Blue burst around tubes, subsequent loss of power?

Discussion in 'Amps and Cabs [BG]' started by Bad Gas, Oct 23, 2004.

  1. Bad Gas

    Bad Gas Guest

    Sep 28, 2004
    I just got a Sunn 300T and was very excited to try it out last night. I have a 4 ohm capable Ampeg cab. I got all my settings the way I like them, plugged into my bass and just started playing on it trying to dial in a particular sound when I suddenly see a blue burst form around the tubes. I continue playing because I didn't notice the sound change until the amp started crapping out on me. Thats when the two red warning lights indicating a fuse problem (I believe?) came on directly behind the tubes. A moment later I realized what had most likely caused the problem. I didn't set the ohm switch to the right ohms before I started playing. It was set to 2 ohms. I have a 4 ohm Ampeg 8 by 10 cabinet. Shortly after the two red warning lights came on the amp just completely shorted out and isn't even getting power at all. Anyone know what I may have done?
  2. Bad Gas

    Bad Gas Guest

    Sep 28, 2004

    I removed four fuses and three of the four of them are completely fried... I hope that is the only problem.
  3. Passinwind

    Passinwind I Know Nothing Supporting Member

    Did it look like arcing outside of the tubes, near the bases? That would be a possible outcome from running high power into too high a load. If you have an ongoing problem, you may need to look into replacing the power tube sockets. Make sure you stay with the standard fuse values too, that could be a transformer saver.
  4. Hoping the problem is the fuses? Well if they all fry out like that then the problem aint the fuses. Either you got a short of some type, maybe a ground short or something is fried, like a cap. Probably something is fried because it continued to play.
  5. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    playing a tube amp into to high a load can increase the plate voltage, but usually you're safe up to 100%. Time to consult a tech. :meh:
  6. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    When you set the ohms switch to the correct position and replace the fuses (make sure they're the correct size and type, sorry to belabor the obvious), one of two things will happen. Either a) everything will work fine, or b) the fuses will fry again "right away", in which case it's definitely time to consult a tech. Tube amps are different from solid state, for tube amps a higher impedance can be "worse" for the amp. In a high power rig like a 300T, the output tranny is already operating right on the hairy edge of burnout, and impedance mismatches can cause increased heat and voltage spikes and all kinds of other bad things to happen, over and above the transformer's baseline stress level. So that's why they put lots of fuses in there, to protect the circuitry, and hopefully they did their job in this case. A blue flash may or may not mean anything, a blue glow is normal when tubes are operating, but if you're saying blue like lightning-bolt blue-and-white, then maybe arcing is a possibility. If you're talking about an electrical flash like that, then a visit to the tech is definitely in order. That's not the kind of thing you want happening on a regular basis. :D
  7. Oh man, I feel bad that you're having problems, Ben. Before I shipped it, I did try it at a 2 ohm that's why the switch was set at 2 ohms. I definetly didn't play it for long, maybe 10 min. I should have set it back, sorry about that. I hope the amp is an easy fix, honestly I never had any problems with it. Keep me updated.

  8. notanaggie

    notanaggie Guest

    Sep 30, 2003
    High load impedance leads to high spikes on the plates, and possible arcing in the tubes.

    The blue glow usually indicates a high voltage. A white glow can be arcing, which you might have seen just as it crapped.

    I'd figure the tubes are possibly fried, if arcing occurred. No guarantee on that.

    Do the fuses. (Tubes don't die as fast as transistors.) Then see what is up.
  9. Bad Gas

    Bad Gas Guest

    Sep 28, 2004
    A few more things:

    Okay, thanks for the information. I want to mention a couple of other things that I recall about this amp when it arrived at my doorstep. The exterior of the amp and all of its parts look virtually untouched. The tubes were tightly wrapped in bubble wrap and, after I removed it, I had to push them back down into their sockets because several of them came loose. I pushed them down as far as they would go and made sure they were secure.

    I want to reiterate that the burst of blue light appeared above the middle two tubes, around their tops, after less than ten minutes of turning on the power. Someone at another site mentioned a possible problem with the output transformer.

    Jason, it's probably just one of those things. Obviously, as you know, the amp appears very, very lightly used. I'm just perplexed about what might be causing this. I just hope it's an easy fix and that I don't have to dump a whole load of cash into getting it to work properly. My first plan of attack is to purchase the necessary fuses on Monday morning and giving it more try. If that's a fix and there are no more problems that surface, cool. If anymore funny stuff occurs, then I'm going to try to find a good tech. Is it possible the tubes are bad... I understand that you bought the amp brand new? It looks it, and if that's the case, I don't know why the tubes would be bad.

    What would cause a problem with the output transformer?
  10. That has nothing to do with anything. When the other posters refer to arcing, they're talking about internal arcing INSIDE the tubes themselves, not arcing across the tube pins.

    All you were seeing is residual ionization of leftover gasses inside the envelope that occurred because the tubes were suddenly seeing a MUCH higher operating voltage due to the impedance mismatch of your speaker. BTW, some makes of tubes have exposed grids that glow blue under NORMAL operation, but I don't think that's the case here.

    There WAS a problem with the output transformer. It "expected" a 2 ohm load and you presented it with a 4 ohm load. I'll link later in my reply as to why that is VERY BAD.

    It was the impedance mismatch. Here's why.

    Good plan. You'd be amazed at how many people are convinced that they can fix any problem with their amp themselves.

    Try the fuses first. The advice you've gotten so far in this thread is solid.

    the mismatched load
  11. Bad Gas

    Bad Gas Guest

    Sep 28, 2004
    I replaced all four fuses today before practice and voila! There were no problems throughout our two-hour practice. In addition, that head is the best sounding amp I've ever played through, hands down.
  12. tubster


    Feb 5, 2003
    Southwest Spain
    What a great example of why this is such a great place to hang out - great advice from players, culminating in Psycho's expert analysis.........and all leading to a happy ending!

    Glad it is back and working. I've got a Sunn too and I am gonna be matching those impedances!