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A question on ohms

Discussion in 'Amps and Cabs [BG]' started by Mesa_Boogie_dub, Mar 19, 2002.


  1. Mesa_Boogie_dub

    Mesa_Boogie_dub

    Mar 18, 2002
    canada
    Hi,in the new month i will order a new bass amp(mesa boogie 400+),and the salesman told me i need to know how much ohms were my cabs.One of them i know its 4 ohms but the other one i dont know.Its a peavy BW 1978 with a 15" in it,and i think its 120watts.If anyone know some info on that cab please help me.thanks.

    Mart
     
  2. Your 1x15 is most likely 8 ohms. Most amps are designed with a 4 ohm load and no lower. But some amps can deal with as low as a 2 ohm load. I'm sure the Boogie could deal with a lower impedance load, but I wouldn't go there. As for your cabs, even if you did connect both cabs up to it, it wouldn't be right because less power would go to the 8 ohm cab. You should have a balanced load with either two 8 ohm cabs or one 4 ohm cab. My suggestion, get another power amp to run your 15, and while you're at it, add another 8 ohm cab to run with it. Then you'll be stylin'!

    Brian
     
  3. Mesa_Boogie_dub

    Mesa_Boogie_dub

    Mar 18, 2002
    canada
    Thanx a lot man...now that i know its most probabably a 8 ohm cab i wont plug it in my new head,i heard its hard on the tubes if it isnt well balanced.For now ill be putting all my cash in that mesa head,maybe in a year ill be looking at a 2x15 mesa cab who knows =)
     
  4. Steven Green

    Steven Green

    Jul 25, 2001
    Pacific NW
    The Mesa 400+ can handle an 8, 4 or 2 ohm load. So it should be fine to run the 4 & 8 together (2.6 ohms). That will be damn loud! Anyone have a different point of view?
     
  5. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    Just that the 400+ is actually a 300W amp, not 400 as the name suggests.
     
  6. Acepiloto

    Acepiloto

    Aug 25, 2000
    I thought that the 400+ was an all tube amp. It may be ok for a little while, but I wouldn't use an 8 ohm and a 4 ohm cab together (2.67 ohms) on that head, the irregular impedance could damage the power tubes (or the power transformer, I can't remember).
    The 400+ with the 2x15 would sound killer:D .

    Too bad I can't afford it.:p
     
  7. Mesa_Boogie_dub

    Mesa_Boogie_dub

    Mar 18, 2002
    canada
    The mesa boogie 400+ amp is an all tube amp.I have also heard it has 500watts of power...anyways guys im new in here,and one thing ive never understood well its ohms,what are they all about???
    by the way you can check out my reggea/ska band at www.fleebaonline.com

    mart
     
  8. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    AFAIK, there is really no such thing as "irregular impedance". Ohms are just arbitrary units we apply to the physical property of impedance. They're useful for purposes of measurement and comparison, but they have no true physical basis. (In the same way, a meter is an arbitrary unit--it doesn't exist in nature, it's a unit humans developed to measure a physical property.)

    It's not as if 4 ohms is somehow normal and nonstressful, and, say, 5.3 is deviant and stress-inducing. Besides, in use, speakers don't present a constant load anyway--the actual impedance wanders around over a range. A speaker rated at 4 ohms isn't exactly 4 ohms at all frequencies at all volumes in all applications.

    An impedance that's not an exact multiple of 2 or 4 won't, *in itself*, hurt the amp or any part of it.
    There are, however, two ways in which a 2.67 ohm load could *possibly* be some kind of problem. (1) If your amp is not rated for loads below 4 ohms, 2.67 could damage it--but it's not because it's "irregular," it's because it's plain too low. (2) When you use an 8 ohm cab with a 4 ohm cab, more of the current will flow to the 4 ohm cab. This may or may not be a problem.
     
  9. Mesa_Boogie_dub

    Mesa_Boogie_dub

    Mar 18, 2002
    canada
    Ok i understand now...so if i plug a 4 ohms 350 watt cabinet with an 8 ohms 130 watts...more power will go to the 4 ohms witch in my case can take more watts...since the mesa boogie 400+ can handle 2 ohms it wont have any problem with 2.67 ohms.So in a month ill have that bitchin amp ordered and everything will be fine :D
     
  10. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    If I were you, though, I'd ditch the 15 and get a more rugged one, preferably 4 ohms. If only because that Mesa amp can easily be made to deliver more than its rated power if you start really winding it up.
     
  11. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    Mesa states "500W of transient peak power", not RMS.

    If you look at other amps with 6L6 tubes, you get roughly 25W power per tube. The 400+ has 12 6L6s, 12 x 25 = 300.
     
  12. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    You're right, of course. I still wouldn't go with that speaker, though!;) Personally, I think it might be a bit counterproductive to spend a bucket of money on an amp and then kind of lowball on the speakers. Besides, suppose he gets in a situation where he has to push the amp kind of hard, and it starts to go into clipping?

    I'm not saying this absolutely won't work, just that it's something I'd give some hard thought to.
     
  13. Mesa_Boogie_dub

    Mesa_Boogie_dub

    Mar 18, 2002
    canada
    hehe youre roght...but im not rich tought...ill start wth a good amp(400+),and ill be looking forward to a new cab in maybe a year,its easy to dream about equipement but getting it is another story :)
     
  14. Even 25 watts per 6L6 is probably optimistic. 15-20 is more typical from what I've seen. The last 400+ I saw on a wattmeter put out 243 wats before the output started getting non-linear. Just another case of inflating specs to sell amps. Does it matter? Not really, the 400+ is still a great amp. Even at about 250W, has anyone ever heard of one not being loud enough? I certainly haven't. Therefore, if it sounds good and is loud enough, it doesn't really matter what a wattmeter says. :D My $0.02....

    Anyway, the 400+ has transformer taps for 8, 4 or 2 ohms. Ideally the nominal impedence of the cab should be exactly one of those values. Unlike Solid State amps, tube amps are set up to deal with specific impedences and not necessarily any impedence above a minimum. However, I wouldn't worry about a 2.67 Ohm load on either the 2 or 4 Ohm settings. That shouldn't be a problem. I'd check with Mesa and see what they say.