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Questions on variations of the blackvibe circuit

Discussion in 'Shock Brother's DIY Amps' started by zorgzorg2, Sep 19, 2020.

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  1. Oct 30, 2020 #61
    zorgzorg2

    zorgzorg2 Tele-Meister

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    OK, so putting contact cleaner on the tube did help in that the crackling isn't as loud anymore. But it's still there when warming up. It's a Frondelli master volume I have, see the schematic above. I will check the soldering joints, I was getting the impression that my ground bus was a bit oxidised, so maybe the AC shunt caps or some bias caps could be having a bad joint indeed.
     
  2. Oct 30, 2020 #62
    zorgzorg2

    zorgzorg2 Tele-Meister

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    Tried again tonight, and the crackle seems to last much less now, maybe 2 minutes. So I think I can live with it. I checked all the solder joints connected to the master, can't see or measure anything wrong...

    Another question I have now: how do I dial the balance pot? I played with it a bit, but I can't hear much difference. The power tubes I have should be matched, so I should in principle dial it midway, right?
     
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  3. Oct 31, 2020 #63
    zorgzorg2

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    OK I read somewhere that it's about minimising the hum, but I can't hear anything...

    Another question about biasing. Robrob recommends measuring the ot voltages indirectly by subtracting each primary voltage to the ct voltage, but why not set the voltmeter directly between the ct and the primary lead?
     
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  4. Oct 31, 2020 #64
    sds1

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    Safety is why he recommends the other way.

    But measure the voltage across the OT directly as you say for better precision. Just be careful.

    50% is a good starting point, but you should be going by the numbers. It looks like you are working thru the bias procedure now. :)
     
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  5. Oct 31, 2020 #65
    zorgzorg2

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    Thanks @sds1! OK, I'll adjust the balance pot using a meter then.

    OK, back to the crackling problem. Tonight it didnt disappear after warmup. The crackle is sometimes mixed with white noise, in a fashion that reminds me of distorted amateur radio signals... Could it be possible that the amp would be catching these?
    How about adding an rfi suppressor cap, in case the noise comes from the mains lines? (they do have Internet signals on the mains nowadays)
    Setting the master volume to full doesn't actually remove the crackle, it just makes it very faint.
    Could a faulty ac cap I added on the bias circuits cause that kind of noise otherwise?

    Still investigating... o_O
     
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  6. Oct 31, 2020 #66
    zorgzorg2

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    One thing I noticed is that switching off the amp kills the crackle immediately, even if the guitar is still ringing. Would it indicate a mains issue?
     
  7. Oct 31, 2020 #67
    sds1

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    Tube and bad solder joint still stand out to me as most likely culprits here.

    That's for both symptoms: the crackling and the sort of radio interference you are possibly hearing.

    This seems like a gremlin type issue, no doubt you'll solve it eventually (there is a solution to every problem inside the amp) but you could chase it for weeks. Let's avoid that if possible.

    If it were me, I would reflow every single joint in the amp and swap out all the tubes, either one by one or all together it doesn't matter. I realize those are both annoying next steps for their own reasons but, to me they are high on the list right now, to avoid leaving something obvious behind.

    I could be right, or I could be wrong! :)
     
    Last edited: Oct 31, 2020
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  8. Oct 31, 2020 #68
    D'tar

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    Use non conductive chopstick and push prod and poke every connection to try and manually duplicate the issue. Sometimes crackling can show as a light spark in a dimly lit room. Worth a look.
     
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  9. Oct 31, 2020 #69
    robrob

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    The crackling is most likely a bad solder joint. Re-flow all the power tube and master volume solder joints.

    The bias balance is used during biasing to match the two power tubes bias current. Alternatively, you can dial in some intentional mismatch to generate some extra power tube distortion to fatten up the clean tone a little. Placing the balance pot mid-way is a good starting place.
     
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  10. Nov 1, 2020 #70
    zorgzorg2

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    Thanks guys for the good advice! I'll start reflowing some solder joints then. I poked around with a chopstick already but couldn't find anything suspicious. I'll dim the lights tonight to check for some arking. Regarding tubes, I don't have an extra set, I'll start hunting for those now.
     
  11. Nov 1, 2020 #71
    tweedy_woodpecker

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    Another vote for a bad solder joint. if you can pull the PI tube and still hear the crackle it must be somewhere in the power amp.

    I had the same issue a while ago and it turned out to be a bad solder connection of one of the filter caps to ground.

    As mentioned pulling tubes starting from V1 is often a first step to finding the origin of the issue.
     
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  12. Nov 1, 2020 #72
    zorgzorg2

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    OK, so I removed the tubes one by one, and the crackle seemed to stop when I removed v4. Then I went to put the tubes back, exchanging the power tubes placés, and when I went to put v2 back, I noticed pin 5 has stayed in the socket... I thought I was careful :(
    Anyway, now I need to order a new PI tube...
     
  13. Nov 1, 2020 #73
    Lowerleftcoast

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    You may have found your crackle. lol.
     
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  14. Nov 1, 2020 #74
    zorgzorg2

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    Yeah, maybe indeed, if that pin was shaky from the start?

    Here the bad tube and broken pin
    20201101_181727.jpg
     
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  15. Nov 1, 2020 #75
    sds1

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    That has literally never happened to me.

    Should never happen!
     
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  16. Nov 1, 2020 #76
    Lowerleftcoast

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    It was just a TAD weak.

    :lol::lol::lol:
     
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  17. Nov 1, 2020 #77
    sds1

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    bwaa

    I bet the microphonics were super low before the pin fell off.
     
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  18. Nov 1, 2020 #78
    zorgzorg2

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    Haha, that will teach me to buy cheap tubes. Just ordered some jj instead for both preamp, pi, and power.
     
  19. Nov 3, 2020 #79
    zorgzorg2

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    Alright, so while I'm waiting for a new set of tubes, I tried to go further on the crackle problem.

    Leaving the rectifier tube in, removing the preamp tube and just leaving the power tubes, the crackle can still be heard (v5 (rectifier) in, v3 and v4 in (power), v1 (preamp) removed and v2 (pi) removed)

    Removing v4: still crackle
    Removing v3: no crackle
    Putting back v3: crackle
    Swapping v3 with the other 6l6 tube: no crackle
    A bad tube?
    Removing v3, putting the "bad" tube in v4: crackle
    Swapping v4 with the "good" tube: no crackle

    Well well... Either I'm a tube breaker, or TAD tubes have a 40% failure rate in my case...
     
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  20. Nov 3, 2020 #80
    BigDaddy23

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    Jeepers.....that’s no good if it’s the latter. Did you rule out bad/dodgy cathode or screen connections on the power tube sockets?
     
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