4 ohm speaker input into an 8 ohm cab?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by obscured, Feb 27, 2008.

  1. obscured

    obscured Member

    Messages:
    79
    Joined:
    Sep 25, 2007
    Location:
    Oakland, CA
    Have a question...was talking to an amp tech today and he said that I could use the 4 ohm speaker input to run my 8 ohm cab to get a bit more volume and headroom. Is this true, and if true is it safe? Thanks!
     
  2. pgissi

    pgissi Member

    Messages:
    2,487
    Joined:
    Nov 23, 2006
    Location:
    Outta Here
    This could not be more wrong. Technically an 8 ohm speaker load into a 4 ohm OT tap is a 100% mismatch.

    A mismatch of 100% results in a power reduction of approx 1/3 and headroom is reduced. Any mismatch is a reduction in efficiency from the stock output.

    Is it safe, it generally is and I use this technique to explore an amps sonic capabilities beyond the stock design. But the question of is it safe needs more examination and there is some agreement on some generalization of the topic in that, brand a can tolerate it more than brand b etc. My personal opinion is that I dont mismatch marshalls but fenders I am more comfortable doing it with, generally

    Any quality amp should have an overdesigned OT and can tolerate the 100% mismatch fine but I would avoid anything beyond 100% which has the OT seeing a higher reflected impedance, could cause it to fail.

    Additionaly, my personal opinion, I will not mismatch an amp that outputs more than 40 or 50 watts, too much heat generated in the OT in high wattage amps, you cant sink it adequately.

    I use a mismatch with my Rivera R30, 30 watt, 16 ohm cab in the stock 8 ohm output, smooths the tone, flattens the freq response, more complex overdrive probably due to the stressed tranny, allows me to push the amp harder because of the reduced output, getting more from the gain stages, tone stack with the only compromise being a reduction in headroom which I am not missing.

    You can recover some headroom by using a speaker that is rated for higher wattage and has a later breakup

    You can also break down a mismatch in this way-

    -a lower than stock mismatch, works the tubes harder (4ohm load 8ohm tap)
    -a higher than stock, works the tranny harder (16 ohm load 8 ohm tap)

    I prefer the sound of the OT stressed over the Output Tubes stressed more especially since biasing hotter can accomplish the latter and most amps have plenty of Overdrive already so for me its piling on whats not needed.

    With all of the talk of OT upgrades these days, a mismatch seems to be an often overlooked option that is and has been used by many.
     
  3. obscured

    obscured Member

    Messages:
    79
    Joined:
    Sep 25, 2007
    Location:
    Oakland, CA
    Thank you so much for your post! Man I love this place.
     
  4. pgissi

    pgissi Member

    Messages:
    2,487
    Joined:
    Nov 23, 2006
    Location:
    Outta Here
    Your welcome and I agree, TGP is a great place

    I have learned much and am sharing all that I know, knowledge accumulated over 30 years and no matter how much you may know, there is always more to learn
     
  5. Free

    Free Member

    Messages:
    1,305
    Joined:
    Jun 18, 2005
    Have you experienced any greater degree of lowered output going out to 16 ohm or 4 ohm, from an 8 ohm tap/amp? Great post btw - really covers a lot of bases, yet gets down to the essentials.

    Thanks, Pgissi.
     
  6. pgissi

    pgissi Member

    Messages:
    2,487
    Joined:
    Nov 23, 2006
    Location:
    Outta Here
    Yes but I would not say greater only because I compensate at the master volume (rivera r30 30 watt), where I used to set it between 3 and 4 is now between 4 and 5 to recover it. The output reduction does occur at both mismatches either lower or higher and is generally a 1/3 reduction so were talking the difference bewteen a 20 and 30 watts using either mismatch, barely discernable.

    Any mismatch reduces output for a tube amp and for this amp, it allows me to really push it to places that previously were really loud even for a 30 watt but now it seems that it cuts better and pushing it to those same high levels is more pleasing. Thats how you know, if it sounds and feels good loud and you dont find yourself cringing, you got something and you want more and cant stop playing it.

    The lower mismatch is clearer with more top end, lower branch inductance using parallel speaker wiring and thats what I didn't need since modern amp designs have plenty of top end, too much IMO and then coupled with overly bright and spikey speakers today, that has been my experience having used marshalls and fenders for my 27 years of playing,

    Too much causes fatigue, makes you hold back and you get told to turn down more, an acoustic byproduct that is due to beamy highs

    There is nothing more common than the too bright, too mid or too harsh in general description/complaint today, you read it in many posts and hear it on stages, have to run from it and its due to todays high gain hot rod amps and since much of the OD is found in the upper mid and top, bringing back the cream while retaining the OD is a challenge so I reasoned I had to alter the freq response in a general way which then made it more in line with yesterdays non master volume designs getting power tube OD since I am running it hotter and using the higher mismatch seems to counter the harshness by flattening the top overall.

    Have you ever noticed while Soldanos sound so good, thick, creamy and aggressive without the beamy or harsh highs, that was a template of sorts for me.

    With a 16 ohm 2x12 series, the inductance is greater due to 2 speaker coils in series, this smooths the top end (the timbre) and also flattens the top and bottom end some also (the response drops slightly), I just compensate with eq and what I like the most is I can run the treble hotter, makes it thicker overall and less harsh.

    So the bottom line is that a mismatch is acting like a parametric eq treament for the output in some way and I cant say it will always be good for a given amp and speaker/cab but it is worth experimenting with if you are in harshville.

    Just follow my warnings, dont do it on your prized Deluxe or Super etc and try to avoid doing it on higher output amps above 30 to 40 watts and be aware it may not work with a specific speaker but may be better with another.
     
  7. Free

    Free Member

    Messages:
    1,305
    Joined:
    Jun 18, 2005
    Great stuff here, Pgissi. Thanks.
     
  8. Groovey Records

    Groovey Records Member

    Messages:
    3,146
    Joined:
    Nov 19, 2007
    Location:
    Jackson Heights Birthplace of Johnny Thunders
    Thanks from all us lurkers too!:AOK
     

Share This Page