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Debugging faulty RobRob Blackvibe build

Discussion in 'Shock Brother's DIY Amps' started by bmoney, Apr 11, 2021.

  1. bmoney

    bmoney TDPRI Member

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    Hi there,

    I successfully built a 5e3 Deluxe Micro by RobRob and tried to follow it up with a Blackvibe build. All seemed well as the build seemed to work but the amp seems to have a problem specifically with the power tube (12AU7) or the socket or something around there.

    When I turn it on, it takes maybe 5 min to warm up enough to get enough headroom. Prior to that it sounds almost silent and you can coax out some distorted sounds by cranking the master volume and turning up the preamp volume. After 5 min, it sounds fine. But after playing for maybe 20 min or so, something happens to the 12AU7. The output goes down a lot, and you can crank the master to around 7-9 and it'll make noise, but very quiet. And the tube while it still works in other amps, it's not the same. I've put in a new 12AU7 and the same situation repeats itself. I've gone through 3 tubes so far.

    I confirmed without the power tube, the voltages in B+1, B+2, B+3 are what I expect them to be. When I put in the power tube instead of reading 395V DC at B+1 it seems to be more at 345V DC. Also I thought there should be no DC voltage at the grids (pin 2 and 7) on V2 and V3 (phase inverter and power tube) but I'm getting around 40V DC there. I thought that meant I had leaky coupling caps so I replaced those with new ones but the old ones read fine and no change. I also re-soldered connections to the power tube.

    At this point, I don't really suspect any components on the turret board so much as connections to the power tube socket or the socket itself. My problem with the 5e3 Deluxe Micro build was around heater wiring and soldering. This time seems to be fine, but soldering to tube sockets seems to be where I run into the most trouble.

    If anyone has any ideas what might be wrong or any tests to confirm / deny any possible issues, I'd appreciate it. Thanks for reading.
     
  2. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Can we have a layout/schematic with voltages? They are invaluable.

    Also mention the reference. E.g. the long tail of the phase splitter will cause a DC wrt ground.
     
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  3. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    ^^^ this. Voltages across the sockets, B+, and heater voltages will help.

    So would some well-lit photos, ideally landscape, and for wide shots, with the amp oriented like the layout diagram... :) FWIW you can drag JPG files right into this composition box, or use the 'upload a file' button and hit Full Image. Also, PDFs don't show inline, so as a demo I'll upload a JPG for the layout (I guess) we're talking about...

    RR763_BLACKVIBE_MICRO_Layout copy.jpg
     
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  4. bmoney

    bmoney TDPRI Member

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    Thanks. Here are some gut shot photos and details of the different areas. The wires may obscure a bit.

    blackvibe_gutshot.jpg blackvibe_turret_board.jpg
     
  5. bmoney

    bmoney TDPRI Member

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    Here's detail on the V1-V4 sockets
    blackvibe_v1_v2_sockets.jpg blackvibe_v3_v4_sockets.jpg
     
  6. bmoney

    bmoney TDPRI Member

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    I'm also using the following schematic. It's the BlackVibe Micro with EZ81 rectifier. Voltages coming right up. And all with respect to ground.
    RR763_BLACKVIBE_MICRO_EZ81_Layout.jpg
     
  7. bmoney

    bmoney TDPRI Member

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    Here are the voltages. So as I mentioned when I turn the amp on with the power tube, B+1, B+2, B+3 all are at higher DC voltages but then drop over time until they settle down. I'm listing the voltage they settle down at after about 5 min.

    With power tube installed:
    V1 Preamp 1 & 2:
    Pin 1 (plate): 212.8 VDC
    Pin 2 (grid): 0 VDC
    Pin 3 (cathode): 1.5 VDC
    Pin 4 (heater): 3.3 VAC
    Pin 5 (heater): 3.3 VAC
    Pin 6 (plate): 210.2 VDC
    Pin 7 (grid): 0 VDC
    Pin 8 (cathode): 1.5 VDC
    Pin 9 (heater): 3.2 VAC

    V2 LTP Phase Inverter:
    Pin 1 (plate): 206.1 VDC
    Pin 2 (grid): 27.7 VDC
    Pin 3 (cathode): 48.1 VDC
    Pin 4 (heater): 3.3 VAC
    Pin 5 (heater): 3.3 VAC
    Pin 6 (plate): 200.9 VDC
    Pin 7 (grid): 30 VDC
    Pin 8 (cathode): 48 VDC
    Pin 9 (heater): 3.2 VAC

    V3 12AU7 Push-Pull Power Tube:
    Pin 1 (plate): 344.1 VDC
    Pin 2 (grid): 45 VDC
    Pin 3 (cathode): 46.7 VDC
    Pin 4 (heater): 3.3 VAC
    Pin 5 (heater): 3.3 VAC
    Pin 6 (plate): 344 VDC <- (edited, I had 0 before but wasn't making a solid connection with the probe)
    Pin 7 (grid): 46.5 VDC
    Pin 8 (cathode): 46.8 VDC
    Pin 9 (heater): 3.2 VAC

    V4 EZ81 Rectifier:
    Pin 1 (linked from pin 2 through diode): 157.8 VAC
    Pin 2 (high voltage): 296.1 VAC
    Pin 3 (B+ DC out): 356.8 VDC
    Pin 4 (heater): 3.2 VAC
    Pin 5 (heater): 3.3 VAC
    Pin 6 (high voltage): 295.2 VAC
    Pin 7 (linked from pin 6 through diode): 156.6 VAC

    B+1: 349.1 VDC
    B+2: 311.6 VDC
    B+3: 308.5 VDC


    Without power tube
    V1 Preamp 1 & 2:
    Pin 1 (plate): 245.8 VDC
    Pin 2 (grid): 0 VDC
    Pin 3 (cathode): 1.7 VDC
    Pin 4 (heater): 3.3 VAC
    Pin 5 (heater): 3.3 VAC
    Pin 6 (plate): 242.9 VDC
    Pin 7 (grid): 0 VDC
    Pin 8 (cathode): 1.7 VDC
    Pin 9 (heater): 3.2 VAC

    V2 LTP Phase Inverter:
    Pin 1 (plate): 237.9 VDC
    Pin 2 (grid): 32.5 VDC
    Pin 3 (cathode): 55.6 VDC
    Pin 4 (heater): 3.3 VAC
    Pin 5 (heater): 3.3 VAC
    Pin 6 (plate): 232.8 VDC
    Pin 7 (grid): 35.3 VDC
    Pin 8 (cathode): 55.6 VDC
    Pin 9 (heater): 3.2 VAC

    V3 12AU7 Push-Pull Power Tube:
    Pin 1 (plate): 404.3 VDC
    Pin 2 (grid): 52.6 VDC
    Pin 3 (cathode): 0 VDC
    Pin 4 (heater): 3.3 VAC
    Pin 5 (heater): 3.3 VAC
    Pin 6 (plate): 405 VDC
    Pin 7 (grid): 54.2 VDC
    Pin 8 (cathode): 0 VDC
    Pin 9 (heater): 3.2 VAC

    V4 EZ81 Rectifier:
    Pin 1 (linked from pin 2 through diode): 163.6 VAC
    Pin 2 (high voltage): 302 VAC
    Pin 3 (B+ DC out): 405.5 VDC
    Pin 4 (heater): 3.2 VAC
    Pin 5 (heater): 3.3 VAC
    Pin 6 (high voltage): 302 VAC
    Pin 7 (linked from pin 6 through diode): 163.3 VAC

    B+1: 403.8 VDC
    B+2: 360.6 VDC
    B+3: 356.6 VDC
     
    Last edited: Apr 13, 2021
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  8. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I expect ltp cathodes to be at 44V, so that is ok.
     
  9. Commodore 64

    Commodore 64 Friend of Leo's

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    [​IMG]

    For your second amp, this looks better than mine did. However:

    Somethings wrong with wiring at your power tube, with the tube in you've got a short or something, there's no voltage on one of the plates.

    Is that black wire shorting pin 6? Even if it isn't I'd clip those nasty leads. The leads in your tube sockets look terrible. Frays and dangles everywhere. Those sockets are tiny, you need to be a bit more diligent there.

    Also, you've posted no schematic, you've only posted layouts. Schematic would help.

    Inlining a robrob:

    [​IMG]

    On chassis mounted tube sockets, it isn't like they are having 3lbs of transformer hanging off them. I would invest in some very short,blunt tip, self tapping metal screws to stick those on. You've got big keps nuts and screw threads taking up space where you have dangles and frays all over the place. Just makes it easier for something to short or touch something it shouldn't. Aforelinked screws shown below with 9-pin socket for scale.

    [​IMG]

    You might want 5/16 or 3/8 if you are using those beltons with a little gap. 1/4 might not be enough.
     
    Last edited: Apr 13, 2021
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  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Absolutely, exactly, re pin 6. Plate voltage is everything to tube function, and the no-tubes voltages suggest it’s getting there, so a short somewhere is sending it into something else there when the tube's plugged in.

    BTW nice work on the pics and voltages, bmoney.

    Although I agree self-tapping screws are nice, I wonder if they’d be a pain to fit and tighten now. Before I went there, I think we need to rewire those sockets anyway, especially V2 and most of all V3, and sucker up / trim off extra solder and stray strands. Some of the wires can be shortened or rerouted to be less in the way.

    if I were rewiring 'em, I’d mount the ears / flange outside the chassis (bottom mount). Less nice grounded metal for stray wire to contact.

    I’m also hoping there’s some vertical clearance between the nuts and some of the stray bits. So be careful bending down the solder lugs too much, especially when top-mounting the socket. Ideally the lugs stand mostly upright and the wires approach low to the floor and 'plug in' from outside to inside so wire ends stay high and far from the chassis. This pic grabbed at random off Google shows the basic idea.

    747601F6-ED96-426C-9969-9206882BB067.jpeg
     
    Last edited: Apr 13, 2021
  11. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I don't see what you state.

    Edit: Now I do o_O. Why do I need to post something stupid before seeing the obvious?!
     
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  12. Commodore 64

    Commodore 64 Friend of Leo's

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    Yeah, I doubt he could retrofit those tube sockets with self tappers. I meant for future projects. :)

    LOL, Tom. I thought you were trying to trick me.
     
    Last edited: Apr 13, 2021
  13. bmoney

    bmoney TDPRI Member

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    Thanks for all the advice. My suspicion was definitely that most of the problem is at the socket wiring. I'll try cleaning it up tonight and see what happens.

    Is it normal with the power tube in that the voltage drops slowly for 5 min to the levels I'm reporting with the power tube? I thought the reference voltages on the layout at 395V for B+1 were what I was supposed to expect at steady state with a power tube in, but what I get is more in the 350V range.

    Edit: I checked and pin 6 on the power tube isn't shorting. I was mis-reading with the probe (not getting a solid connection). So it basically reads the same as pin 1.
     
    Last edited: Apr 13, 2021
  14. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    It'll drop from above-nominal values seen at the initial surge as the amp/tubes warm up and start to conduct, but that's over seconds, far less than a minute, and then only very small drops (a few volts) may kick in over the next minutes. Your slow warm-up to make sound and later drop in voltage (and sound?) *likely* reflects something (tube socket solder?? or ???) warming up and first starting to function more (short less?) but then to malfunction more (short more?).
     
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  15. Commodore 64

    Commodore 64 Friend of Leo's

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    If the short is drawing current, it can (will) sag your voltage depending on how much. If it draws enough it will blow the fuse.
     
  16. D'tar

    D'tar Friend of Leo's

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    "Shorting" hundreds of volts to zero would likely produce what is commonly called "Magic Smoke"!!

    I'd be looking for poor solder connection (open circuit), perhaps at the splice in the OT wiring?
     
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  17. bmoney

    bmoney TDPRI Member

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    Yes, the next area I thought might be suspicious is the extender wires I spliced for the output transformer and choke. I'm pretty bad with stranded wire and I think a lot of my issues are soldering with stranded wire vs. solid core which I mostly used in my first build. Pre-tinning on stranded thread is fine but then it won't bend or hook once the solder dries.
     
  18. Commodore 64

    Commodore 64 Friend of Leo's

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    And yet, there are millions of stranded solder joints in the wild that are tidy, bent and hooked. :p Be patient. And practice.
     
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  19. dan40

    dan40 Friend of Leo's

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    The very long warm up time made me think of a bad heater connection on one of the sockets, most likely the power tube. Once the tube finally warms up fully and begins drawing current, the voltages fall into their normal range.

    46.8vdc across the 12au7's cathode seems a bit high also. Is this measurement correct?
     
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  20. bmoney

    bmoney TDPRI Member

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    I measured again just now and got 44.4 VDC.

    I ordered some freeze spray so I can see if freezing any areas causes a drop in headroom (my primary symptom). I also ordered a couple cheap replacement 12AU7's as I've noticed when I placed a fresh JJ 12AU7 in there, it sounded pristine, clean, a bit loud (too much voltage to the power tube?), but after 20-30 min playing something seems to burn out or change in the 12AU7 without breaking it. It then has lower headroom and behaves like I've described now.
     
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