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Question about all tube amps

Discussion in 'Amps and Cabs [BG]' started by hetsscaryguy, Nov 12, 2004.


  1. hetsscaryguy

    hetsscaryguy

    May 22, 2004
    Ireland
    ok i am once again trying to gather information so i dont make a major **** up when buying my amp next year. I have pretty much made up my mind 100% on the head i am buying and if this question is answered to my satisfaction that will be my cab decision made. The head in question is a fnder 300h all tube head, now my question is:
    Is it possible to connect an 8ohm and a 4ohm cab up to this head at the same time. That will givea load of 2.67 ohms and the head is rated to work at 2ohms, so will it work? and if so i would have to set the switch on the back at 2ohms right?

    Please help, i know its probably a stupid question to some of you but do you really want tosee such a lovely amp ruined because some idiot (me) connected cabs of bad resistance to it?
     
  2. Ralphdaddy

    Ralphdaddy Supporting Member

    Nov 6, 2003
    Chicago, Illinois
    Yeah, that'll work just fine man. Anything at or above 2 ohms will be fine with that amp.
     
  3. playmybass

    playmybass

    Mar 28, 2004
    Tucson, Az
    actually, I think you can only use the exact amount of ohms, so if you set it to 2, you have to use 2. if you set it to 4, you have to use 4, etc.
     
  4. hetsscaryguy

    hetsscaryguy

    May 22, 2004
    Ireland
    someone else please back one of these guys up, and discount the other. I looked at both of your profiles and not to be mean or anything playmybass but i think ralphdaddy mite have a bit more experience here and i think he might be right. Sorry that is i hope he is right.

    Actually now that i have sort of cleared that up, what would your opinion be of a 2x15 and a 1x15 rig, whats your opinion of 2x15 and a 2x12 rig, and finally, your opinion of a 2x15 and 4x10 rig. Which is best?
     
  5. I've also heard something similar to this. I'd wait for one of the tube gurus to pipe in.
     
  6. Tim__x

    Tim__x

    Aug 13, 2002
    Alberta, Canada
    It's true that you must match impedances with a tube amp (playmybass was right) but their usually fine with as much as a 100% mismatch. It's ok to run a amp at 2.67ohms using the 2 ohm tap
     
  7. SnoMan

    SnoMan Words Words Words Supporting Member

    Jan 27, 2001
    Charleston, WV
    http://www.talkbass.com/forum/showthread.php?t=144244


    from Ivanmikes post it sounds like it may actually be better to set the amp at 4 ohms and run at 2.67 instead of 2 ohms(since tubes prefer to be run on lower ohms instead of high)
    It should be safe since most sources say its ok to run tube into up to 100% difference.
     
  8. Nick man

    Nick man

    Apr 7, 2002
    Tampa Bay
    Since it's a tube amp, you are safer running it with the amp set at 4 ohms than 2. You want the same or LOWER impedance cab when you are talking tube amps.

    With solid state its the opposite.
     
  9. hetsscaryguy

    hetsscaryguy

    May 22, 2004
    Ireland
    so i set the switch to 4ohms then? Everone agreeing with that?
     
  10. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI
    Any given speaker cab has a relatively wide range of impedances across the frequency spectrum - if you measure the impedance with pure sine tones, you'll measure different values at different frequencies. The rated impedance is just a general average - then on top of that, they round it to a nearby whole number anyway. I'm pretty sure it's not that critical. I don't know if I'd want to push the average to half or double the amp's optimum, though!

    When it comes to specifics as to whether it's best to go to 2 or 4 I'm less sure. 2-Ohms is closer for sure - that reason alone leans me toward saying "two"...

    I think electronically-speaking, when it comes to running a tube amp up near max-output: pushing a lower than optimal impedance is harder on the power tubes (over-current), and running into a higher Z puts some voltage stress on the output transformer windings (over-voltage).

    Joe
     
  11. Nick man

    Nick man

    Apr 7, 2002
    Tampa Bay
    Check on TB, for a solid state amp, that would be true, but for a tube amp its harder on the amp to push a cab with a higher impedance.
     
  12. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI
    Yeah - I guess that makes sense. Besides, you'd rather have a bit-shorter tube life than to short the windings of your output transformer - that's by-far the most expensive single component in a tube amp!

    I s'pose you could always add a resistance to make the Z perfect: eight Ohms in parallel with the whole works would bring the 2.667 down to 2.000 (hey, that's it.. run three cabs: an 8, 8, and 4! That'd give you 2). As for the watts rating of the resistor... dang! I forgot the power formula... Anyone??

    Wait-a-sec..(!) When you mix impedances like that in parallel, doesn't one cab hog twice as much current, since there's the same voltage across both impedances?! Yes it would - it would! Your 4-Ohm cab will be hogging twice as much current than the eight. (Let's see... I-squared-R.. It's all coming back to me now): twice the current would mean four times the power - the four will be screamin', and the eight will be whisperin', right?? Can anyone do a sanity-check for me on this one?

    Joe
     
  13. Tim__x

    Tim__x

    Aug 13, 2002
    Alberta, Canada
    The four will get twice the current, same voltage, and twice the power. It will be 3db louder than the others neglecting effeciency, but it doesn't really matter.
     
  14. hetsscaryguy

    hetsscaryguy

    May 22, 2004
    Ireland
    right so in plain english, is it or is it not possible to run 8 and 4 ohm cabs together
     
  15. Nick man

    Nick man

    Apr 7, 2002
    Tampa Bay
    Yes, its not ideal, but if you do you should set the amp to 4 ohms.
     
  16. Ralphdaddy

    Ralphdaddy Supporting Member

    Nov 6, 2003
    Chicago, Illinois
    Having spoken with a rep from Yorkville in regards to the YBA200 he told me it was more than acceptable to run the amp set to 4 ohms, at a TWO ohm load. The amp performs perfectly well under these conditions. I may not know dick about any other tube amps but I know about my own and it runs perfectly at 2 ohms according to numerous sources including Yorkville.
     
  17. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Tube guru here! (been doin' tubes for about 40 years now)...

    My take is: the general rule with tube amps is that you can use anything between half and double the rated impedance, and still be safe from an electrical standpoint.

    So, if you have a 2.67 ohm load, and you set your amp to the 2 ohm position, everything will be just peachy. :bassist:

    The "at or above" rule only applies to solid state amps. For tube amps, a "too low" impedance is better than "too high", especially if you're going outside the safety range mentioned above. However the difference between 2 and 2.67 isn't enough to matter, from an electrical standpoint, and it shouldn't noticeably affect your overall volume either (the multiple speaker cabs should give you a pretty good boost in stage volume, but that's due to the amount of air the speakers are pushing, and it's not due to any electrical differences in the impedance loading).

    There's a fine point (which probably doesn't apply in this specific case), which is that older tube amps tend to be more robust than newer tube amps, when it comes to impedance mismatches. This is because the newer tube amps tend to be built with "cost consciousness", meaning in practice that in some cases, the output transformer is already spec'd at or near the point of self-destruction (in other words, sometimes there isn't much headroom or wiggle room in the impedance area). Impedance mismatches in tube amps tend to create some funky behavior in the output transformers, so if your trannie is good quality, it won't be a problem, but if your trannie is one of those cheap Chinese jobs, it might tend to overheat a little earlier than a good quality trannie will. But I don't think that's an issue with the 300h, that's a good amp and it'll easily withstand a 25% impedance mismatch.

    Edit: oh yeah, reading the earlier posts like Nick man's, I forgot to mention that according to the above rule, the 4 ohm setting on your amp would also be acceptable with a 2.67 ohm load. Either the 2 ohm or the 4 ohm setting should work fine.
     
  18. iualum

    iualum

    Apr 9, 2004
    60453
    Yep.

    And among the cab choices you listed, my fav would be the 215 & 410.
     
  19. notanaggie

    notanaggie Guest

    Sep 30, 2003
    I'll throw in a ringer here....

    It depends on the amp. I don't remember the Fender, but....usually the more power the amp has, the closer you want to match things.
    The harder you play, the close you want to match.
    The more you will clip it, the closer you want to match.

    I have run and seen run 60W amps like Mesa's at double or half the rated, with no problems.

    But, with high power stuff, the ratings are probably being pushed, AND the consequences of problems are a lot worse.

    The high impedance tends to increase internal voltages. New tubes are NOT as good as older tubes and may not be properly tested and flashed off to prevent arcs. Arcs will destroy tubes, and maybe other parts too.

    If teh Fender 300 is 300 W, I'd be a lot more cautious with it than I would with a smaller amp.

    I would go towards 2.7 on the 4 ohm tap, if I had to pick one and use it that way. I'd rather match 4 to 4 though.
     
  20. hetsscaryguy

    hetsscaryguy

    May 22, 2004
    Ireland
    Thanks very much for that, hopefully i will soon be joining the rest of you all tube head players. I cant wait.



    BTW, any more opinions on those speaker combinations?

    And now taht im on the subject, i looked at the specs on the eminence speakers that avatar are using and i cant help but notice that the frequency response is very poor in comparison to say an ampeg or eden. The 15s frequency response is only 2.5 or somethin as opposed to the ampegs 18 or edens 20. Does this make a gig difference or would it still be a good cab with plenty of highs?