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Dead AL811H modded with HV glitch resistor + fuse

Discussion in 'Amateur Radio Amplifiers' started by W0OPW, Sep 17, 2021.

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  1. W0OPW

    W0OPW Ham Member QRZ Page

    I worked on an dead AL811H for a friend and was surprised to see a blown 1 amp 250v fuse in the HV line, in front of a glitch resistor.
    Surprised because the whole idea of the glitch resistor was to absorb any HV spikes and to fall on the knife, so to speak, and thereby save other more valuable components.
    So why the fuse? A fuse would have been the first thing one would think of instead of a resistor when the idea came about some years ago.
    In this case, there's now a component whose only task is to save the glitch resistor.
    So, we now have three ways of stuffing the fire: fuse, glitch resistor, or both. But something's redundant here.
    The amp now works fine with a a new fuse, but I'm suspicious.
  2. G0JUR

    G0JUR Ham Member QRZ Page

    glitch resistor to limit current, fuse to open hv line during flash over, thats my understanding of it.
    Of course I could be wrong.
  3. N3RYB

    N3RYB Premium Subscriber QRZ Page

    I'd be more concerned about why the fuse blew rather than it being there to start with. Does it have 811a tubes in it? (See other threads regarding stuffing 572b's in them if so). I'd worry about it flashing over again.

    Regarding the fuse protecting the glitch resistor, it's sure easier to replace the fuse than replace the glitch resistor.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    A 250V fuse in a 1600V line is a poor idea, though.

    The reason fuses have a "voltage" rating (and not just a current rating) is for the fuse to function normally and safely, it shouldn't be used in series with a conductor carrying a potential voltage above its rating: The fusing element can break (as it's supposed to with overload) and then simply arc from one contact to the other, allowing current to flow even after the element breaks.

    There are high voltage fuses available for such applications.
    N8VIL, AC9YY and N3RYB like this.
  5. GM3SEK

    GM3SEK Ham Member QRZ Page

    Exactly right. It needs both the resistor and the fuse, because they operate on different timescales and each fills in for the weakness of the other.

    A fuse provides a permanent shutdown; but until it has actually blown and the arc has been quenched, a fuse provides almost no current limiting at all. A lot of damage can be done in those few milliseconds. For the resistor, it's almost the exact opposite: the resistor provides instantaneous current limiting, but it can't hold out forever... which is why it also needs the fuse.

    Agreed that a 250VAC rated fuse is being used far outside its guaranteed ratings. For a small PA, the right kind of 250V fuse will probably work... but it depends how happy you are with "probably".

    73 from Ian GM3SEK
    W0OPW, W3SLK and N3RYB like this.
  6. W0OPW

    W0OPW Ham Member QRZ Page

    It doesn't sound like the glitch resistor never comes into play. As I stated above, the fuse seems to be protecting the resistor.
  7. G0JUR

    G0JUR Ham Member QRZ Page

    Yes it is, the resistor is limiting the massive surge of current that occurs during flashover destroying most of the amplifier in the process.
    Both are needed.
    GM3SEK likes this.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'll be the contrarian.

    First, the glitch resistor protects against "ion arcs" (a phrase Eimac used a lot, especially when Bill Orr was writing the material) which are common in lots of tubes but don't represent a short circuit. The resistor absorbs the arc energy, doesn't self-destruct but limits the energy available for the internal tube arc and protects everything. Usually, an amp with a proper glitch resistor can do this lots of times and the operator wouldn't even know it was happening.

    Second, I don't really see the need for a fuse in the B+ line when one in the B- line that's very close to ground potential can accomplish the same thing while using a common, ordinary and cheap fuse that can be mounted in a panel fuseholder for easy replacement. Henry amps almost all had this, labeled as a "cathode fuse."
    W0OPW likes this.

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