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HT fuse located between center tap and chassis ground?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Jan 13, 2021 at 12:06 AM.

  1. itsGiusto

    itsGiusto Tele-Holic

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    I just completed my Ceriatone OTS FM build (Dumble #102 clone), though I haven't really tested it at all yet, so there's still probably a lot left to do. But I have a question about strange behavior I'm seeing, and the HT fuse.

    Here's the layout:

    [​IMG]

    As you can see, the HT fuse is not like many other amps, in that it's not located between the rectifier diodes and the filter caps. Instead, it's located between the chassis ground and the high voltage winding center tap.

    I kind of understand how this is supposed to work, in that current needs to be able to flow back to that winding in order to complete the rectified circuit. But I don't fully understand it.

    The really weird thing is that when I switch on the power and standby on the amp, even if that fuse in not in place, the filter caps still start charging up, and I start to see DC voltage if I measure between, for example, chassis ground and the first or second filter cap.

    Is this behavior incorrect? I sort of feel like it could make sense, maybe the transformer can maybe still produce voltage but not current? I'm not sure if I'm just excusing faulty behavior though. I mean, thinking about it another way, if the fuse can't stop the high voltage and turn off the amp, then what's even the point of it?
    Can anyone advise? Thanks!
     
    Last edited: Jan 13, 2021 at 12:14 AM
  2. itsGiusto

    itsGiusto Tele-Holic

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    Hmm, I just found this:
    https://el34world.com/Forum/index.php?topic=19440.msg201234#msg201234

    Sounds like because I ended up using a different PT than Ceriatone intended, it's dangerous to leave the fuse on the center tap as is. The PT I ended up using has the bias tapped from the HT winding, not on its own winding, like in the diagram above. It sounds like if the fuse is blown or not inserted, the bias tap can end up serving as an unbalanced center tap, resulting in damage to the bias filter cap. I think tomorrow I will move the fuse to instead come between the diodes and the standby switch, it should be an easy change to make. And instead, I'll ground the HT center tap directly against the chassis.

    I just hope I didn't damage the bias filter cap when I was measuring the voltage and experimenting. It seems okay, still reads 100uf on the multimeter, still does not read continuity between its two leads, doesn't show any burn marks, and is not leaking any fluids. So maybe it's not damaged...
     
    Last edited: Jan 13, 2021 at 12:43 AM
  3. 2L man

    2L man TDPRI Member

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    I think it is much better to put fuse to potential which is higher than earth! Signal Ground is often connected to earth/chassis so fuse on zero voltage output does not protect PT if current flows to Chassis.

    Without fuse the current still flows to Earth thru mains cable Earth! so it charges capacitors and amp should keel operating. Now If Earth wiring fails there comes high voltage to amp chassis which technicaly has direct connection to guitar strings.

    Neutral wire is often connected to Earth wire on building electric cabin where it is lead outdoors where often a copper pole is pushed in to Earth so,there is no Earth cable outdoors on power lines where actual Earth is used as zero potential.
     
    Last edited: Jan 13, 2021 at 10:18 AM
  4. 2L man

    2L man TDPRI Member

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    Its layout seems confusing when text say "star grounding" but there seem to be many chassis "lugs" where black ground wires go?

    On Star Grounding amp sections have their own "stars" where that section currents are first connected and then each section have wires which are connected to PT CT or SS bridge rectifier negative where all current paths "end".

    I don't remember what is the best place to connect Signal Ground to Chassis when Star Grounding is used? Either at input jack or at near PT CT so if anyone knows I'd like to know? I think HiFi tube amp builders know ;)
     
  5. itsGiusto

    itsGiusto Tele-Holic

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    I think that signal ground-chassis connection is best near the input jack, if I recall from http://www.valvewizard.co.uk/Grounding.pdf
     
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  6. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    2L man and itsGiusto like this.
  7. itsGiusto

    itsGiusto Tele-Holic

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    Very interesting. But unfortunately, I don't think I have room for adding another fuse to have one on each leg. One, after the rectification will have to do for now, though maybe I could consider the inline fuses, which I did not know existed. But at least this amp has 3 diodes on each side, which I think should make complete failover of one side more unlikely.

    Also wondering, if you go with the route of having two fuses, one on each leg, what should the rating be? Would it be half of the rating of the rectified fuse? So 250ma per leg?
     
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