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Blown fuse?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Ardis, Aug 17, 2005.

  1. Ardis

    Ardis Member

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    I'm sure this is a stupid question, but here goes: how do I know if a fuse is blown or not? What does a blown fuse look like?

    I'm getting no output from my Clark amp. All the tubes seem to be glowing properly, but I'm getting no signal whatsoever. Even the idling hum is absent. So could this be a sign of a blown fuse?

    Thanks, as always, for your input.
     
  2. John Phillips

    John Phillips Member

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    It sounds like a HT (B+) fuse could have blown. Does the amp have one? If not it's something more serious.

    (Although, have you checked that the speaker is connected properly?)

    If there is an HT fuse, have a look at the wire inside the glass. If it's melted, burned, or even slightly broken, the fuse is blown.

    Sometimes it's hard to tell - and sometimes a fuse which looks good can still be dead - but there's a way to to test them without any equipment other than another (working) amp and a guitar cord.

    Plug the cord into the amp and set it so you can hear the usual buzz when you touch the tip of the plug. Then hold the fuse between your fingers so you're only holding the metal cap at one end, and touch the other end to the tip of the plug. If you get the same buzz the fuse is OK.

    If it is an HT fuse which has blown, chances are a power tube has as well - that's the usual cause.
     
  3. Ardis

    Ardis Member

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    Wow. Thanks a million, John, for your informative response.

    By HT fuse, do you mean the fuse inside the chassis? This amp has two fuses--one attached to a spring-loaded plastic cap accessible from the control panel and the other in the inside of chassis. The one attached to the cap looks fine, but the one inside the chassis has no visible metal wire inside the glass at all; there are also a couple faint sooty black marks on the inside of the glass. This is probably a blown fuse, huh?

    Assuming that it is a bad fuse and that this is the problem here, should I look for a particular brand or type of fuse? Are there better ones than others?

    I can't thank you enough, John. I'll report back after I get a new fuse.
     
  4. John Phillips

    John Phillips Member

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    Yes, that sounds like an internal HT fuse which has blown. You should be able to read the value on it - it's usually marked on the end caps if there isn't a sticker on the amp next to it. It will be something like "T0.5A" or "F250mA", where T stands for Time-lag, also called Anti-Surge (AS) or Slo-Blo, and F stands for Fast-blow. Use the same value! (remembering that 0.5A = 500mA, etc.)

    Chances are the fuse has not just "failed" though - they sometimes do, but sooty marks are usually the sign that something has caused it to blow, most likely a blown power tube.

    Fuses are cheap, and unless you have other measuring equipment it's just as easy to trouble-shoot by wasting a few if you have to. Pull the power tubes (and the rectifier if the amp has one), and replace the fuse. Turn on (fully, including the standby if the amp has one, after it's warmed up). If the fuse blows again, you have a really serious problem and you need a tech! - chances are it won't though.

    If it doesn't, turn off, replace the rectifier if it was a tube one, and try again. If the fuse now blows you have either a bad rectifier - or maybe a shorted filter cap or something like that. Assuming it still doesn't blow, replace the power tubes one at a time, powering the amp up each time and seeing if the fuse blows. When it does, you've just inserted the faulty tube.

    It's possible that it still won't though... power tubes sometimes fail only partially or intermittently, and this can make it difficult to diagnose. But if you do suspect you have a bad tube, it's best to change the set anyway. Don't throw out the old ones, give them to a tech - he'll probably be able to identify the bad one and either give you the other(s) back as a spare or just use it for testing, if you don't feel like trusting it.

    Hope that helps!
     
  5. Ardis

    Ardis Member

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    It sure does help, John. Thanks again and again.

    Is it okay to power up the amp without the power tubes inserted? I assume that it is since that's exactly what you're suggestiing I do. I'm just being cautious because I know nothing about electronics.
     
  6. John Phillips

    John Phillips Member

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    Yes, it is. If it's a cathode-biased amp it may not be a good idea to run it for long with less than the full number though - I should have mentioned that.

    Strangely enough it can sometimes be unsafe to power an amp up without the preamp tubes in - the reason will probably sound like Greek to you :) - because without the voltage drop down the B+ chain caused by the preamp tube current draw, the voltage at the far end will rise to the full B+, which may be well over the filter cap ratings in that part of the circuit.

    If you don't know a lot about electronics, BE CAREFUL in there! There are potentially lethal voltages inside which in some amps persist for a long time after the power has been turned off - in the area of the HT fuse, especially.

    I learned all this stuff the hard way before the internet, BTW - I'm just trying to put a little back :). It's not rocket science!
     
  7. Ardis

    Ardis Member

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    John,

    So I bought a new fuse and inserted it and everything appears to be working fine now. I'm getting regular output and tone.

    I didn't realize this yesterday, but tracing back what I was doing with the amp when the fuse blew, I think I know what happened--especially after reading your explanation of what might happen if you power up without the preamp tubes. I had to take out preamp tubes for some reason and I accidentally powered up the amp without reinserting the preamp tubes. It was after this that the amp lost its signal. Given your explanation, it makes sense that the fuse would have blown, no? Is it possible I could've done other damage to the amp? Judging from its sound now, it seems to be operating okay. But could there be some silent damage I wouldn't know about?

    Thanks again, John. You rock!
     

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