12 amp 115v slow blow fuses for KPA 500

Discussion in 'Amateur Radio Amplifiers' started by KB4OIF, Jul 14, 2018.

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  1. KB4OIF

    KB4OIF Premium Subscriber QRZ Page

    Hello. I have been looking my eyeballs out for fuses for my KPA 500. Recently purchased from a good friend. he ran 220 and had 6 amp fuses in the fuse holder. I do not have 220 in my shack. It's 110. Manual say use two 12A fuses for 115v. The tech at elecraft said use slow blow one. Can't find them anywhere. I am open to suggestions. Need resources for them. I ordered some 12 amp slow blow one but did not look at the volts. They are 230V.

  2. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Hi John,

    The fuse size is 3AG and you are looking for a 12A Slo Blo rated at 250v. You can find these at Home Depot, Lowes, many hardware stores, etc.

    If not, you can order them from Digikey>

    Click here to pick from this page. The postage may be more than the darn fuses... :p
  3. K7MH

    K7MH Ham Member QRZ Page

    You can use a fuse with a higher voltage rating than the circuit is rated for but NOT less. The proper current rating is important but you already know and have that that.

    Your shack should be 120Vac. That has been the standard for many years now. You could look up the history of that if interested. Similarly, it is 240 Volts not 220 but many that were around back in that time still refer to it as 220 out of habit.

    Glass fuses are seemingly okay for 120 Volts but they usually suggest ceramic fuses for 240 Volt amplifiers.
    Last edited: Jul 14, 2018
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I don't think anyone makes 12A 120V cartridge fuses anymore; there's no need, since 240V fuses will work exactly as well and can be used in either 120 or 240V circuits.
  5. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I have seen people pick up 32 volt fuses by mistake. Not the same thing as 250v 3AG. :oops:
    W7UUU and AI3V like this.
  6. N7WR

    N7WR Premium Subscriber QRZ Page

    NAPA Auto Parts usually have them
  7. W1QJ

    W1QJ Ham Member QRZ Page

    Why SLO BLO?
  8. W7UUU

    W7UUU Principal Moderator Lifetime Member 133 Administrator Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    The manual calls for FAST blow fuses!!! Switching to slow-blow could potentially cause serious damage should they be called upon to "blow"

    Manual link: http://www.elecraft.com/manual/E740150 KPA500 Owner's man Rev D1.pdf


  9. W1QJ

    W1QJ Ham Member QRZ Page

    Dave, this is exactly why my above interrogation!! I’ve taught a few basic electronic classes and always advised my students to use fast blow fuses in sensitive radio type devices and save the slow blow fuses for motor starting type devices and the like. I’m glad you pointed this out with a link to the manual and should be a lesson that not all technicians know what they are talking about. Hence my questioning. I’d urge the OP to follow the advice of the manual.
  10. G3YRO

    G3YRO Ham Member QRZ Page

    Yes, you should definitely use normal quick-blow fuses.

    Even a quick-blow fuse takes a while to blow . . . so will usually withstand the high switch-on current pulse when turning on a high power amplifier.

    Slow-blow fuses usually have a small spring at one end of the fuse wire inside the glass . . . so they are easy to identify.

    Roger G3YRO

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