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OMG not another ohm question! but yeah

Discussion in 'Amps and Cabs [BG]' started by basscooker, Jan 27, 2017.


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  1. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, ChopShopAmps
    Alright, here goes. Over on the Marshall forum a poster asked about hooking up some stuff. The replies tend to agree, but I admit I'm still questioning.

    Ok. He has an 8 and a 16 ohm cab. He has taps for 16,8,4. The guys there are telling him put the 8 ohm on the 4 ohm tap and the 16 ohm on the 8 ohm tap, and the power will distribute evenly between the cabs, as well showing proper load to taps.

    I'm completely lost. This is definitely out of my league to say one way or another. I think I remember reading here when connecting a load like this in parallel, the 8 ohm output is the way to go (for this example), but factoring in different taps, let's just say I'm"tapping out" ;)
     
    Last edited: Jan 27, 2017
  2. john m

    john m Supporting Member

    Jan 15, 2006
    I think you have to pick one tap only. Assuming the cabs are in parallel, I'd use the 8 ohm tap. Tube amps generally like the greater load (lower z), solid state amps need a load equal to or less than their rating (ie a cabinet with a higher z).
     
  3. Don't do a lot with tube amps myself, so more of a question to add to the conversation than an answer.
    Don't you want to connect the speakers to only one tap at a time? For example, hook your one 8 Ohm cab to the 8 Ohm tap and be done with it. Likewise you could hook two 8 Ohm cabs to the 4 Ohm tap.

    Wouldn't doing what the Marshall crowd suggests be like hooking a 4 Ohm cab to the 8 Ohm tap?

    I can't help but wonder if using the Marshall Plan would set up some strange impedance relationships. Do the windings (taps) on the transformer interact differently with the voice coils, than just hooking things up Ohm for Ohm?

    Or is this one if those, "Hey duuuude... listen to what happens when I do this!" "Bitchin' am I right man?" :bassist:
     
    Last edited: Jan 27, 2017
  4. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, ChopShopAmps
    I have no dog in this fight, so to speak. I'm just one of those guys that likes to help. This doesn't seem to me like good advice, which is why I brought the question here. There are smart, experienced professionals hanging out over there, too, but the suggestion made me balk a bit.
     
    Old Garage-Bander likes this.
  5. /\/\3phist0 likes this.
  6. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, ChopShopAmps
    I pinged him in another thread. I was going to in this thread, but I don't wanna stalk the guy, lol.
     
    agedhorse and catcauphonic like this.
  7. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, ChopShopAmps
    Maybe @Mr. Foxen will be around later with some Marshall -specific knowledge to condone or discourage such a connection.
     
  8. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    When you plug multiple things in, the amp sees it as all one impedance, so you can't do magic to make an awkward load work when the amp only has 4/8/16 ohm taps. Plus Marshalls blow up way easy if you connect them up wrong.
     
  9. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, ChopShopAmps
    I wonder should I stop back into that thread and mention that? I'll dawn the flame suit, I guess. :(
     
    catcauphonic likes this.
  10. Seems plausible to me, although I've never really thought about doing such a thing.

    Imagine the amp wants to see some load 'X'.

    The four ohm tap provides a ratio of 4:X
    The eight ohm tap provides a ratio of 8:X.
    The sixteen ohm tap provides a ratio of 16:X.

    So with an 8 ohm load on the 4 ohm tap the amp is seeing 2X.
    And with a 16 ohm load on the 8 ohm tap the amp is also seeing 2X.

    put those both in parallel and you are back to X.

    maybe....
     
  11. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    People don't use Marshall amps because they want to avoid having them shoot fire out.
     
  12. bassdude51

    bassdude51 "You never even called me by my name." Supporting Member

    Nov 1, 2008
    Central Ohio
    Annoying ohm question.................."how many ohms does it take to screw in a light bulb?"
     
    agedhorse likes this.
  13. iamprowla

    iamprowla

    Nov 28, 2016
    UK
    My guess is that 2x the power will go to the 4 ohm compared to the 8.
     
  14. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, ChopShopAmps
    Or "how many posters a day don't read the OP and reply to the title only"
     
    Sartori likes this.
  15. monsterthompson

    monsterthompson The Eighth Note Wonder Of The World Supporting Member

    Nov 25, 2008
    Hollywood
    From the Barefaced site, regarding their 8 or 12 ohm option Two10 cabs

    With valve/tube amps you should use the 8 ohm or 16 ohm tap with a single 12 ohm cab, 8 ohm or 4 ohm tap with two cabs, 4 ohm tap with three cabs, 2 or 4 ohm tap with four cabs, 2 ohm tap with five or six cabs.
    So, if they suggest and 8 or 16 ohm tap for a 12 ohm load, it seems there's some wiggle room to go high or low.
    Two 12 ohm cabs would be a 6 ohm load, and they say 8 or 4 ohm tap could work. Again, wiggle room to go high or low.

    If that is the case, then an 8 & 16 pairing (in parallel) will yield 5.3 ohms with a 2/3 vs. 1/3 power split.
    (Speaker Impedance)
    In series, you'd get 24 ohms.

    Many 310 cabs are 5.3 ohms. Epi's is rated at 5.3. Demeter's is rated at 6 (probably not far off 5.3)

    I've read about this issue in other places, but haven't retained that information very well. Nor is this my field of expertise. It seems you could argue it from a few angles. I've generally seen taps say "X ohms minimum" so if I had to, I'd pick a tap with a lower number than the combined cab rating. I agree that you probably will find the manual saying you don't get to pick multiple taps on this head.
     
  16. monsterthompson

    monsterthompson The Eighth Note Wonder Of The World Supporting Member

    Nov 25, 2008
    Hollywood
    Not if you're using a tube/valve head.
     
    Sartori likes this.
  17. iamprowla

    iamprowla

    Nov 28, 2016
    UK
    Thanks - appreciated!

    What's your guess?
     
  18. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, ChopShopAmps
    ;)

    The distribution is even worse considering the 8 ohm cab is a 2x12 and the 16 ohm cab is a 4x12.
     
    Last edited: Jan 28, 2017
    monsterthompson likes this.
  19. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Use either the 4, 8, or 16 ohm tap. Not more than one at a time. It isn't just about impedance matching.

    The secondary of most amp output transformers is a coil with loops or taps pulled out that offer different impedance options. Speakers connected to different taps at the same time will draw different currents. The load of each speaker will be different, they will interact and affect the way that the transformer performs. This can exceed the design limits of the transformer and overheat it. Flames and smoke turn out to be not so good.

    A transformer with separate windings for for each impedance is different. They are designed to work in the way proposed. These type of transformers are quite rare in musical instrument amps.
     
    Last edited: Jan 28, 2017
    agedhorse likes this.
  20. I would really like @agedhorse to chime in on the concept of using two taps at the same time on a tube amp. Sounds like a good way to make magic smoke!
     

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