Yet Another 5F1 Build

Discussion in 'Shock Brother's DIY Amps' started by brackp, Feb 18, 2019.

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  1. brackp

    brackp TDPRI Member

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    Hey,

    It's me again, since ye were all so helpful with my build I figured I'd start another thread to help me with my friends build also. I'll try to get him more involved with this thread because with you guys on board with my one I learned a lot more about how the amp works and how to diagnose issues. Assembly is one thing, knowing what's going on is another.

    So with that said, we finished wiring it up yesterday and got a little puff of smoke from somewhere around the pilot light (ish) I didn't see it so I'm not 100% certain.

    I took the following readings:

    5Y3
    2: 321 DC
    3: 281 AC
    5: 281 AC
    8: 321 DC

    B+1: 323
    B+2: 209
    B+3: 0

    (Clearly a problem right there)

    6V6
    2: 3.2
    3: 314
    4: 209
    5: 0
    7: 3.2
    8: 12.5

    12AX7
    This is a weird one, nothing except for the 3.2AC to the filaments coming through
    I measured and wrote down 518DC on pin 3. I reckon that was maybe the meter was set to auto range and that was probably mV

    I checked the fuse, all good. The 10K and 22K between B+1 and B+3 are measuring correctly. I checked the filter caps for ground and they're good. I had a general poke around for grounds and it seems ok to me.

    What would cause that big drop from B+1 to B+2 and then the drop to 0 at B+3?

    The only (intentional) difference between the wiring of his and mine is that we followed robrob A/C wiring, going to the fuse first. My live from the mains goes to the pot then back to the fuse, I understand that's not as safe so I'll be changing mine around. I'm not sure why it's safer though, someone might explain that to me also.

    Thanks!
     

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  2. D'tar

    D'tar Friend of Leo's

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    A direct short to ground in the B+3 string between b3 and 100k plates.

    Lets see the underside jumpers.
     
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  3. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Once the board is pulled as D'tar indicates, I think I see a different problem with the V1A cathode resistor. It appears to be grounded through the input jack near the 68k input (grid stopper) resistor instead of the actual cathode ground. It should go to the same terminal as the cathode-bypass capacitor.
     
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  4. brackp

    brackp TDPRI Member

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    Makes sense. Gonna take it out for a look now... he's gonna owe me a few pints for this
     
  5. brackp

    brackp TDPRI Member

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    Thanks a lot! Yeah the cathode resistor is completely wrong, should have spotted that. I got some pics of the back and think there's more than a few potential shorts there. Sorry they're not so clear, I just removed the input jacks, pot and pilot light so I could try get a look-see.

    I guess he could put in taller stand-offs, some of that solder is definitely big enough to reach I would reckon. Truth be told he might be better off taking it out and tidying up the back completely eh?

    Either way, it'll be a few days before we do anything, he's busy with small kids at home and work etc. Thanks for the help again guys!
     

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