Orange AD200 MKIII - high buzz coming out but nothing else

Discussion in 'Amps and Cabs [BG]' started by p-keisari, Aug 17, 2023.

  1. p-keisari


    Oct 6, 2022
    Hi! I was playing with my friends Orange AD200 MKIII bass head the other day when all of a sudden it went mute. Just like that. Power stayed on but no sound was coming out of the cab. I tried to smell out if there would have been an electrical burn, but I couldn't smell anything too strong. When I looked behind the head, there were these two LEDs where it read "Failure fuses" and the other one was lit.

    I took the other one out that was under the lit LED and it was burned for sure. I took the burned fuse, went to an electricity shop and bought a similar one. When I got back to replace the fuse, I noticed that the fuses that were there in the head in the first place were T (slow) fuses, even though the amp says it needs F (fast) fuses.

    The only fuse I had at that point was a T fuse so I put it in anyway. The failure light went off, but there were still no sound coming out from the cab except this kinda high pitched buzz. The buzz gets louder as I raise the volume, so at least the amp responds to something.

    Any ideas that could have gone wrong and what could be the issue will be very much appreciated. Thank you!
  2. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    I can think of "about" 167 possible causes, all of which require knowledge and test equipment to correctly identify.
    AudioTaper, Peter Torning and Wasnex like this.
  3. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    The tube fuses are designed to protect the amp in the event of a power tube failure. They will allow operation on two power tubes until the amp can be serviced. If one blows the best course of action is to bring the amp for service at your earliest convenience, rather than continuing to play, but the idea is you can make it through a gig on half the power tubes if needed.

    If you’ve had a failure of a power tube which damaged the tube fuse and have no signal I would also check the two HT fuses. If one or both of these is damaged it may explain why you have no signal.

    Either way, same answer, have the amp checked by a tech.

    Sorry you said only getting a buzz, not silent output. Same answer still, blowing fuses is a sign of a serious issue. Have it checked by a pro.
  4. Wasnex


    Dec 25, 2011
    IMHO, Turn the amp off and take it to a qualified technician.


    The output tubes are fused in pairs. If one of the Failure Fuses blows, it normally means one of the output tubes has failed or surrounding circuitry has a problem. If one tube fails, normally the associated fuse should blow, but the amp should continue working.

    This is why the tubes are fused in pairs: Two tubes are on the push side of the output transformer and two tubes are on the pull side. Normally the current on each side of the transformer should be balanced, so if one tube fails it turns another tube off to keep the current balance in the output transformer. The idea is so the amp can limp through the gig at reduced power and the remaining pair of tubes.

    I have a modest background in electronics and I am fairly comfortable working on basic tube amps. I have some basic tools, including a variac, an inline bias-meter, multi-meter, and an oscilloscope. I would open the amp, pull the tubes, do a visual inspection for obviously burned parts, bring the amp up on a variac, and check all of the operating voltages.

    Set aside the tubes associated with the burned fuse. If the operating voltages check out, I would install a pair of the bias meter bases in the amp with the two assumed good tubes...taking care to install one tube on each side of the output transformer.

    Then I would bring the amp up on the variac and monitor the bias meter to see how the tubes are biasing and adjust as necessary to keep tube current in a safe range.. If the amp is still making noise, I would trouble shoot where the hum is coming from. If the hum is gone, the tubes hold bias, and the amp operates as normal, I would install a fresh matched set of tubes and set the bias. You normally let the tubes run for awhile and recheck the bias to make sure they are stable.

    The fact that the amp is making noise suggest some collateral damage may have occurred. However, I believe it's possible that one of the output tubes has developed a partial short that is stressing the power supply and injecting noise into the preamp. So if you are lucky, you may just need a fresh set of output tubes....this sort of fault can cause collateral damage so turn the amp off.

    There are lots of other possibilities as well. For example it's possible the output tubes are/were fine and the actual fault caused them to experience a momentary surge that blew the fuse. The output tubes may have been damaged by the event, and it would be a good idea to check them in a tube tester.
    Peter Torning likes this.
  5. Wasnex


    Dec 25, 2011
    FYI the Failure Fuses only disconnect the cathodes of a pair of tubes. All of the other working voltages are still applied to the tubes: 1. heater voltage, 2. grid voltage, 3. screen voltage, and 4. plate voltage. So if a tube has an internal short, it can still cause collateral damage even after the Failure Fuse has opened up.
    Peter Torning likes this.
  6. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    I do not recommend the OP attempt this, unless they are suitably qualified AND understand proper safety procedures.
    Peter Torning, Wasnex and kbakerde like this.
  7. Wasnex


    Dec 25, 2011

    I agree 100%.
    Peter Torning and agedhorse like this.
  8. Milo's

    Milo's Supporting Member

    Jan 23, 2008
    New Hampshire
    This is the only answer. Professional service can save your more than your gear, it can save your life. Those amps can store up some seriously scary charges. Play it safe and let a pro diagnose it. Good luck man, killer amp and it deserves to be well taken care of.
    agedhorse and Wasnex like this.
  9. mmon77

    mmon77 Supporting Member

    Jul 9, 2008
    As has been said, take it to a pro. Especially since the amp doesn't belong to the OP, and is his friend's amp. You don't want to risk making it any worse than it already is!
    agedhorse likes this.