezrider

    unezrider Tele-Meister

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    IMG_4138.jpg IMG_4139.jpg
     
  6. unezrider

    unezrider Tele-Meister

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    & a look before it went in

    IMG_4076.jpg
     
  7. elpico

    elpico Tele-Holic

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    The most important voltage in the amp is the negative bias voltage, which is pin 5 on the 6v6. The schematic say pin 5 should be -31, which you're pretty far off. You should never plug power tubes into an amp without first verifying the negative bias voltage is in the correct range so the first thing to do is pull those out and work on getting the bias voltage right.

    As a side note, you don't measure heater voltages to ground. Measure them pin to pin on the tube socket. If the heater winding is center tapped the two methods *should* give the same results, but if there's any problem with the center tap connection it may not... whereas pin to pin is always accurate.
     
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  8. unezrider

    unezrider Tele-Meister

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    Yeah, I totally dropped the ball on that one.
    So I leave in the 5y3, take all other tubes out, unplug the speaker, turn it on, then can test from the pins? (that would be one in pin 2, & one in pin7?)
    & aside from the heaters, everything else I test to ground, correct?
     
  9. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I think you have your PT center tap running to your bias circuit, which would be no bias voltage. Red/White wire. Which PT did you use?
     
  10. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I think the brown wire should go to the circuit board -> diode -> pot section. Red/White should be grounded. But, but please post a diagram or let someone else advise on that before you change anything and damage it.
     
  11. unezrider

    unezrider Tele-Meister

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    The colors are a bit off, but they're in the right spots

    IMG_3990.jpg
     
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  12. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Yes. (Speaker can stay plugged in.)

    IDK if you are using a light bulb limiter. If you are it will give odd voltage readings. (If you are using one you can stop now.)

    Now as @elpico said the bias voltage needs attention. With power tubes out, adjust the bias pot for the most negative number you can get. The bias resistor(s) value need to change if the pot can't provide sufficient negative voltage.

    Since the tubes are out, make sure the pins make good contact in the sockets. If you have deoxite use it. You can also gently scrape the pins with an exacto blade to remove any oxidation. The crackling popping noise can come from poor contact of tube pins.
     
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  13. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Why is the 100k resistor from the speed pot connected to the depth pot?
     
  14. tubegeek

    tubegeek Tele-Afflicted

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    bad ground copy.jpg


    Either shadows are playing tricks in this pic, or one of the most important solder joints in the entire amp is kinda suspect. (I think that wire is the power transformer ground connection, right?) It looks like the flux has not boiled completely out of the joint and like the wire strands are not all completely wetted with solder. Unscrew the lug from the chassis (so it will heat properly when you go to solder it) and make sure you heat it until you get a smooth, shiny joint. Or use a crimp-on terminal after freshly stripping the wire.

    While you're at it, look at every solder joint with a magnifying glass and make sure they are ALL smooth and shiny, and the terminals and wire are all completely wetted. Nothing more aggravating than chasing bad solder joints!
     
  15. unezrider

    unezrider Tele-Meister

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  16. unezrider

    unezrider Tele-Meister

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    & thank you guys for the help here. I do appreciate it.
     
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  17. dan40

    dan40 Tele-Afflicted

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    Do you have your meter set properly when taking your readings? Everything from the PT (filament wiring and high voltage to rectifier tube) will be an AC voltage reading. For all of the other voltage checks, you need to set your meter to DC to get the proper readings. Forgive me if you were already aware of this but I just wanted to be sure you were measuring correctly.
     
  18. unezrider

    unezrider Tele-Meister

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    No offense taken. I was aware of that, but all help is welcome.

    I removed the power tubes & got the readings from the pins as suggested.

    Heaters were 6.9vac
    5y3
    pin 4 & 6 both 303vac
    pin 2 - 5vac
    pin 8 - .5vac

    6v6
    pin 3 - 407vdc
    pin 4- 405vdc
    pin 5- 1.8vdc

    So something is definitely off along the power chain. & not sure if pin 8 on the 5y3 should be reading anything other than 5.
     
    Last edited: May 1, 2020
  19. dan40

    dan40 Tele-Afflicted

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    Those numbers look about right without power tubes installed. They will run a bit high without a load on the power supply but your earlier numbers definitely looked strange.

    Edit: I missed your pin 5 voltage. Definitely a problem in the bias circuit as the fellas mentioned earlier.
     
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  20. Nickfl

    Nickfl Tele-Afflicted

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    Start looking at your bias circuit. Figure out where you have negative voltage and why you don't have it at the 6v6 grids.
     
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