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  • Heaters blow fuse

    So I have built this amp: http://jussii.tk/tmp/6v6.JPG
    Power supply is from "plexi 6v6" by m. huss

    Currently I don't have the internal PT secondary fuses installed at all because I wan't to test only heaters first.

    If I pull out all tubes and start the amp the heater voltage is 6.3V and led is glowing. If I install tubes and start the amp: led flashes and main fuse (1A) blows. I don't understand what could be the problem.

    From this picture you can see how my heaters are wired and I've checked they are the same in the build: http://jussii.tk/tmp/heaters.png

    Any ideas?

  • #2
    Could be a bad tube or bad socket causing a short when a tube is inserted. Try inserting tubes one at a time. You can do this with the power on. When you find a tube that pops the fuse, either that's a bad tube or a bad socket, or possibly just the sum of the currents got big enough. So mark that socket, put in a new fuse and try another socket/tube. If more than one tube/socket location pops the fuse, it will be harder to debug.

    While you're at it, look up "light bulb limiter" to save you some time and fuses.
    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    • #3
      Are you using the same power transformer as the one in the huss drawing? If not, what current is your 6.3V winding rated for?
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


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      • #4
        Thanks, R.G. I'll try that tomorrow.

        g-one: Nope, I use this transformer: https://edcorusa.com/p/608/xpwr015
        It's rated for 3.5 A

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        • #5
          Were the high voltage secondary fuses installed when the line fuse blew? If the center pin on a 6V6 power tube is missing, it's possible to insert the tube so that the heater connects the plate (pin 3) to the cathode (pin 8). Since this is a new build, it would be possible to mis-wire the socket in such a way as to cause a short when the tube is installed. Measure the resistance of point A to ground, then install the tubes. Did the resistance change? The heater of a 6V6 will measure about 7 ohms when it's cold, a 12AX7 about 40 ohms.
          WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
          REMEMBER: Everybody knows that smokin' ain't allowed in school !

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          • #6
            Thanks to everyone. The problem was a faulty JJ 12AX7 (new).

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