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Tube glows red hot, blows fuse

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damagefactor

Member
2008-01-30 1:38 am
  • #1
    Okay, forgive my total and complete ignorance when it comes to electronics.

    I have a Peavey Classic 60 power amp, worked before I brought it in for service (new tubes).

    Got it back from the local tech, and one of the tubes started glowing red hot to an extreme- I turned everything off and got a RMA on the matched tube set.

    Put the new tube set in (Groove tubes, supposedly no bias needed for the same power rating). And immediately the same socket's tube began to glow like a red lightbulb, a strong hum came from the speaker, and then the fuse popped. All in about 5 seconds.

    I can't imagine I had two bad tube sets in a row, I also read that not all the socket's contacts may not be seated properly.

    I don't want to bring the unit back to the tech who serviced it, he either missed something or charged me for work he didn't do. Claimed he checked everything out before the retube and bias.

    Any ideas what it could be? Thanks for your help!
     
    Miles Prower

    Miles Prower

    Member
    2005-05-06 3:50 am
    USA
    www.dolphin-hsl.com
  • #3
    damagefactor said:
    I don't want to bring the unit back to the tech who serviced it, he either missed something or charged me for work he didn't do. Claimed he checked everything out before the retube and bias.
    Click to expand...

    Why not? He obviously did something wrong, and you are owed at least a real fix, or your money back. Let him get away with that, and he'll just do the same thing to someone else.

    "Groove tubes, supposedly no bias needed for the same power rating"

    I don't know about Groove Tubes, but this is nonsense. Replace the finals, and you will need to check the bias. Always. If that's their advice, then I'd shop for a new supplier.
     
    P

    PRNDL

    Member
    2006-10-17 3:39 pm
    www.naturdoctor.com
  • #4
    Unfortunately, it not uncommon for this to happen ... new tubes last a short time and then go bad. The tech may have been honest when he stated that the amp worked fine.

    The bad tube may have taken out a grid resistor or something else, which is now causing the new tube to burn up.

    Fixing it yourself involves taking the amp apart and checking the resistors for both power tubes, as well as checking the tube socket for proper contact.

    I'd recommend calling the amp tech and asking how much it would cost to bring it back in. If he's reputable, the repair should be free or the cost of parts (unless something was broken unrelated to his original repair.)

    Also, specify a better quality tube, such as EH.
    Most people agree that GT's don't live up to their advertised hype.
     
    HollowState

    HollowState

    Member
    2006-08-22 6:43 pm
    Taxland, New Jersey
  • #5
    damagefactor said:
    (Groove tubes, supposedly no bias needed for the same power rating). Any ideas what it could be? Thanks for your help!
    Click to expand...

    With one output tube burning up, you have a loss of negative control voltage on the tube's grid. This voltage is what's refered to as bias. All output tubes need it to some degree. The most likely cause is either a bad resistor that feeds the grid, or there is a very leaky or shorted coupling capacitor going to that tube.

    This is not a complicated repair. I would return it to the original tech and have it made right.

    Victor
     
    Hi_Q

    Hi_Q

    Member
    2007-10-21 3:39 pm
    Watford,Herts
  • #9
    Yes he should have soak tested it for a while after service, it's standard thing really, sort of confidence check that the item repaired won't bounce back! It may be just bad luck of course that a coupling capacitor chose the wrong time to go short circuit or the transport back moved a connection and is shorting something, like the cathode bias chain directly to ground. I would imagine that the tech would be happy to have a no-cost look at it again due to the fact that you have not been able to use the amp since he last repaired it. Don't run the amp too long like this as it could lead to output transformer failure and that will cost a lot of $$$.
     
    anatech

    anatech

    Administrator
    Paid Member
    2004-06-06 8:31 pm
    Georgetown, On
  • #12
    Hi damagefactor,
    As a repair shop owner, I am agreeing with the previous comments. Most good technicians would be concerned that you had a failure soon after repair and want to investigate this.

    Understand too, that these days, technicians are very wary of scams. They don't want to be ripped off either. With this in mind, approach him openly and ask nicely if he will have a look at it. His response will probably be that it might be chargeable. Nothing iffy about that. Trust is going two ways and he simply wants to not get stiffed either. Whether he is honest or not, you can't tell at this stage.

    He may charge for damage caused by a faulty new tube, it should be a minor charge. Most shops are at a half hour minimum, although some are 1 hour. If the technician doesn't charge you at all, that's even better! If you get a high estimate (make sure he estimates before going over a reasonable limit), get a second opinion unless this can be explained in a reasonable fashion with proof. If he comes across upset or excited, then he is either inexperienced or possibly dishonest. This is assuming you stay calm.

    If you get excited or abusive (demanding), most people would become very defensive. Remember that you want him to help you out. Most techs will give you the benefit of the doubt as long as they are not threatened. If you go on the attack, be prepared to pay what we sometimes call "asz hole" tax. Every industry has a variation on this. :devilr:

    -Chris
     
    anatech

    anatech

    Administrator
    Paid Member
    2004-06-06 8:31 pm
    Georgetown, On
  • #16
    Hi Trout,
    It varies in relation to how much of an idiot the shop thinks you are, and possibly by the value of what you drive in with. :devilr:

    The worst thing a good technician can do to you is very simple.

    They can refuse to deal with you as a customer.

    I've done this once or twice, and it is far more costly to the customer than anything else I can think of. The best part is that the bad customer doesn't know what exactly has been done to him. :devilr:

    Same thing with car mechanics or any other specialty.

    You know, none of this would be necessary if everyone simply dealt with each other honestly. A little respect goes a very long way.

    -Chris
     
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