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As high ohm number as possible?

Discussion in 'Amps and Cabs [BG]' started by jock, May 6, 2001.


  1. jock

    jock

    Jun 7, 2000
    Stockholm, Sweden
    I´m going to buy an old Ampeg v4b. It´s rated at 100W@ 2, 4 or 8 ohms. I read somewhere that it´s better to run the amp at 8 or 4 ohms than 2 ohms. Is that true?
    So, will it tare more on the amp if I run two 4 ohm cabs (total 2ohms)?
    Will I have to change tubes more often?
    I´m a bit clueless here so please help me out.

    /Jocke
     
  2. Yes you can run it at 2 ohms if it says you can. However running your amp at lower ohms produces more heat, since your head will be working harder, but it does put out more power - you never want to go below the min impedance of the head. (this should be on your head or in your manual)

    for instance my head is min 2 ohms, but I personally will never go that low, I like to stick on no less than 4 ohms.

    however I dont understand how it could be 100 w at 2,4 and 8 ohms????
     
  3. Reid

    Reid

    Aug 25, 2000
    Oakville, Ontario
    it's tube, not ss
     
  4. embellisher

    embellisher Holy Ghost filled Bass Player Staff Member Supporting Member

    Exactly.

    rexspangle, tube amps use a special matching transformer so that they can supply their full output power at 2, 4, or 8 ohms.

    jock, running the amp with a lower load does produce more heat.

    I personally would not run it at 2 ohms.

    I would run it at 8 ohms into 1 cabinet, or for a little more volume run it at 4 ohms pushing 2 8 ohm cabs. While it won't put out any more power at 4 ohms, the additional drivers would equal a little more volume.
     
  5. a tube amp won't run any hotter at 2 ohms than it will at any other impedence, since the nominal impedence seen by the power amp is always the same (from the primary winding of the op transformer). This means the amp will produce the same amount of power at all of its rated impedences. Solid state amps run hotter at lower impedences because they produce more power at the lower impedences. Running your V4-b at 2 ohms won't increase wear on tubes as long as the amp is properly biased (you need to see a tech for that with that particular amp) and the output impedences are matched.
     
  6. got it! that makes more sense
     
  7. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Spacegoat ...

    I don't think that's correct, but I'm probably wrong.

    Power=current squared times resistance (impedance in this case). For the same power output, lower impedance necessitates higher current somewhere. I want to say higher current is going to generate more heat.

    Now here's why I'm probably wrong. Power is measured in watts, which is a measure of heat. Same power output, same heat generation. I have a hunch that circuit efficiency is going to play a role in the final outcome.

    All that said (a blend of ignorance, wisdom, and gut feel), I wouldn't run any amp at 2 ohms, even if it's rated for it.
     
  8. MikeyD

    MikeyD

    Sep 9, 2000
    I side with Space on this one. You could take another transformer, attach it to the speaker output, and drive a .001-ohm load if you wanted without stressing the amp (in theory). That's what a transformer is for. You step up the current output by a factor of N while reducing the voltage output proportionally (1/N). This is called a "step-down" transformer (because it's stepping down the voltage, but stepping up the current). Net result: same wattage delivered (in theory). So, if you have a tube amp by a reputable manufacturer, and it has a transformer tap rated for a 2-ohm load, I doubt there will be any problem whatever. Yes, the secondary winding will see more current (and I squared R loss means more heat on that side), but the primary side will see less.
    - Mike
     
  9. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Well, if you say so ...
     
  10. MikeyD

    MikeyD

    Sep 9, 2000
    Trust me: don't believe anything I say. I'm untrustworthy.
    - Mike
    ;)
     
  11. Munjibunga,
    I understand where you're coming from on this, and your comments prompted me do do some reading. Here's what I found. What happens in the output section of a tube amp is this: After the power tube stage, the transformer is needed to convert the high voltage - low current energy found in the power amp to low voltage - high current energy suitable for speakers. For example the voltages in SVT power amps are on the order of 600 V. Typical speaker voltages would be an order of magnitude or so less. The amp still only sees the impedence of the primary winding and therefore is unaffected by speaker impedence, because it is effectively decoupled from the speakers. So the amp itself doesn't work any harder at low impedences. On the secondary side of the transformer the taps corresponding to each impedence have a different number of windings. This means they have different voltages at a specific power level. The currents of each tap are proportional to the voltages so power is conserved. The lower impedence taps would have lower voltage and higher current than the higher impedence taps. This doesn't make much difference to the speakers. There's probably more heat lost in the speaker and cable at the lower impedences (due to the higher current) but this is negligible with respect to the power generated by the amp. None of this applies to solid state amps of course. I wouldn't reccomend running most SS amps at 2 ohms either. It just makes me nervous:D tube amps are OK as long as there's a 2 ohm tap on the transformer.

    Good comments Mike!
     
  12. I think I'm the one who said it was better to run at 8 or 4 than at 2 with a tube amp. The reason being I^2R losses in the wire inside the transformer and the speaker cables and the speaker voice-coil. Not that it's that big a deal, I mean the amp is designed to handle it, but it is better to get your power in the form of higher voltage and lower current than lower voltage and higher current. That's why the power company transmits at extemely high voltages. They can then just push a few amps and still get really high power, with no heating of the wire. Current heats wire, voltage does not.

    Plus, that way none of your solid state buddies will want to borrow your cabs. :D

    Chris
     
  13. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Thanks, Spacegoat and throbbinut. MikeyD too. And everybody else. I (re) learned some basic electronics. Believe it or not, I used to teach electronics in the Marines. But that was 30+ years ago. I've lost a few neurons since then. Now I'm just a simple country geologist.
     

  14. So I guess you play ROCK music? :D:rolleyes:

    Sorry, I couldn't pass it up.

    Chris
     
  15. Ah you're full of schist. I'm sorry, that wasn't very gneiss, I'm taking you for granite. :rolleyes: :p

    Apologies for the lame-o geology humor, but I can never resist a bad geology joke.... I had a Geology prof who used those and far worse jokes on a regular basis.

    Thanks for actually reading our technobabble rants!
     
  16. Yeah, every field has its running gags. I'm an electrical engineer, and as we always said, you can't spell geek without EE.

    Chris
     
  17. A slight correction to RexSpangle's post, the amp will create more heat when you run it with a lower draw, but the amp is running more efficiently, not harder. If you think of it this way, when comparing 8 ohms to 2 ohms, an 8 ohm load would be like sucking liquid through a stir-stick and the two ohm load would be similar to using a McDonald's shake straw to suck the same liquid.
     
  18. I know I may be sounding like a fool after all of the technical jumble up here but then again I am a fool so it all works out. Just a simple question. Which is more resistance, 2 Ohms or say 16 Ohms? Also how do you Equate Ohms? I know 4 + 4 = 2 but what does 8 + 4 ? or 16 + 2 = ?
     
  19. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    I take back all the nice things I ever said about you guys ... if I did. Hey throbbinut ... ever hear of the Nixon FET? Unimpeachable source, Watergate, fiscal drain. I read this on the pooper stall wall in the restroom in the math building at San Diego State in 1977, I think.
     
  20. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    OK, you're right. 4+4=2. So 4+8=2.667. 16+2=1.778. 2+2=1. 8+8=4. 16+8=5.333. 4+2=1.333. 16+4=3.200. And so forth. You should see a pattern emerging here. If not, that'll hold you for a while anyway.