HELP! My amp is down!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Paul86, Jun 11, 2006.

  1. Paul86

    Paul86 Member

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    Hello everyone,
    I hope you can cure me of my depression. My Alessandro Bloodhound is down. I took it to a local tech who told me the PT is blown, but I have my doubts. There was a burning smell at the time, but I looked inside and nothing looks burnt. I tried replacing the fuse, but they blow as soon as I turn the amp on. What advice would you give me?
    P.S. I like driving the power section hard, and I've been using a Marshall Power Brake. Is the PB to blame here?
     
  2. John Phillips

    John Phillips Member

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    Do you mean it blows immediately on switching the main power on, or when you turn the standby on?

    Is it a tube-rectifier amp? If so, pull the rectifier tube and power it up. If the fuse still blows, it is almost certainly a PT issue. (It could also be a filament short, but they're pretty rare.)

    If not, put the rectifier back in and pull the power tubes. If the fuse now doesn't blow you most likely have a shorted power tube; if it does it could be the rectifier, the OT, or a filter cap. Try replacing the rectifier.

    The Powerbrake by itself is unlikely to be the direct cause unless it's faulty. But running an amp really hard with an attenuator is quite stressful for it in any case - depending on the attenuator, maybe more so than just cranking the amp up full at high volume through speakers. The PB is quite good in that it duplicates the impedance of a real speaker quite well, and I've used one for proof-testing amps I've worked on for many years without any trouble (except for a Marshall DSL50, which doesn't really count ;)). I do really thrash the amps through it too - dime everything, but usually only for a few minutes.
     
  3. Paul86

    Paul86 Member

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    Hi John,
    It blows when I switch main power on. No, it does not have tube rectifier.
    You said pull the power tubes. Sorry if this sounds dumb, it is a genuine question, won't I hurt the amp this way? And, if pull the tubes and the fuse doesn't blow, I'll just have to replace tubes (I have my fingers crossed)?
     
  4. John Phillips

    John Phillips Member

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    If it blows immediately on power-up and the amp does not have a tube rectifier, the most likely cause is a shorted rectifier (solid-state) diode. This is far more common than a blown PT, but it's still a bench job since they're inside. The good news is that if it's this, it should be cheap to fix - just labor and about a dollar for the parts at most.

    It's unlikely to be a tube issue, unless one has a filament short, which is rare, although not impossible. A 'normally' shorted power tube won't blow the fuse until you flip the standby to play.

    It is safe to run the amp without the power tubes in almost all cases - there are only one or two I know of where it's likely to be a risk, and they're old and rare. It's not always safe to run it without the preamp tubes though - which may sound surprising, but it's because the preamp tubes are part of a voltage supply chain which gets lower as it goes down, and without the tubes the voltage at the far end may rise to above the filter cap rating. The power tubes are at the start of the chain, and although pulling them does make the voltage rise as well, it's not by as much and the caps have higher ratings there.
     
  5. Paul86

    Paul86 Member

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    Hi John,
    I tried pulling the output tubes and the same thing happened. I took it to a local tech and he told me the PT is blown, but I have my doubts. This is the same guy who found a "short circuit" in my amp that magically disappeared when he changed tubes! Where are the rectifiers, so I can show them to him and have them tested?
    Thank you
     
  6. 1guitarslinger

    1guitarslinger Member

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    Not to step on any toes, or muddy the waters...but depending on the circuit, which I am not familiar with, couldn't it also be a shorted filter cap on the hot side of the standby switch? Lifting the 2 rectifier diodes that connect to the PT would rule in or out the rectifier and those filter caps. Of course your tech may already have done this and other troubleshooting to lead him to the PT.

    I'm going to risk being blunt here. Please don't take this the wrong way, as I am not putting you down, and I think it is great for us all to learn everything we can, everywhere we can.

    If your tech does not know what a rectifier diode is, then you need to find a new tech. If he does know what it is, which I would imagine he does, then he probably won't appreciate "back seat troubleshooting" by a customer who has been looking for message board fixes from people who have not actually seen the amp, and do not know what the tech has, or has not done to troubleshoot it. You mentioned a burning smell, but nothing looked burnt. Diodes are easy to test, easy and cheap to replace during troubleshooting, and easy to see if "crispy". This leads me to suspect that the shorted PT could be correct.

    I suggest letting the tech follow through and fix the amp, or take it to another tech if you are not confident in him, or fix it yourself with help from the board. Doing 2 or more of those suggestions at the same time will just make a simple repair turn into a mess.

    Paul
     
  7. John Phillips

    John Phillips Member

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    Yes, but a rectifier diode is many times more likely... I'd say at least twenty to one in my experience, possibly more.

    I've almost never seen one look even slightly unusual when shorted.

    I'm not saying what the fault definitely is - it's not that conclusive on this evidence - just what in my experience is (by far) the most likely cause, which also luckily happens to be far less serious than the first possiblility mentioned.

    But it could certainly still be the PT.


    Paul86 - I'm not certain where the rectifiers will be, I'm not at all familiar with that amp. Probably on the main board, but not necessarily. But if you're uncertain about this tech's abilities, I agree with the other Paul... it may not be a great idea to either trust him to do it right or even to suggest where to look, as that may be construed as interference. I also to be honest don't trust techs with limited diagnostic skills to do a good job in terms of workmanship. The two things aren't necessarily related, and I've come across many the other way round (great technical knowledge and experience, awful workmanship) - but it still doesn't bode well.

    If you're unsure, I would contact Alessandro before you go any further. If it does need a new PT you're best to anyway.
     
  8. mr coffee

    mr coffee Gold Supporting Member

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    (or has) lifted the red leads to the diodes or the diode output to the cap to verify that
     
  9. 1guitarslinger

    1guitarslinger Member

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    I agree completely. If the tech who has looked at the amp has even a fraction of a clue, he would have gone straight to those diodes. And for all we know, he did.

    BTW, have a single ended amp right here that blew up when the customer had it cranked. The power tube blew and both the OT and PT fried. The rectifier diodes are perfect. You don't know how much I wish it were different as replacing diodes is much easier than tracking down a PT that will physically fit the thing with little modification. In addition, the customer is whining about the potential cost. So I really wish it just needed diodes so it would already be out of my hair!

    My point is that shtuff happens with electronics, regardless of probabilities.
     
  10. John Phillips

    John Phillips Member

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    ...that sinking feeling when you finally can't avoid the conclusion that something really expensive and a total pain to change has died, when at first it looked like being a 20-cent part in easy reach...

    :)
     
  11. 1guitarslinger

    1guitarslinger Member

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    LOL...exactly!

    I can't speak for everyone, but I think most techs want the fix to be fast and easy, only requiring parts that are on hand, at least as much as the customer does.

    You know John, I was thinking. I have seen blown up diodes, but only in lightening struck industrial equipment. Not in guitar amps...yet anyway!
     
  12. John Phillips

    John Phillips Member

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    I do actually like the challenge of doing a really good job on a really nasty repair, and the feeling of quiet satisfaction when it's done, even when you know that the effort you put in won't be seen by anyone else... but often (partly because the customer doesn't quite see it the same way ;)) it's difficult to really charge the true cost of the job, and it's more financially rewarding to just stick to the simple stuff.

    Your SE-amp job sounds like a "good" one. I hate seeing bodged replacement transformers with extra holes drilled or chassis cut-outs enlarged... I'm not an 'originality purist', I just like to see the work done right. I think you know what I mean :).
     
  13. 1guitarslinger

    1guitarslinger Member

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    I agree. A troubleshooting challenge is great. Searching out impossible to find parts, or figuring out the repair is going to cost an amount that causes the customer to balk etc. is not so great.
     
  14. Paul86

    Paul86 Member

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    Hi everyone,
    Thank you all for your comments/suggestions. I want to thank this forum for 1) being a place where you can vent things (this helped me clear my head), 2) being a place where you can find sensible, honest advice (thank you 1guitarslinger). I decided to just send the amp back to George to have it serviced.
    I'll report back soon (I hope).
    Thank you all again.
     

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