Crap, I think I broke my Tech Soundsystems G 308 combo

Discussion in 'Amps and Cabs [BG]' started by phxlbrmpf, Mar 13, 2005.

  1. phxlbrmpf


    Dec 27, 2002
    It happened during rehearsal -- all I'm getting out of it now is a farty, distorted noise with very little sustain, it happens with both of my basses and cables, so I think I can rule out empty batteries or broken cables. Does anyone know what could be wrong, exactly what I broke and how to fix it? I don't think it's the speaker, it definitely sounded different when I messed up my old 50 watts practice amp's speaker.

    I really loved that amp, it had a good, clean sound with nice highs and lows. It only has a 12 inch speaker, but I hooked it up to a no-name 2x15 cab for a serious low end boost. It's a solid state amp, by the way, and I think it should be about seven years old by now. Any advice? Lots of thanks in advance.
  2. Were you running both the combo's 12 inch speaker AND the 15 inch speaker at the same time?

    If so, the combined speaker impdeance may have been too low for the amp to handle, and you fried your amp :(
  3. phxlbrmpf


    Dec 27, 2002
    Crap. I usually unplugged the 12 inch speaker when I used the 15 inch cab, but I think on the last few rehearsals, I may just have forgotten to do it.

    Here's another detail that I thought was irrelevant at first:
    I own an Akai Unibass, and last Thursday we decided to try out what it would sound like with the effects signal run through our guitarist's amp. The Unibass has two seperate output jacks for highs and lows that allow you to do this. It didn't sound too great, but we played a bunch of songs with this setup.

    I don't know much about speaker impedance, but could it be possible that we fried my amp this way? After all, this means I had two extra 15 inch speakers and two extra 12 inch speakers connected to my amp.
  4. I doubt the effects pedal did much harm, other than effectively diverting too much bass EQ into the combo amp.

    But here's how speaker impedances work. You have one speaker that's 8 ohms, and you add another speaker that's 8 ohms. The resulting impedance is now 4 ohms. The speakers will now draw more current...if the amp cannot supply the current that the speakers are asking for, the amp's output transistors can cook. Giving lots of bass EQ will ask the amp to provide even more power....not good!! Sorry to bear bad news...
  5. phxlbrmpf


    Dec 27, 2002
    Thanks a lot for the explanation, nashvillebill. Bah, and there I was, saving cash for a trip to the US this summer ...

    On the back of my amp it says "200 watts, min. 4 ohms load. Would you recommend me to do the "smallish combo + extra 2x15 inch cab" thing again? The combo was pretty light and I only used the cab when I needed the extra low end. I really don't want to lose another amp like this.
  6. Well, if your amp was rated into 4 ohms (which it says it is) and if both the internal speaker and the external speaker were 8 ohms, then you should have been OK...

    First, do you have the pedal still plugged in? Unplug it and try it without the pedal, if so.

    Otherwise, maybe it is the internal 12 inch speaker has started to go...
  7. phxlbrmpf


    Dec 27, 2002
    Nope, I don't have the pedal plugged in right now. By the way, when you fry a transistor, doesn't it usually start to smell? I didn't notice anything and neither did my bandmates.

    I don't really know the specs of my 2x15 cab, it's in our rehearsal room right now. I don't have to work tomorrow, so I can go check on them tomorrow morning.

    By the way, I'm pretty sure the distorted sound was also audible through the cab. My bandmates and I thought my preamp battery was empty.
  8. Transistors may not necessarily smell burnt when they go...

    I just noticed the second cab was a 2x15. That cab may (probably) have been a 4 ohm cab, so when you combine a 4 ohm cab and an 8 ohm internal speaker, the resulting load is 2.67 ohms....again, too low for the amp to handle....

    Unfortunately, I can't give you much more to go on. Diagnosing amp problems in person can be tricky enough, doing it long distance via a forum isn't the best way.

    For your info: to absolutely tell if the amp is fried, my approach here would be to hook it up to a dummy load (disconnecting the internal speaker) with the output parallelled to the oscilloscope, then drive the amp with a sine wave from the signal generator. Observing the amp's output on the scope will immediately tell whether the amp is OK or not.

    If you chose to have the amp repaired, or maybe at least get an estimate on it, take it to a qualified amp repair place, NOT your local Guitar Center or Sam Ash. (Look in the phone book) Good luck!!
  9. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    I've never seen a 2x15 cab that was rated at 8 ohms. Odds are it was a 4 ohm cab, and that coupled with the internal speaker made it well below a 4 ohm load, and that is why he fried his amp.

    Kids, never EVER plug cabs together without checking the impedances first!!!
  10. phxlbrmpf


    Dec 27, 2002
    Crap. And you guys really don't think we broke the amp when we connected my guitarist's 2x12 inch combo to it via the Unibass? Just asking because it's always feels better if you can blame someone else because it was my guitarist's idea to do it. :p

    By the way, could someone recommend me a combo amp that does roughly the same thing as my old one for roughly 600 Euros/USD? I.e., 200 watts, 4 Ohms, decent EQ, tweeter, no 15 inch speaker if possible. Is the GK backline series any good? I really have no need for a huge amp right now, my rock band needs an extra guitar player and it doesn't look like we'll find one any time soon so we won't be able to play gigs for quite some time.
  11. phxlbrmpf


    Dec 27, 2002
  12. phxlbrmpf


    Dec 27, 2002
  13. Passinwind

    Passinwind I Know Nothing Supporting Member

    As far as troubleshooting the problem yourself, you can substitute known good components methodically, until you isolate the general area of the problem. Unplug the 12 inch speaker, plug in one that you know is not broken. Don't trust that other cab until you hear for yourself that it's OK. Alternately, plug that 12 into a different amp. If there's a power amp input, plug a working preamp into it. Use a CD player or something similar if that's all you have on hand for a signal source that you can trust. Try your EFX out or preamp out into another power amp. Use a different bass, and different cables. After you've done all that, let us know what you've found out.

    Bill's mention of a 'scope is right on, but you can often at least get an idea which part of the signal chain is creating the problem without using scary technical tools. :cool:
  14. Bassliner


    Mar 15, 2005
    Most probably not ;) I do the same except I take the 2nd line from my Line 6 DL4 pedal. My best guess is you messed up the ohms, and your amp needed to provide power below the 4 Ohm limit.
    It's a bit like this:
    8+8=4 ohm, 4+4=2 ohm
    The formula for total impedance: X= amount of speakers, Y=impedance (ohm rating) Y divided by X = total impedance. So let's say you had the internal speaker which was 8 ohm, and the 2x15 was 4 ohm. Think of the 4 ohm as two 8 ohm speakers:
    So X = three speakers. Y = 8 Ohm.
    8 divided by 3 = 2.67 Ohm which is not good if the minimum is 4...

    There was a nice article in Bassplayer mag. a couple of years ago. Maybe they should always include 'gear basics' somewhere in the magazine... :eyebrow:
  15. phxlbrmpf


    Dec 27, 2002
    Gah. Thanks again, guys. Like I said, I'm pretty bad with amps, guess you just gotta learn some things the hard way.