Join Strat-Talk Today

Calling all Amp experts !

Discussion in 'Amp Input - Normal or Bright' started by AxRookie, Aug 10, 2019.

  1. AxRookie

    AxRookie Strat-Talk Member

    Age:
    56
    Messages:
    90
    Joined:
    Jun 14, 2019
    Location:
    Las Vegas, Nevada
    I have a speaker wiring question, I have a 4x12 cab and 2 each Fender "special design speaker" 16 ohm speakers and 2 each Celestion G12L-35 8 ohm speakers that will be driven by a little Marshall MG15MSIIthat has 2 "16ohm loudspeaker outputs" and it states "do not use with a load of less than 8 ohms".
    What would be the best way to wire up the speakers and should I use all 4 or just wire up two of the speakers and leave the other two disconnected, Or something else???
    Also, this I what I have and I want to make it work so please no advice on buying something else...

    Here is what I have...

    [​IMG]
    [​IMG]
    [​IMG]
     
  2. JB74

    JB74 Senior Stratmaster

    Messages:
    1,343
    Joined:
    Mar 22, 2018
    Location:
    Australia
    impedance mismatch is the issue here.
    a 16Ω cab is 4x16Ω drivers in series-parallel wiring.

    you amp output transformer wants a 16Ω load, but can cope with an 8Ω load - while the word "cope" means you can use it, it's simply not adviseable. Without going on for eternity about impedance and reactance, the bottom line is you could feasibly drive it right down to 2Ω but it will be very shortlived indeed.

    the 16Ω impedance is there for a reason.

    So, the mix n match approach is going to net you some issues with heat in the voice coils and unequal SPL. you can't series wire 8&16Ω speakers together with a view to any longevity whatsoever. you can series-parallel them, to have "3" x 8Ω speakers (although it's actually 4) in parallel. this would be the only way to equalize the load, even then it's not great - Also it's a 5.3Ω load which is below the minimum rating specified by your amp.

    so the answer in short is No, don't do it.

    Instead, wire both 16Ω speakers in parallel, and leave the 8Ω speakers as dummies.

    my advice is to sell or trade the two 8Ω speakers for a pair of 16Ω speakers, then series-parallel wire for a 16Ω cab when you have budget and time.
     
  3. AxRookie

    AxRookie Strat-Talk Member

    Age:
    56
    Messages:
    90
    Joined:
    Jun 14, 2019
    Location:
    Las Vegas, Nevada
    both 16Ω speakers in parallel will give a 16Ω load to the amp?
     
  4. Kayden33

    Kayden33 Strat-O-Master

    Age:
    35
    Messages:
    590
    Joined:
    Apr 23, 2017
    Location:
    Knoxville, TN
    ,,,
     
    Last edited: Aug 10, 2019
  5. JB74

    JB74 Senior Stratmaster

    Messages:
    1,343
    Joined:
    Mar 22, 2018
    Location:
    Australia
    mind fried. I made a mistake

    2x16Ω in series =32Ω
    2x16Ω in parallel = 8Ω

    I have attached a spreadsheet. use the 1st sheet in the workbook to do your calcs.
     

    Attached Files:

    Last edited: Aug 10, 2019
    Studio420Ben and s5tuart like this.
  6. AxRookie

    AxRookie Strat-Talk Member

    Age:
    56
    Messages:
    90
    Joined:
    Jun 14, 2019
    Location:
    Las Vegas, Nevada
    I thought something was off, I checked it and it was 7.4 ohm.

    What I'm going to try first is run one 16 ohm speaker to one input jack and run the two 8 ohm speakers in series to the other input jack on the cab and see how those two setups sound...

    Thanks it does!
     
  7. CB91710

    CB91710 This is a Custom Title Gold Member Strat-Talk Supporter

    Messages:
    1,464
    Joined:
    Feb 24, 2019
    Location:
    SoCal
    DC resistance is not the same as impedance.
    8ohm impedance will generally measure between 6 and 8 ohms on a meter.

    Series adds.
    Parallel divides.

    The closest you'll be able to get with those speakers is to series one Celestion and one Fender, which will give you two 24ohm strings, then parallel those to give you a 12ohm cabinet.
    But the power levels on each speaker will not be the same. The Fender speakers will dissipate more wattage than the Celestions. As long as all speakers are rated for more than 50% of the output of the amp, you'll be OK.
    But honestly, I'd buy two more Celestions to build the 4x12 series-parallel for 16ohms, then build a 2-12 narrow slant front cabinet and series the Fender speakers for smaller venues.

    One thing to remember...
    In general, within reason, lower impedance will not damage a tube amp.
    Higher impedance will not damage a solid state amp.
    The impedance mismatch will impact the tone.

    Double check your manual and see what they recommend for using the speaker outputs. If the jacks are paralleled (likely they are) then plugging in two 16ohm cabinets will put the total load on the amp at 8ohms. Do not run an 8ohm and a 16ohm cabinet at the same time, that will put your total load below 8ohms at 5.3. You could series the Celestions and series the Fenders, then parallel those, which would put your final load at 10.6 ohms.

    Tube amps are voltage-regulating devices. A low impedance increases current draw, which pulls the voltage on the plates down. Too high impedance (or an open output) will cause the voltage to increase and can cause arcing in the tubes.
    A shorted output will increase current to the limits of the voltage... you'll get a hot output transformer, but no permanent damage after a short term exposure. That's why the speaker jacks on tube amps normally short to ground (either directly or through a resistor) when no speakers are plugged in.

    Solid state amps are current-regulating devices. A low impedance increases current draw and will quickly overheat the output transistors. A shorted output will likely cause immediate, permanent damage unless there is good thermal protection (higher end amps and PA systems have this).
    A higher impedance reduces the current draw, but will not cause internal arcing due to the way the transistor is made. You can run a solid state amp into an open output all day without hurting it.
     
    Last edited: Aug 10, 2019
    pazman6, JB74, Stratoskater and 2 others like this.
  8. JB74

    JB74 Senior Stratmaster

    Messages:
    1,343
    Joined:
    Mar 22, 2018
    Location:
    Australia
    The marshall outputs are 2x16Ω in parallel, meaning their lowest rated impedance is 8Ω which is why they state min 8Ω on the head's output jacks.

    I didn't want to write an essay on why mixing the impedance is a bad idea, but I'll keep it short:

    ohm's law is required understanding if you're going to start messing with speakers and amplifiers.

    If you are wanting to get 'nerdy, then you will need to understand the Thiele/Small parameters with regards to speakers - stuff like Zmax, Fs, Vas etc.

    Let's try and maintain a KISS principle:
    You’ve got one cab with 8Ω impedance, and one with 16Ω – and you want to connect them both to an amp's speaker output at the same time.
    Is that even possible?

    To put it simply, yes. BUT!

    You need to know the total impedance you’ll have if you’re running the two speakers in PARALLEL.
    This is because most (not all, but most) amp outputs are MONO. If they have multiple output jacks, they will be in Parallel.
    Some amps have separate windings for low impedance speakers on the OT (or a second OT if it is stereo), but using your marshall as the example, it has TWO 16Ω outputs in PARALLEL. i.e. both outputs are connected to the same winding on the output transformer.

    Another ground rule here: if you’re connecting resistors in parallel, then the total resistance is always going to be smaller than the smallest partial resistance. In this case, it means you're going to end up with a TOTAL resistance less than 8Ω.
    To be more clear, you can calculate the precise figure using the following formula (which, by the way, only works in cases where you have two parallel resistors!):
    Parallelresistancecalcs.png

    Or, in your case, that’s (R1xR2) / (R1+R2)= (8×16) / (8+16) = 128/24 = 5.333 Ω


    Based on this, you can think about whether you would prefer to use this combination with a tube amp output of 4 or 8 ohms.

    It also demonstrates that it is BELOW the MINIMUM impedance of the marshall which is 8Ω

    It’s certainly technically feasible on any amp which has a 4Ω or 8Ω tap on the OT, and the difference in volume between the two cabs will also be less than most people would think. And by the way, you should avoid combining 4 ohm cabs if you can, because you’ll end up in a red zone of under four ohms in total (for example if you go for the following ohm combinations: 4+4, 4+8, 4+16). Most amps aren’t designed to go this low, and you could cause some quite serious damage.

    We could go on and on and on and on.

    the bottom line is simple. You should never mix impedances (no series connection of 8Ω and 16Ω speakers) because it will damage the voice coil due to the current requirement of the lower impedance speaker and the higher resistance of the high impedance speaker.

    Keep your speaker impedances matched. i.e. keep the 8Ω speakers together. keep the 16Ω speakers together. wire them up individually as single speakers or wire them in series or parallel - however you want, just don't mix speakers of different impedances in the same circuit.

    so again to KISS:

    2x8Ω stereo (separate / individual speaker connection) that's 8Ω per speaker connection - you need two ¼" cables, one for each speaker.
    (Your Marshall amp has a 16Ω output with two parallel wired jacks = 8Ω minimum. Two 8Ω speakers in parallel is 4Ω - therefore you cannot use an 8Ω speaker in BOTH jacks, but you can use ONE 8Ω speaker in One of the output jacks).
    2x8Ω series mono = 16Ω one cable, two speakers in series. (OK)
    2x8Ω parallel mono = 4Ω one cable, two speakers in parallel. (NO)

    2x16Ω stereo (separate / individual speaker connection) one cable per speaker (OK - one speaker per output connection)
    2x16Ω parallel mono - 16Ω one cable, two speakers in parallel. (OK)

    There is literally an encyclopedia on this stuff and it's hard to not get bogged down in the math of dynamic impedance of speakers under load and why impedance is never the same as it's advertised on speaker bells, but the fundamentals are straightforward.

    Use Ohms law. Use the parallel resistance equation, use the series resistance equation.
     
    Last edited: Aug 10, 2019
    s5tuart and CB91710 like this.
  9. Stevem

    Stevem Senior Stratmaster

    Messages:
    1,089
    Joined:
    Sep 3, 2014
    Location:
    NY
    Also note that the change in inductance caused from wiring 2 or more speakers in series or parallel , or in the case of 4 speakers in series parallel or parallel series will effect the high end responce and how tight the bottom / low end will be.
     
    s5tuart likes this.
  10. AxRookie

    AxRookie Strat-Talk Member

    Age:
    56
    Messages:
    90
    Joined:
    Jun 14, 2019
    Location:
    Las Vegas, Nevada
    I was only planning on plugging into one jack or the other but not both at the same time, Both jacks will have close to a 16-ohm load with one jack having the two Celestion speakers running in series and the other jack having only one of the Fender 16 ohm speakers connected and the other will just be a disconnected dummy.

    Like I can plug my amp into this...

    [​IMG]

    or this...

    [​IMG]

    Either one will be a 16-ohm load...
     
    Last edited: Aug 11, 2019
  11. AxRookie

    AxRookie Strat-Talk Member

    Age:
    56
    Messages:
    90
    Joined:
    Jun 14, 2019
    Location:
    Las Vegas, Nevada
    On closer look of the manual, it turns out it's a 8 ohm amp that can use a 16 ohm load so I'm going to wire the 16 ohm speaker pair in parallel on on jack for a 8 ohm load and on the other jack I'm going to use only one of the 8 ohm speakers for a 8 ohm load and maybe wire in a switch to put the other 8 ohm speaker in series for a 16 ohm load with the switch flipped...

    [​IMG]
     
  12. AxRookie

    AxRookie Strat-Talk Member

    Age:
    56
    Messages:
    90
    Joined:
    Jun 14, 2019
    Location:
    Las Vegas, Nevada
    I've got the switch mounted in the jack plate and I've got the wiring fig out to switch between one 8 ohm speaker OR two 8 ohm speakers ran in series for 16 ohms...

    [​IMG]
    [​IMG]
    [​IMG]
     
  13. AxRookie

    AxRookie Strat-Talk Member

    Age:
    56
    Messages:
    90
    Joined:
    Jun 14, 2019
    Location:
    Las Vegas, Nevada
    I was thinking today about what I could do with the unused side of switch I added to the jack plate...

    [​IMG]

    and it hit me, I'll use it to disable the other jack if the switch is flipped for a 16 Ohm load that way even if both jacks were plugged into an amp the amp will only ever see a 16 Ohm load.

    In that way the cab can only ever be a 16 Ohm load max OR an 8 Ohm load min no matter what is plugged into it...
     
  14. AxRookie

    AxRookie Strat-Talk Member

    Age:
    56
    Messages:
    90
    Joined:
    Jun 14, 2019
    Location:
    Las Vegas, Nevada
    I have all the jack plate wiring done, I used much heavier gauge wire...

    [​IMG]
    [​IMG]

    And my kit of new wire connectors for connecting up the wires to the speakers came today, Good quality connectors with nice soft silicone jackets!

    [​IMG]

    Now to add all the speaker wire connector to the wires and reassemble the cab...
     
  15. AxRookie

    AxRookie Strat-Talk Member

    Age:
    56
    Messages:
    90
    Joined:
    Jun 14, 2019
    Location:
    Las Vegas, Nevada
    The cab is all done...

    [​IMG]

    And I plugged in the speaker cable and tested the Ohms load with my handy Ohms meter...


    [​IMG]

    first on the side with the 8 Ohm / 16 Ohm switch and they work just how they were supposed to!

    [​IMG]
    [​IMG]

    Then I plugged into the other side and it works to!

    [​IMG]

    And the cab will still work with my Crate G1500 which can handle a 16 Ohm / 8 Ohm load and even a 4 Ohm load!

    [​IMG]
    [​IMG]
     
    Dudeman7 likes this.
  16. Dudeman7

    Dudeman7 Strat-Talk Member

    Age:
    38
    Messages:
    42
    Joined:
    Jul 21, 2019
    Location:
    Around
    Awesome! Interested to hear how you like the sound.
     
  17. Groovey

    Groovey Most Honored Senior Member Strat-Talk Supporter

    Messages:
    7,051
    Joined:
    Nov 17, 2016
    Location:
    WV. USA
  18. AxRookie

    AxRookie Strat-Talk Member

    Age:
    56
    Messages:
    90
    Joined:
    Jun 14, 2019
    Location:
    Las Vegas, Nevada
    I'm on to the next thing which is how I'm going to power my n-tune onboard chromatic tuner that I want to install on my new guitar that's being built right now, It's powered by a 9 volt battery which there is no place to put it without modding the guitar which I don't want to do so I came up with this little setup using one very small Li-Po battery and a tiny step-up voltage regulator that takes the Li-Po's 4.0 volt and outputs 9 volts and I know I can find someplace to put them in the controls cavity...

    [​IMG]

    The other tiny board is the battery charger that plugs into any USB outlet and you plug in the little connector to charge the battery at 0.5 amps which should be fairly quick...
     
  19. Triple Jim

    Triple Jim Senior Stratmaster

    Messages:
    3,345
    Joined:
    Feb 27, 2018
    Location:
    North Central North Carolina
    Will you have a switch between the battery and converter so its idle current doesn't drain the battery when not in use?
     
  20. AxRookie

    AxRookie Strat-Talk Member

    Age:
    56
    Messages:
    90
    Joined:
    Jun 14, 2019
    Location:
    Las Vegas, Nevada
    Yes, it's part of the volume pot, you pull the volume knob out to engage the tuner and the same time it mutes the output and engages the power.

    Right now I'm running A test to see how long the battery can be connected to the reg before the battery is drained to the minimum Li-Po safe voltage of 3.0 volt, So far it's looking very good!

    I'm using a slightly larger battery than pictured with the former being a 100mA and the current being a 300mA battery...
     
    Last edited: Aug 19, 2019 at 3:33 PM
    CB91710 and Triple Jim like this.
  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.