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Thread: Fuse on cathodes or OPT primaries for added protection? Anyone ever try this?

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    Fuse on cathodes or OPT primaries for added protection? Anyone ever try this?

    Just got an old Ampeg Gemini VI restored to good working condition, which turned out to be a LOT of work. The amp was pretty much destroyed when I got it(for free.) It had a shoddy AC filter 'repair' job from a previous owner, suggesting a prior power supply failure. Whoever 'fixed' it did a lousy job, with lower-voltage caps and diodes, complete with a 25A fuse instead of a 3A fuse. When a power tube shorted it took down the filter caps, diodes, some resistors, and the power transformer--which is why I got it for free. From what I've read, these amps frequently had power supply reliability issues, and that is the main knock against them(It sounds GREAT.)

    So my question is..has anybody ever fused the output tfx primaries or the power tube cathodes of an amp? I have heard of some home hi-fi guys doing this. The 3A main fuse is necessary to charge the filter caps on startup, but would allow tons of current to pass through it if a tube ran away and started conducting heavily. By my seat of the pants calculation, a 0.5A fuse on either the cathodes of the power tubes, or on the primaries of the OPT would not interfere with normal operation but would protect components in case of a power tube shorting. Thoughts or suggestions from anybody who has ever done this would be appreciated. Thanx!

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Cathode fusing... I think it is a probably a waste of time to go through all that.
    Also, I'd be willing to bet your diodes shorted first and blew the filter caps and the power tubes... not the other way around.
    Just fuse the high voltage after the filter caps if you are really worried. Maybe something like a fast blow 500ma or 1a.

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    Bruce

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    I favour (sorry favor) using a fast-blo fuse in the HT after the first filter cap.

    My reason for preferring this over cathode fusing is that if a valve shorts to the cathode and the fuse in the cathode blows then you will have a valve with the cathode potentially sitting at the HT voltage. This can break down the cathode heater insulation and impose the HT on the valve heaters, which mucho potential for colateral damage. I've seen this in many amps, so is not an hypothetical occurence.

    Shutting off the HT is a far better solution.

    By the way I had 5 (yes 5) Ampeg Gemini in the workshop recently. Great amps, although some of the previous work done on them was not of the highest quality......

    How to wire a mains earth to a US amp with 2 prong cord. NOT! | Guitar Amp Repairs, Custom Amp Builds, Speakers - JPF Amplification

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    Quote Originally Posted by jpfamps View Post
    I favour (sorry favor) using a fast-blo fuse in the HT after the first filter cap.

    My reason for preferring this over cathode fusing is that if a valve shorts to the cathode and the fuse in the cathode blows then you will have a valve with the cathode potentially sitting at the HT voltage. This can break down the cathode heater insulation and impose the HT on the valve heaters, which mucho potential for colateral damage. I've seen this in many amps, so is not an hypothetical occurence.

    Shutting off the HT is a far better solution.
    Thanx for the info. That is a much wiser solution. As for the fuse after the first cap, at 500ma, would it not potentially blow when powering up?

    Quote Originally Posted by jpfamps View Post
    By the way I had 5 (yes 5) Ampeg Gemini in the workshop recently. Great amps, although some of the previous work done on them was not of the highest quality......

    How to wire a mains earth to a US amp with 2 prong cord. NOT! | Guitar Amp Repairs, Custom Amp Builds, Speakers - JPF Amplification
    And yes, they call these the 'poor man's vibro-verb' for a reason. Great sound, but cheap and under-engineered power supplies. I rebuilt the power supply with some pretty stout 1400V 3.5A NTE fast-recovery diodes, a 700V 80uf filter cap and a 525V multi-cap from Mallory. I also beefed up the bias supply, with a 10w resistor and a higher voltage-rated cap. I didn't want the bias supply to heat up and potentially go open, letting the tubes run away. The power tfx I used was pulled from a Hammond A-100(6V6-12AX7-12AU7 chassis.) I didn't want to pay $130 for a stock replacement when the stock tfx was a pile of garbage IMHO. Got the Hammond tranny for 10 bux from a buddy. The supply voltages I read are within 1-2% of spec(on the Ampeg schematic) and the new tfx is 2X the size and 2X as heavy as the old one. My intuition(lacking hard data) tells me that the new tranny should be a little more resistant to damage and should sag less than the stock unit. The amp was in non-working condition when I got it so I didn't have any reference to start from as far as voltages.

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