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Ohms... Help! I'm friggin' confused!!!!!

Discussion in 'Amps and Cabs [BG]' started by HamInChains, Oct 10, 2002.


  1. I'm sure people post questions like these all the time, but I can't seem to find the answers I need!!

    I have an Ampeg SVT Classic that has a setting for either 4 or 2 ohms. My cabinet is an Ampeg PR-410HLF rated at 4 ohms. I currently have the ohm setting on my Ampeg set to 4 ohms while playing through my 4 ohm cabinet... is this the correct way to do it?
    Also, I'm thinking of getting an Ampeg PR 2x10H, but this is rated at 8 ohms... is this a wise combination with a 4 ohm cab? I've been told two different things: would this total 6 ohms??? or 2.6??? I don't get it!!!
    Either way, would this combo of an 8 and a 4 ohm cabinet be a good idea? I really don't want to push my SVT too hard.

    ...and how does this work: let's say i have an 8 ohm head and I run it into a 4 ohm cabinet... is this good or bad? Should the ohms on the amp be lower or greater or the same as the ohms on the cabinet?

    Thanks!!

    P.S. If it IS a good idea and you are or know of someone with an AMpeg PR-210H gimme a ring!!
     
  2. zoran

    zoran

    May 10, 2002
    croatia
    it will be 2.6 ohms. Check if amplifier can continuosly run that resistance
     
  3. Johnny BoomBoom

    Johnny BoomBoom Supporting Member

    Jun 8, 2001
    Glasgow, Scotland


    Running a 4 ohm head into a 4 ohm cab gets you maximum power (you get this when the head and amp have their impedance (ohms) matched) - so you're currently doing the right thing.


    When you add 2 cabinets, you need to add the reciprical of their impedance, in your case its:-

    1/Rtotal = 1/8 (new cab) + 1/4(current cab)

    1/Rtotal = 1/8 + 2/8

    R total = 8/3 = 2.67 ohms

    This is because cabinets are wired in parallel, and this is how you calculate the resistance/impedance in a parallel circuit!


    If the head outputs into 8 ohms, and you attach a 4 ohm cabinet, be prepared for your head to go into meltdown and self destruct! Basically this is very bad thing. Always ensure that the head can handle a lower resistance (ohms) than the cabinets or combination of cabinets( the best results are when the total resistance of the cabinets equals the output resistance for the head)

    Hope this helps!

    Also - check out the FAQ in this forum - their are people who have already worded this more eloquently tha I have
     
  4. lamarjones

    lamarjones Supporting Member

    Aug 27, 2002
    Raleigh, NC
    Ok, so help me out....

    I got an ampeg svt 3 pro, and 2 bse 4 10's, one 8 ohm, and one 4 ohm. It appears in the back that I have two speaker jacks. I have plugged both in without meltdown, but am I running a risk at all? Anyone know?
     
  5. Johnny BoomBoom

    Johnny BoomBoom Supporting Member

    Jun 8, 2001
    Glasgow, Scotland

    What does the SVT Pro handle is it 4 ohms or 2 ohms?
     
  6. lamarjones

    lamarjones Supporting Member

    Aug 27, 2002
    Raleigh, NC
    Specifications:
    Dimensions : 19 x 3.5 x 15.5
    Weight : 26 Pounds
    Preamp (Tube or SS) : T(3x12ax7)
    Driver Tubes : 12AX7, 12AU7
    Power Amp : MF
    RMS Power Output (8 Ohms) : 275
    RMS Power Output (4 Ohms) : 450
    RMS Power Output (2 Ohms) : n/a
    Tone Controls : 3 band
    Mid Tone Control : 5 pos
    Graphic EQ : 9 band
    Ultra Low/High Boosts : Yes
    Balanced Line Out : Yes (Transformer)
    Effects Loop : Yes
    Preamp Out/Power Amp In : Yes

    Other Features: Input Peak LED
    Input Pad & Bright Switch
    Mute Function
    Tuner Out
    Variable Line Out Level
    Time Delayed Turn-On
    DC Supplied Filaments
    Variable Fan Speed
     
  7. It's like Johnny says. The total impedance of the two cabs that the amp sees will be 2.6 Ohms. It doesn't necessarily follow that the amp will self destruct if you run it into 2.6 Ohms. However, if the amp's not designed to feed that load (say it's designed to run into a maximum of 4 Ohms) you are certainly running an increased risk of destroying the output stage. And the louder you play it, the more risk you run.

    I don't know what the Ampeg will run into so can't say. But it shouldn't be too difficult to find those facts.

    John
     
  8. Johnny BoomBoom

    Johnny BoomBoom Supporting Member

    Jun 8, 2001
    Glasgow, Scotland
    OK, looking at the specs, it doesn't handle 2 ohms. I'd say from that that you are risking a burning smell and no more amp! I wouldn't take the risk. The lower impedance means that the amp output isn't matched, this will cause lots of heat to be generated and reduce the longevity/long term reliability of your amp!


    Unless it's a stero amp???? What I mean is that both jacks can independently handle 4 ohms - in which case you would be OK! I'm not too well genned up on bridging amps and stereo amps and that kinda stuff!

    Hope this helps!!! Maybe some of the other amp gurus can help you - they might even prove me wrong!
     
  9. OK from the post, it seems the Ampeg will NOT run into 2.6 Ohms. The best advice, then, is not to parallel connect the 4 and 8 Ohm cabs. Just use the 4 Ohm cab.

    John
     
  10. This is a tube amp so the rules are a bit different. Basically, you should only run this amp under 2 or 4 Ohm loads, beacuse the output transformer only has 2 and 4 Ohm taps. The 4 and 8 Ohm box give a total of 2.677777 Ohms. You probably could run this combinmation on either tap (2 or 4) without much worry because the impedence mismatch isn't too bad. But, officially, I wouldn't reccommend it!

    Get yourself another 4 Ohm cab. When you run both cabs you'll have a load of 2 Ohms, which the amp has a setting for. This way, you won't void the warranty and you can be sure that there won't be any problems.

    LamarJones: I'd be a bit worried running the 3Pro at 2.67 Ohms. It's not rated below 4.