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Help with amp blowing fuses?

Discussion in 'Glowing Bottle Tube Amp Forum' started by Asmith, Nov 28, 2017.

  1. Asmith

    Asmith Friend of Leo's Ad Free Member

    Age:
    21
    Nov 27, 2014
    Morley, England
    Relatively new to the tube amp experience and so far I've enjoyed it until my AD30r cut out.

    To break it down it's blowing 2 fuses verified using a DMM,

    The HT fuse (500mA) and the mains fuse (2A), they are the correct value as stated on the chassis and I'm not going to use other fuses for obvious reasons.

    When turned on I get some noise out of the guitar for a very short time (during the tube warm-up period until the fuses blow, I'm using slow blows). I've only swapped the fuses once to see if the amp would blow them again the second time.

    From what I can tell the heater fuse must be going first and then the mains instantly after.

    The amp is an Orange AD30r, it uses 3 ECC83s (12ax7s) (preamp, phase inverter, reverb) and 4 EL84s and a GZ34.

    Here's a schematic just in case,

    http://www.prowessamplifiers.com/schematics/images/OrangeAD30_1.pdf

    I'm really hoping that this just needs a tube swap and not a trip to a tech because this thing weighs a lot.
     

  2. Wyatt

    Wyatt Tele-Holic

    856
    Nov 3, 2004
    HT=high tension (the plate voltage feed), not heater

    A blown power tube is the most likely culprit. Pull all the tubes, replace the fuses, and turn on and see if the fuses blow. If a fuse blows with no tubes in, it's serious and time for a tech.

    If the fuses remains intact, then you turn off, insert he GZ34 and test again. Finally, turn off and insert the power tubes and test again.

    But note, if fuses keep blowing it's because there is something wrong and if it's a serious short, you are risking further damage.
     

  3. Asmith

    Asmith Friend of Leo's Ad Free Member

    Age:
    21
    Nov 27, 2014
    Morley, England
    You learn something new everyday thanks for correcting me, I'll give that a go and if it's tech time it's tech time. :rolleyes:
     

  4. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    You should be in good shape - that's almost always a bad rectifier or power tube.

    If it is a power tube there's no way for you to know which one. It's best to replace them as a matched set anyway.
     

  5. Asmith

    Asmith Friend of Leo's Ad Free Member

    Age:
    21
    Nov 27, 2014
    Morley, England
    Thanks, after this set back I'm thinking I need an amp with fewer valves, it's not cheap when it needs doing but that's the nature of the beast. I still a love this amp and having it out of action has made me realise how much I'd miss it.
     

  6. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    Sorry for a very late reply - had an extra strength flu shot knock me back into the stone age for 2 1/2 weeks...

    The number of tubes in an amp should *not* be a deterrent - at least in the long run.

    If you install good tubes the cost may (*not* "will") be higher up front - but you shouldn't have to replace good-quality tubes for years unless you constantly push the amp hard.

    I service mostly vintage Fenders, Marshalls, Ampegs, Vox, Silvertones, Magnatones and so on. Many of them come to me for a 3rd, or 4th or 6th filter cap replacement :D (the "grin" meaning *that* type of service means the owner is usually concerned with proper maintenance)...

    ...yet many still have some - even all - the original tubes. It's very common in (some) Fenders, Magnatones, (some) Ampegs and Silvertones where power tubes in particular are not hit with sky-high voltages and currents. Original "Fender" branded RCA and GE tubes are often found working just fine in blackface amps - even some browns and tweeds.

    Newer Fenders I work on, though (newer being 1990's amps and newer) either have replacement tubes - or usually need them. The Russian tubes normally installed at the factory I find - for the most part - to be decent sounding, but even "rugged" ones like the old wafer-base Sovtek 5881's don't seem to last long if pushed often.

    You don't need to spend $400 on a pair of power tubes, but if you educate yourself my reading plenty of information about vintage guitar amp *tubes* - not amps - (and not by reading guitar magazine reviews of new tubes) and shopping carefully you can find decent deals on matched, good-quality used tubes AND some NOS tubes. If you are careful about your sources (and understand what you're doing - which isn't all that hard) you can find good deals on US-made tubes that will last FAR longer than just about any new imported tubes.

    Time spent learning about vintage tubes can save you a bunch of money - and get you much better tone to boot.

    Hope that helps.
     
    Thin69 and BobbyZ like this.

  7. the fatch

    the fatch TDPRI Member

    Age:
    73
    14
    Dec 19, 2017
    LEEDS UK
    Following on from Wyatt's advice, test with no tubes, install the GZ34 and test again, then install the 12AX7s and test again. If you get this far and the fuses haven't blown, you've isolated the fault to the output stage. With your DMM measure the resistance between chassis ground and pin 3 of each of the output tube sockets (on top of the chassis, locate the gap where there appears to be a pin missing, and count counter-clockwise from there). If you don't read approximately 150 ohms at every pin 3 you should take it to a tech, but if you do it's almost certain that you have one or more bad EL84s. If it were my amp I'd risk installing a new set of EL84s. (The 150 ohms is specific to this amp, it's the value of the cathode bias resistors.)
     

  8. Asmith

    Asmith Friend of Leo's Ad Free Member

    Age:
    21
    Nov 27, 2014
    Morley, England
    thanks for the advice that gives me something to do between now and next payday. I am missing this amp a lot, going a while without has done some good. I was thinking about selling and maybe getting a fender amp (probably a blues junior III or a hot rod deluxe), I probably would have loved the fender but would have definitely regretted selling the orange.

    Cheers thanks for the Cathode resistor check tip.
     

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