Can i some how run a 16ohm cab and an 8 ohm cab simultansously with 6505 head?

Discussion in 'Gear & Equipment' started by Gmork, May 6, 2015.

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  1. Gmork

    Gmork SS.org Regular

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    can i some how run a 16ohm cab and an 8 ohm cab simultansously with 6505 head? this is just a stupid question isnt it lol?
     
  2. desmondtencents

    desmondtencents Plays With Wood!

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    A 16 ohm load in parallel with an 8 ohm load will result in a total load of 5.33 ohms. You can use the 8 ohm output safely. The impedance mismatch means you won't get the full 120 watt power transfer though.
     
  3. Insinfier

    Insinfier SS.org Regular

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    Can one of the cabs be rewired?
     
  4. mr coffee

    mr coffee SS.org Regular

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    It's not ~ideal~ but probably wouldn't burn out your OT. You would have a noticeable volume difference. I found this on TGP:




    -m
     
  5. vick1000

    vick1000 SS.org Regular

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    You risk blowing your OT.
     
  6. Promit

    Promit SS.org Regular

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    Modern OTs are generally going to be okay with a 2:1 mismatch in either direction. That is, it won't kill an OT to put an 8 ohm cab on its 4 or 16 ohm taps. At least, not quickly. It will create physical stresses and excess heat, which could combine with other environmental factors (high ambient temperatures, poor ventilation) to produce some bad end results in principle. Obviously this is not ideal. But it is not fatal the way people tell you it could be. The safest mismatch setting is to use an amp impedance that is higher than the cab. That is, 8 ohm output for 5 ohm cabs and 16 ohm output for 12 ohm cabs. The way I remember this is, what do tube amps hate more than anything? Not being plugged into anything, aka infinite impedance. So 8 ohms to infinity is the wrong direction.

    This is from the manual of a Mesa Dual Rectifier:
    Note that "running cooler" isn't a good thing as such (it's irrelevant, mostly). This setting will generate flyback voltages in the OT if it's not designed to cope with that. Vintage transformers would not accept this. The Mesa IS designed to handle it, as the amplifier does not have a 16 ohm output.

    I've noted similar comments on mismatching in the manuals of various other amplifiers. But I again want to emphasize, this only applies to modern day modern design amplifiers with modern output transformers that are designed not to generate large flyback voltages. Don't try it with a JMP or something.
     
  7. TheWarAgainstTime

    TheWarAgainstTime "TWAT" for short

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    What about running the head at 4 ohms and connecting to both cabs? That way each cab is getting 8 ohms, which is a match for the 8 ohm cab and a safe mismatch for the 16 ohm cab.
     
  8. glpg80

    glpg80 √εvil

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    All of the information on this page is incorrect. Absolutely all of it. If you have any questions what so ever about mismatched loads, please read the link provided below.

    Impedance Mismatching, Improper Loading, and More.

    Incorrect. Should a mismatch occur, the amplifier must see a load equal to or higher than it's intended setting. Meaning only the 4 ohm tap should be used for a 5.33 ohm load - and that is also not ideal for maximum power transfer nor do I recommend it.

    This is wrong, but not only is it wrong, your overzealous impedance suggestion is backwards. The amplifier must see a load which is higher, not lower. Impedance is not resistance, impedance is measured in the S plane and therefore a loading effect occurs which is frequency dependent.

    An 8 ohm cabinet must only be used at 8 ohm setting for maximum power transfer and to prevent impedance reflections of certain frequencies causing the EM field of the transformer to collapse which will cause damage to the primary windings of the OT. If you HAVE to mismatch the load to the source, the load should be a higher impedance value than the amplifier's setting. The link above has information regarding a full explanation I have written out for further proof of concept. I suggest you read all of it.

    Absolutely incorrect. It is people like you that do not know what you are talking about that cause significant damage to expensive amplifiers. If you do not know what you're talking about do not reply.

    The Truth about Mesa's User Manual

    Mesa is in a business to make analog amplifiers seem easy to use because complications affect their bottom line sales figures. Textbook electrical engineering does not apply to a user's guide and you should never reference a user's guide for anything more than what it actually is.
     
  9. mr coffee

    mr coffee SS.org Regular

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    You forgot to shoot down the idea I found...I'm more curious than anything. I would think a properly rated dummy load would be a safe option, although I don't grasp the theory well enough to say how it would perform.

    -m
     
  10. desmondtencents

    desmondtencents Plays With Wood!

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    glpg80 beat me to it. I just realized I should have said to use the 4 ohm output.
     
  11. glpg80

    glpg80 √εvil

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    Yeah no, that would not work either. Although the amplitude of the signal going to each cabinet is ideal, the voice coil is seen as an inductor to the amplifier. A change in current through the inductor induces a voltage across it which is multiplied by the henries of the inductor. That current through it is frequency dependent because a guitar signal is a complex waveform of AC - it's nonlinear. Although the resistance of the loads are equal at idle, when you are playing your guitar through the loads the impedances are changing drastically as if there is still a missmatched load. Not ideal, and still dangerous.

    Once again keep it simple - matched impedances in parallel and you're golden. The transformers used for decreasing the primary impedance to an impedance the speakers can see/use are not designed for complex loads like active bass amplifiers, and therefore you run the risk of damage/overheating the plated laminate coatings or the insulation between the windings when running loads that are not matched. The extra energy from a loss in maximum power transfer has to go somewhere - it will be dissipated as heat. if you read your recommendation (which is once again incorrect) he tried to cover up this fact by saying "A high wattage resistor". Otherwise it would overheat and fail.
     
  12. Gmork

    Gmork SS.org Regular

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    i had the chance to buy a 2x12 that is 8 ohms. i decided not to go with it anyway. so problem mostly solved. im going to make my own. so what exactly makes a cab run on a certain ohm? ive noticed on the eminence site that the swamp thing comes in an 8ohm ver. and a 16ohm ver. so is it as simple as just making sure i buy 16ohm speakers thus making my home made 2x12 16 ohms?
     
  13. Insinfier

    Insinfier SS.org Regular

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    [​IMG]

    It's the speakers used and how you wire it.
     
  14. mr coffee

    mr coffee SS.org Regular

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    If you want a 2x12 to be 16ohm, you need two 8ohm speakers in series.

    -m
     
  15. Gmork

    Gmork SS.org Regular

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    or 2 16ohm speakers in parallel. any reason i would go with series rather than parallel or vice versa?
     
  16. desmondtencents

    desmondtencents Plays With Wood!

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    Nope, two 16 ohm speakers in parallel will give you an 8 ohm load.
    If you want a 16 ohm 2x12 cab, the only way to do it is to wire two 8 ohm speakers in series.
     

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