I need to be ohm schooled

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cheameup

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Heya

So I have my 3 channel DR which I use a 2 x 12 recto cab

I have two 16 ohm vintage 30s from my old amp that's fried. Got a guy building a cab for me to put the old speakers in so I can use 4 x 12 when needed

My best option for ease would be to just use the 2 8 ohm outs on the head. Easy to setup and not mess up

Is that doable with series or parallel wiring or will the 16 ohms need a different ohm rating ? Obviously Im a bit retarded when it comes to ohm's and cabs. everything I read just confuses me more

I think the 2 x 16 ohm speakers in parallel will use the 8 ohm out but not totally sure
 
You are correct, two 16 ohm speakers wired in parallel will yield an 8 ohm total load.

When you run two 8 ohm cabinets together you need to plug them into the 4 ohm jacks. The speaker jacks (both 4 and 8 ohm) in the amp are wired in parallel. When you plug in two 8 ohm cabinets they will combine in parallel for a total of 4 ohms. If you had two 16 ohm cabinets you would plug them both into the 8 ohm jacks.

Dom
 
Ohm Schooled (since you asked):

Here are the formula's to calculate total speaker cabinet impedence:
(R = speaker impedence in ohms)

For SERIES wiring
1R = R1 + R2 + R3.... Simply add the values.

Ex.1
If you were to wire your two 16 ohm speakers in series the formula to find "1R" is:
16 + 16 = 32 ohms (1R = 32)


For PARALLEL wiring
1R = 1/R1 + 1/R2 + 1/R3.... The total impedence of a set of speakers in parallel is found by adding up the reciprocals of the impedence values, and then taking the reciprocal of the total (we're going to make fractions, add them up, and flip over the answer) * .

Ex. 1
With your 16 ohm speakers wired in parallel the formula to find "1R" would look like this:
1/16 + 1/16 = 2/16, reduce to 1/8, flip to 8/1 = 8 ohms (1R=8).

Ex. 2
If you had four 16 ohm speakers the formula to find "1R" would be:
1/16 + 1/16 + 1/16 + 1/16 = 4/16, reduce to 1/4, flip to 4/1 = 4 ohms (1R=4).

Ex. 3
If you had two 16 ohm and two 8 ohm speakers the formula to find "1R" would be:
1/16 + 1/16 + 1/8 + 1/8 = 6/16, reduce to 3/8, flip to 8/3 = 2.66 ohms (8 divided by 3 = 2.66, or 1R = 2.66)

* When dealing with identical impedence values in parallel you can shortcut the formula, simply take the ohms of one speaker and divide it by the number of speakers.

Ex. 4
If you had four 16 ohm speakers the formula is:
16 ohms divided by 4 = 4 ohms (notice the result is the same as Ex. 2 above)

Ex. 5
For your cabinet with two 16 ohm speakers wired in parallel the formula is:
16 ohms divided by 2 = 8 ohms

BTW- this all goes for resistors as well.

Dom
 
domct203 said:
You are correct, two 16 ohm speakers wired in parallel will yield an 8 ohm total load.

When you run two 8 ohm cabinets together you need to plug them into the 4 ohm jacks. The speaker jacks (both 4 and 8 ohm) in the amp are wired in parallel. When you plug in two 8 ohm cabinets they will combine in parallel for a total of 4 ohms. If you had two 16 ohm cabinets you would plug them both into the 8 ohm jacks.

Dom


Ahh. Thanks

That's what was confusing me when i was reading up other threads . Having to plug the 8 ohm cabinets into the 4 ohm outputs




Would plugging the 2 cabs into the 8 ohms do damage to the amp or just not sound right ? Cause I did it that way the other day for about 20 mins to test with a mates cab .hopefully no damage is the answer :oops:
 
If you didn't have the amp cranked right up, no harm will be done. It might have sounded a bit softer and more compressed than normal, that's all.

If you did crank it, you might have shortened the tube life just a tiny bit. Still not much to worry about.
 
Nah I didn't crank it. It was a bit softer and less defined than usual Knew something wasn't right . It was where it should be loud as he'll but wasn't quite there and it felt like it lacked a bit of guts

Thanks for the info guys
 
Tubes tend to be pretty forgiving about impedance within a certain range. Putting a 4 ohm load on an 8 ohm output won't hurt things too bad. Putting an 8 ohm load on a 4 ohm output won't either.

Solid state amps don't like to be UNDER loaded (4ohm load on an 8ohm output) and tube amps don't like to be OVER loaded, but within a factor 2..............tubes don't care.
 
kick6 said:
Tubes tend to be pretty forgiving about impedance within a certain range. Putting a 4 ohm load on an 8 ohm output won't hurt things too bad.

+1

You will just wear out the power tubes quicker.

kick6 said:
Putting an 8 ohm load on a 4 ohm output won't either

That's a well known "mojo" to change the tone and response of a speaker cabinet. I believe this will actually reduce the load on your entire output section.

Dom
 
so following from all this



If running 2 x 8 ohm cabs and you wanted 1 to be more dominant in the mix than the other could you run a mismatch with the louder one at 4 ohms and the softer at 8 ? would that do damage or hammer your power tubes in the long run ?

I ask cause the 2nd cab I'm getting is open back for now with a view to change it to closed if I want . Wanna see if the open back adds a bit more openness to the high mid /treble to the mix but I'd like the mesa recto cab chunk to be the bulk of the tone in the room with the other complimenting it
 
cheameup said:
so following from all this



If running 2 x 8 ohm cabs and you wanted 1 to be more dominant in the mix than the other could you run a mismatch with the louder one at 4 ohms and the softer at 8 ? would that do damage or hammer your power tubes in the long run ?

I ask cause the 2nd cab I'm getting is open back for now with a view to change it to closed if I want . Wanna see if the open back adds a bit more openness to the high mid /treble to the mix but I'd like the mesa recto cab chunk to be the bulk of the tone in the room with the other complimenting it


You're going to want to connect the cab you want to be quieter to a 8ohm output, and the one you want to be louder to an 4ohm output. This shouldn't really hurt things.

It sounds backasswords, but the research I've read suggests that tubes decrease their power output when their impedance load is both less AND more than expected, unlike a solid state amp where the power output only drops as the loud increases.

If that doesn' work, reverse the two cabs.
 
thats what i thought .

though This shouldn't really hurt things. isnt really what i'm keen on . rather hear "def won't hurt things "


anyways I dont have the extra cab yet . it'll be finished in the next day or so for weekend studio rehearsal. Hopefully the mix sounds great with both an 4 ohms and I dont have to worry about it
 
Sorry - not trying to hi jack a thread - but I do have a question... I have a Marshal 1960 lead cab.. it's wired I think for stereo and mono with switch - has numbers all over the place - 4 ohm on top left over left jack (for mono I believe) and 16 ohm on top right of right jack (mono as well) the both bottom corners of jack has 8 ohms, which I assume is 2 of the 12 inch speakers running in stereo? I also have a fender 4x12 cab that handles 300 watts at 8 ohms... first, what is my best option for the best sound from my Marshall cab - 2nd - can I hook both cabs up at same time, with mismatched loads? Not too knowledgeable here about that - appreciate any help. -
 
The Marshall cab with the switch set to mono has two wirings - the left jack is the 4ohm wiring, and the right one is 16ohm wiring. You can run your DR from the 16ohm output into the 16ohm (I think it has a 16ohm output, yes?). If there is no 16ohm, run it from 8ohms, that's a safe mismatch.

If you flip the switch to "stereo" it has a left and a right half which are both 8ohms.

And yes, you probably could hook both cabs up but I do not know how the jacks on the DR are wired so I can't know how the final impedance would calculate... I'd check if someone had a similar question.
EDIT according to the MESA manual, you can hook up a 8ohm and 16 ohm cab by using the 4ohm output for the 8ohm cab and the 8ohm output for the 16ohm cab..
 
cheameup said:
thats what i thought .

though This shouldn't really hurt things. isnt really what i'm keen on . rather hear "def won't hurt things "


Due to the fact that tubes are analog, its hard to give a 100% assured answer. I will go so far as to say that I wouldn't concern myself with it if I was doing the smart thing, and was carrying a spare pair of power tubes when gigging.
 
Thanks for the help - I have a triple rec, not thinking volume is an issue..lol - appreciate the help. I have heard that the marshall cab and a recto cab together make a great combo, but I am a little light with the cash at the moment after buying this amp and putting a pedal board together...
 
kick6 said:
Tubes tend to be pretty forgiving about impedance within a certain range. Putting a 4 ohm load on an 8 ohm output won't hurt things too bad.

I don't completely agree. It's not so much the tubes you have to worry about, it's the output transformer. It is generally not a good idea to have a speaker load that is LESS than the output you are using. We all know you should never run a tube amp without a load, and running it into a smaller load than it is expecting can produce a similar negative effect. Maybe 8>>4 isn't "too bad", and I can't say you would definitely fry your amp by doing this, but I wouldn't try it unless you are OK with that being a very real possibility.

kick6 said:
Putting an 8 ohm load on a 4 ohm output won't either.

This is totally fine, and people do it all the time. I've heard it may wear out your tubes faster, but it shouldn't have an impact on your transformer. You have a bigger speaker load than the amp is expecting to see. This may make things sound a little darker, and can make tone controls, etc., a little less responsive, but some people like that.
 

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