Results 1 to 22 of 22

Thread: How to choose Amplifier ohm rating?

  1. #1
    Forum Member 10-15-14's Avatar
    Join Date
    Jan 2008
    Location
    Ontario
    Posts
    168

    How to choose Amplifier ohm rating?

    My new amp (60w Fender Super-Sonic head)has a selectable ohm rating on the back, 4,8 and 16 ohms. I have a handmade cabinet with two 8 ohm celestion V30 speakers in it. Right now I have it wired in series which would make it a 16 ohm cabinet, but if I wire it in parallel it will be 4 ohms. Does anybody have some advice on which is the best way to go? I see a lot of amps have an 8 ohm rating, is that best? Thanks.

  2. #2
    Forum Member cdw2000's Avatar
    Join Date
    Mar 2006
    Location
    New Hampshire
    Posts
    1,197

    Re: How to choose Amplifier ohm rating?

    There's no hard rule except that to ensure the reliability of your amp, you should match it to your speaker impedance, so use the 16 ohm output for your current speaker configuration.

    Tone-wise, if it's not too much of a pain to do, you can try rewiring the speakers in parallel and connecting to the 4 ohm amp output to see if you like it better. Though the load to the power tubes will end up being the same, the damping characteristics of the amp may be a little different, which you might find more desireable.
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
    "If something has a 1 in a million chance of occurring, 9 times out of 10 it will happen" -- Terry Pratchett

  3. #3
    Forum Member Guitar_Mc's Avatar
    Join Date
    Sep 2005
    Location
    Columbus, Ohio
    Posts
    2,128

    Re: How to choose Amplifier ohm rating?

    What is best is what matches the amp.

    You've already got it set up for 16, so I'd switch it to 16 ohm and go make some music.

    If you really don't like the sound, then I'd switch the wiring to parallel and switch it to 4 ohms. It might sound a little different, but I wouldn't worry about it unless I just didn't like the sound as is.

    If you have another amp you might want to run thru the cabinet too, I'd set it up for whatever load that amp is looking for & switch the Super-Sonic to match.

    Also, if you are planning on having a long run in between the amp and the cabinet, I think the higher ohm load is supposed to work better.
    Music will always find its way to us, with or without business, politics, religion, or any other bull$hit attached. - E.C.

  4. #4
    Forum Member cdw2000's Avatar
    Join Date
    Mar 2006
    Location
    New Hampshire
    Posts
    1,197

    Re: How to choose Amplifier ohm rating?

    Quote Originally Posted by Guitar_Mc View Post
    Also, if you are planning on having a long run in between the amp and the cabinet, I think the higher ohm load is supposed to work better.
    That's true. With the 16 ohm load, you end up with less current going through the wires and therefore less loss.
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
    "If something has a 1 in a million chance of occurring, 9 times out of 10 it will happen" -- Terry Pratchett

  5. #5
    Forum Member Wilko's Avatar
    Join Date
    Jul 2002
    Location
    San Diego, CA
    Posts
    5,105

    Re: How to choose Amplifier ohm rating?

    There is a reason amps/cabs are almost never wired in series. the failure mode of one speaker leaves the circuit open and your amp will see no load and fry very quickly.
    Plus, a 60 watt/8 ohm amp at 16 ohms is actually making 30 watts. You may find that you are losing some "punch".

  6. #6
    Forum Member
    Join Date
    Jul 2005
    Location
    So Calif
    Posts
    3,321

    Re: How to choose Amplifier ohm rating?

    Quote Originally Posted by Wilko View Post
    Plus, a 60 watt/8 ohm amp at 16 ohms is actually making 30 watts. You may find that you are losing some "punch".
    Not quite that simple with tube outputs.

  7. #7
    Forum Member
    Join Date
    Feb 2008
    Posts
    86

    Re: How to choose Amplifier ohm rating?

    If you wire for 4 ohms you will have less inductance and a bit more highs should you need or want to check out how the amp responds at 4 ohms.
    What I use to call the Marshall delema in the early 60s when we where all running our 100 watt head flat out thru one 4-12 cabwas cascading speaker failure! (IE) four 25 watt drivers hook up in S/P, one driver blows which takes out 2 drivers due to the serise path and then you wind up with the two reamining 25 watt drivers hanging on for about 50 seconds and then they blow and you stuck rewiring the cabinet and playing thru the remaining good 12 for the rest of the night! So hopefully their was a spare mic to pump you thru the PA.

  8. #8
    Forum Member Wilko's Avatar
    Join Date
    Jul 2002
    Location
    San Diego, CA
    Posts
    5,105

    Re: How to choose Amplifier ohm rating?

    Quote Originally Posted by JAM View Post
    Not quite that simple with tube outputs.
    The relation of current to resistance and "power" remains constant. That's what Ohm's law is all about.

  9. #9
    Forum Member Guitar_Mc's Avatar
    Join Date
    Sep 2005
    Location
    Columbus, Ohio
    Posts
    2,128

    Re: How to choose Amplifier ohm rating?

    Quote Originally Posted by Wilko View Post
    There is a reason amps/cabs are almost never wired in series. the failure mode of one speaker leaves the circuit open and your amp will see no load and fry very quickly.
    Plus, a 60 watt/8 ohm amp at 16 ohms is actually making 30 watts. You may find that you are losing some "punch".
    There are tons of 112 amps out there. Speaker failure would have the same result. Doesn't seam to stop Fender from making the DRRI.

    The topic of this thread has nothing to do with an 8ohm amp pushing 16 ohms. The amp is either 4, 8 or 16 ohms. If it is switchable between these output loads, I would assume that there are seperate taps on the output transformer. These taps would match the output load to the power tubes.

    There should not be a loss of power. Just make sure the impedence switch matches your load impedence.
    Music will always find its way to us, with or without business, politics, religion, or any other bull$hit attached. - E.C.

  10. #10
    Forum Member cdw2000's Avatar
    Join Date
    Mar 2006
    Location
    New Hampshire
    Posts
    1,197

    Re: How to choose Amplifier ohm rating?

    Quote Originally Posted by Wilko View Post
    The relation of current to resistance and "power" remains constant. That's what Ohm's law is all about.
    Wilko, I would agree with you regarding a SS amp. But with tube amps, you have to factor in the output transformer. As long as you are using the correct impedance matching tap, the power output on a tube amp should be roughly equivalent going from 4 ohms to 16 ohms.

    Example: My homemade amp delivers 4 watts RMS into a 4 ohm driver. P=V^2/R so solving for V, it is delivering 4 Vrms to the speaker.

    The transformer has a 5Kohm primary impedance so the impedance ratio is 5000:4. The impedance ratio also equals the square of the turns ratio so this transformer has a 35.355:1 turns ratio which also equals the voltage ratio.

    So ... We know that we are delivering 4 Vrms to the speaker. Which means the voltage swing on the tube plate must be 35.355 x 4 = 141.42 Vrms. P = V^2/R = 141.42^2 / 5000 ohms = 3.999 watts (some roundoff error).

    Now the same transformer has an 8 ohm tap while maintaining a primary impedance of 5000 ohms. So its impedance ratio is 5000:8 which equals a turns/voltage ratio of 25:1.

    Since we have determined the voltage swing at the power tube plate is 141.42 Vrms, the output voltage for the 8 ohm tap is then 141.42/25 = 5.66 Vrms. Once again P = V^2/R = 5.66^2 / 8 ohms = 4.0 watts delivered to an 8 ohm speaker.
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
    "If something has a 1 in a million chance of occurring, 9 times out of 10 it will happen" -- Terry Pratchett

  11. #11
    Forum Member Wilko's Avatar
    Join Date
    Jul 2002
    Location
    San Diego, CA
    Posts
    5,105

    Re: How to choose Amplifier ohm rating?

    All equal, if you are putting an 8 ohm speaker on a four ohm tap, Ohm's law applies.

    And yes, you are right about using the different taps (switching the amp). I just confused the matter.
    Last edited by Wilko; 03-05-2008 at 12:34 PM.

  12. #12
    Forum Member Guitar_Mc's Avatar
    Join Date
    Sep 2005
    Location
    Columbus, Ohio
    Posts
    2,128

    Re: How to choose Amplifier ohm rating?

    .
    Music will always find its way to us, with or without business, politics, religion, or any other bull$hit attached. - E.C.

  13. #13
    Forum Member cdw2000's Avatar
    Join Date
    Mar 2006
    Location
    New Hampshire
    Posts
    1,197

    Re: How to choose Amplifier ohm rating?

    Quote Originally Posted by Wilko View Post
    I never mentioned changing taps.
    Wilko, you got me there ...

    I agree, using the same tap, it will somewhat follow ohms law except that with tubes there is a secondary effect in that the impedance reflected back to the tube(s) will have changed, which alters their output transfer characteristics.

    Transistors are easier, but tubes are COMPLEX! That's why we like them (he says with a wild, maniacal look).

    P.S. Its been one of those days
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
    "If something has a 1 in a million chance of occurring, 9 times out of 10 it will happen" -- Terry Pratchett

  14. #14
    Forum Member Wilko's Avatar
    Join Date
    Jul 2002
    Location
    San Diego, CA
    Posts
    5,105

    Re: How to choose Amplifier ohm rating?

    Yeah there's a whole lot of dynamic going on there with frequency and feedback, and, and.

  15. #15
    Forum Member
    Join Date
    Jul 2005
    Location
    So Calif
    Posts
    3,321

    Re: How to choose Amplifier ohm rating?

    Quote Originally Posted by Wilko View Post
    All equal, if you are putting an 8 ohm speaker on a four ohm tap, Ohm's law applies.

    And yes, you are right about using the different taps (switching the amp). I just confused the matter.
    You better take a look at this graph. (From the RCA receiving tube manual)

    Simple Ohm's law doesnt apply here. Depending where the amps OT reflected impedance is, a higher load actually can give a higher power (at the expense of some distortion, as you can see on the graph.)

    http://members.canonphotogroup.com/j...t%20Curves.jpg

  16. #16
    Forum Member Wilko's Avatar
    Join Date
    Jul 2002
    Location
    San Diego, CA
    Posts
    5,105

    Re: How to choose Amplifier ohm rating?

    That graph doesn't apply to speaker loads. That's plate to plate restance between 2,000-10,000 ohms.

    This about the transformer load of between 4 and 16 ohms. Very different piece of the puzzle.

  17. #17
    Forum Member
    Join Date
    Jul 2005
    Location
    So Calif
    Posts
    3,321

    Re: How to choose Amplifier ohm rating?

    ??? Of course it refers to the speaker load, transformers by themselves dont have an impedance, it's just the reflected load from the speakers. For example, on my 5F6-A clone with a Weber OT, there are 3 OT secondary taps, 2,4, and 8 ohms for a primary impedance, which is the plate-to-plate resistance as seen on the graph of 4K. In other words, if a 4 ohm load (speaker load that is) is connected to the 4ohm secondary tap the plate to plate resistance (on the primary) is 4K. This is a 1000:1 ratio. If an 8ohm load (speaker) is connected to the same tap, the reflected load is now 8K plate to plate (8X1000 - the impedance ratio). From the graph you can see the power output between 4K and 8K doesnt drop.
    Last edited by JAM; 03-05-2008 at 08:43 PM.

  18. #18
    Forum Member NTBluesGuitar's Avatar
    Join Date
    Apr 2006
    Location
    North Texas
    Posts
    5,820

    Re: How to choose Amplifier ohm rating?

    Ohm my goodness...do I have to separate you two?
    "...pray do not imagine that those who make the noise are the only inhabitants of the field;
    that, of course, they are many in number; or that, after all, they are other than the little,
    shriveled, meagre, hopping, though loud and troublesome, insects of the hour."

    -Edmund Burke

  19. #19
    Forum Member
    Join Date
    Jul 2005
    Location
    So Calif
    Posts
    3,321

    Re: How to choose Amplifier ohm rating?

    Quote Originally Posted by NTBluesGuitar View Post
    Ohm my goodness...do I have to separate you two?

  20. #20
    Forum Member
    Join Date
    Feb 2008
    Posts
    86

    Re: How to choose Amplifier ohm rating?

    Sorry , I got off the track of the question back there!

  21. #21
    Forum Member cdw2000's Avatar
    Join Date
    Mar 2006
    Location
    New Hampshire
    Posts
    1,197

    Re: How to choose Amplifier ohm rating?

    I think we completely scared off the original poster. He hasn't checked back in.
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
    "If something has a 1 in a million chance of occurring, 9 times out of 10 it will happen" -- Terry Pratchett

  22. #22
    Forum Member Toneseeker's Avatar
    Join Date
    Oct 2007
    Location
    Bo'ness, Scotland
    Posts
    496

    Re: How to choose Amplifier ohm rating?

    Interesting stuff though.

    Pete
    Failure is simply the opportunity to begin again, this time more intelligently.

    Henry Ford

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •