Speaker question

[masterofsparks]

Please pardon the newbie question, but I'm doing a speaker change in my IIC+ but have never done one before. Could someone let me know what I need to do to make the change? From looking at the amp it looks like I need to go through the front. Is that right?

Also, I know that if I have a 15 watt speaker wired in series with an 80 watt speaker, the 15 watt speaker will be o.k. with the amp's 75 watts. If I put a 60 watt speaker in the combo and run it with the same cabinet at the same time, is there still a danger of blowing the speaker in the combo? As a practical matter, I'll never run it loud enough to blow it, but just in case that Madison Square Garden gig comes through. . .

Thanks in advance.

Scott

 

[stokes]

You say "wiring the speakers in series",but just remember that if you plug into the output jacks,either the 4ohm and 8ohm or 4ohm and 4ohm,your speakers are now in parallel,not series.I think your wattage would be okay but I cant say for sure each speaker is going to react the same.You have to consider the efficiency of each and if you mix the rated impedence you are not going to get equal power to each.You are right about mounting the speaker,it mounts thru the front of the combo.Why are you changing the combo speaker?It isnt clear as to exactly what you are trying to achieve.

 

[masterofsparks]

Right. I think I get it about the speakers being in parallel, but wouldn't I still need to wire the 2x12 cab in parallel so the two 16 ohm speakers in the cab will be 8 ohms?

I was changing because I wasn't crazy about the sound of the EV that was in there. Found out why when I took it out - there was a one inch tear in the cone :roll:

I'd like to run the combo speaker along with the extension cab simultaneously. Will it work?

Thanks again.

Scott

 

[stokes]

Okay,you never mentioned the fact that you were using a 2x12 ext.cab with 2 16ohm speakers.Yes the 2x12 has to be wired in parallel to get 8ohms to match your 8ohm speaker in the combo.In your first post you made reference to the extra spkr being 15watts.If what you are now saying is that each speaker in the ext.cab is 15watts then that cab is now capable of handling 30watts.Added to the wattage of the spkr in the combo it should be able to handle the wattage the amp puts out.Unless you have a 100watt version of the IIC+.If it has 4 power tubes it is the 100watt version.In a 100watt Marshall cab they used 4x 25watt speakers to get the 100watt rating,but they are using 4 equal speakers,I cant say for sure offhand if mixing different watt spkrs if you get a sum of the 2 wattages or if you are going to get an average between the 2 power ratings.I do know if the speakers are different impedence ratings,say a 4ohm and an 8ohm,the 4ohm will get twice the power since it is half the "resistance".As I said before you probably have to figure the efficiency of each speaker into the formula as well.

 

[masterofsparks]

Actually, my mistake - the two speakers are each 8 ohms (it was a 16 ohm cab - they were wired in series - think that's what confused me) so I'll wire them in parallel to get a 4 ohm load. The speakers are rated at 80 watts (G12H80) and 15 watts (Alnico Blue) and the cab speaker is 60 watts (Vintage 30) so I think I'm o.k. power wise. I'm a little confused with the wiring diagram I found explaining how to wire in parallel, though. Can't tell if there are one or two wires coming off each of the jack posts (I'm sure that's not the correct term. . .).

 

[stokes]

For parallel wiring the + on one spkr goes to the + on the other spkr.,the - goes to -and then connect one speaker to the output on the amp.But this will give you 4ohms,if your other speaker is 8ohms,you havent made that clear,you will get more power to the 4ohm as there is less impedence,in other words the 4ohms doesnt impede the current flow of your outputas much as the 8ohm.Current will always take the easiest path,one of the basic electronic principles.Also if that is the case,that you are mixing 8 and 4ohm,when you connect them both in parallel you are going to get an implied impedance of like 2.6ohms,that may not be good for your power tubes and your OT.Even if it is two 4ohm loads I wouldnt think it is a good idea that will give you 2 ohm load.The two 4ohm jacks are for using two 8ohm loads.The tear on your EV should be fairly easy to repair,depending on where it is,I have seen spkrs with a tear repaired with a piece of tape and sound okay,depends where the tear is,and if there is no paper missing.Try smearing a little glue over the tear to close it up.might work,got nothing to lose.

 

[masterofsparks]

I'll give it a shot. BTW, both speakers are 8 ohms so I think I'm good to go. Can I run a 4 ohm load and an 8 ohm load at the same time since the two 4 ohm jacks are for two simultaneous 8 ohm loads? Would I use a 4 ohm jack and an 8 ohm jack? I would think so, but I don't want to damage anything.

 

[stokes]

Assuming your spkr in the combo is 8ohm and your ext cab is 4ohm,I would say this could be a bad thing.If you look at the schem.the 8ohm tap is at the end of your secondary winding,the 4ohm is tapped earlier in the winding.As I explained before if you ran the two mismatched loads from the same tap the 4ohm would get twice the curent your 8ohm would,and you would be running a 2.6 implied load.This is halfway to a dead short,and would cause your tubes to run hotter.By putting the 8ohm load on the 8ohm tap and the 4ohm load on the 4ohm tap,you would get even less power to the 8ohm load since the current will always take the easiest path,the 4ohm load "impedes" the current half as much,and is now at a point before the 8ohm load on the secondary.See?I would not mix the spkr.impedences,although it might not cause an immediate failure over time it wont be good for your tubes or your tranny.

 

[Ken j]

On top of the impedance mismatch if you run all three of those in parallel the most I would feel comfortable pumping into them woul be 45 watts. The 15 watt speaker isn't going to shut itself off when it gets to its limit. If they were all the same resistance the power would divide equally between the 3 speakers. Once you got to 45 watts you would be over driving the 15 watt speaker. If one is 4 ohms and the other two are 8 ohms the 4 ohm speaker will probably get to its max much sooner. This is a poor mix. You would be better off just running the two 12" speakers until you can fix the EV.

 

[masterofsparks]

I was on my way to my tech to make sure I was doing the wiring correctly and figured I'd give the guys at Mesa a shout. He suggested keeping the cab at 16 ohms and running it through the 8 ohm jack and running the combo speaker through the 4 ohm jack. Apparently not the best, but it also won't damage the amp.

Maybe the best idea is to get a couple of 16 ohm speakers in the cab, run them in parellel, and use both 4 ohm jacks for the cab and the combo speaker. . .

Thanks again for all the help.

Scott

 

[Ken j]

The older Mesa product manuals don't address mismatched cabinet loads but the newer ones do. Go to pages 34 through 39 here: http://www.mesaboogie.com/manuals/RoadKing2_v2.pdf this should help you understand what you have to do.

 

[stokes]

Read the second paragraph on page 34.Running the 4 and 8ohm in parallel will give you 2.6ohm implied impedance,it tells you right there not to do it.If you read further the only mismatch combination they recomend is 8ohm and 16ohm which will give you 5.3ohm parallel load.If you measure the dc resistance of an 8ohm spkr it will never read 8ohms it always reads between 5 and 8 ohms.So a 5.3 would be safe,but a 2.6ohm is not.As I said before I would not run the 8 and 4ohm loads together.There is probably a good reason the older manuals,and your IIC+ is an older amp,dont mention load mismatches.It is always safe to mismatch an output with a higher load i.e. 16ohm load in an 8ohm output or 8ohm load to a 4ohm out,but running the mismatch in the other direction will make your tubes run hotter and can cause your OT to fry.Even if Mesa says it is okay to do it,I wouldnt.

 

[masterofsparks]

Thanks for the lead on the manual. I left the wiring the same on the cab so it's still a 16 ohm load. So it sounds like I'm safe running the 16ohm load into the 8 ohm out and the 8 ohm load into the 4 ohm out.

Thanks much and I'll be sure to give the manual a read.

 

[masterofsparks]

Thanks for the lead on the manual. I left the wiring the same on the cab so it's still a 16 ohm load. So it sounds like I'm safe running the 16ohm load into the 8 ohm output and the 8 ohm load into the 4 ohm output.

Thanks much and I'll be sure to give the manual a read.

 

[Ken j]

Here is the link for your amp. There is a single paragraph in the 6 page manual for it.
http://www.mesaboogie.com/manuals/Mark%20II-C%20Manual%20-%20Maintenance%20&%20Repair.pdf

 

[Ken j]

If you measure the dc resistance of an 8ohm spkr it will never read 8ohms it always reads between 5 and 8 ohms.

Speaker resistance is a nominal measurement. The resistance changes with the frequency of the signal.

 

[stokes]

Yes this is true,If you read the posts you will see where I refered to the implied impedance,same thing.But this doesnt mean you should ignore this"nominal" measurement.It is and has been an industry standard for a reason.A 2.6 load is not safe to use,It even says so in the manual you posted,second paragraph,page 34.

 

[Ken j]

stokes,

I am in total agreement with you. I just want to make sure it is explained well to masterofsparks before he creates a smoke generator of his 15 watt speaker or worse yet his OT and or power tubes.

 

[stokes]

Sorry Ken,just wasnt clear which way you were going with this.These threads get so long,sometimes you need to keep a scorecard to remember the players,if you dont go back and re-read.Looks like he went back to the 16 and 8ohm combo.Might be safe,but it aint gonna be the best,tonewise.I'm with you,better to get 2 matching loads.And that 15watt spkr is definately questionable.Even though Marshalls have 4X25 or 30 watt spkrs in some of their 100watt cabs,the power is delivered evenly between them as they all match in power and impedance.MIxing these values involves way too much math to prove or disprove,better to go with what has already been proven to work and leave the math to the engrs.