Speaker mismatch

[pmb]

Hi guys
Probably next week I 'll be the owner of a mark III-head
Now the problem is I still have my old cabinet (peavey 5150-cabinet)
It is 16 ohms while the mark III only has outputs for 4 and 8 ohms
When I was looking at the user manual for the mark III it said the amp wouldn't be very sensitive to speaker mismatches
But still
I'm very carefull with a great amp like a mark III
So I would like to ask if anyone knows if it would be bad to use that cabinet
Thanks Frederik

 

[stokes]

Plug the 16 ohm cab into the 8ohm output it will be fine,as long as you like the sound.You will get a slight loss in output watts,but it will be hardly noticeable.Your power tubes will run cooler and therefore last longer.I have a IIC+ combo that I have been running for years with the 8ohm speaker in the 4ohm tap,can hardly notice any difference.

 

[igfraso]

If I recall properly, MB told me that if you run a 100/60 amp in 100 watt and you plug an 8 ohm speaker into a 4 ohm jack, the sound of the amp will be closer to a SimulClass.
Regards

 

[pmb]

Thanks for the replies
Maybe later on I will let someone change my cabinet
But I'm not sure it is possible
I think the speakers are 8 ohms but I 'm not sure
I'm so bad at impedances

 

[stokes]

Is it a 2 or 4 speaker cab.If it is a 4 spkr cab they are most likely 16ohm spkrs,if it is a 16ohm cab.If that is the case you could wire them in pairs in parallel with 1 jack for each pair.Each jack would then go into the 2 4ohm outputs on your amp and you would have a match.It really isnt necessary.I would just pit the 16ohms in the 8ohm output jack and be done with it.

 

[Paul Secondino]

Hi guys
Probably next week I 'll be the owner of a mark III-head
Now the problem is I still have my old cabinet (peavey 5150-cabinet)
It is 16 ohms while the mark III only has outputs for 4 and 8 ohms
When I was looking at the user manual for the mark III it said the amp wouldn't be very sensitive to speaker mismatches
But still
I'm very carefull with a great amp like a mark III
So I would like to ask if anyone knows if it would be bad to use that cabinet
Thanks Frederik

A good rule to remember is always put a equal or "greater" load on your amp. A tube amp's output transformer wants to see "atleast" an equal load or a greater load

If you ever forget, use the muscle car analogy. In the transmission, the lowest gear supplies very little resistance to the power of the motor. The highest gears provide "more resistance. So if you nail the throttle and leave the car in 1st gear, it wont take but a few seconds to over rev the motor and hurt something. Got to shift to a higher gear to provide more resistance to the motor.

Hope that helps.

 

[pmb]

I found this somewhere else on the forum
And as far as I understand this guy says my tubes won't last as long when I put my 16-ohmspeaker in the 8-ohmoutput
This is the opposite of what someone in this thread told me
Greets Frederik

You can run a 16 ohm cab through the 8 Ohm tap. It won't damage the amp, but will wear the tubes a tad quicker. The term "Cheap" and "Good Tone" usually do not coexist. I would say a Mesa 2X12 or 4X12 standard would be the best match. A great alternative would be the G-Flex 2X12. As far as balancing the three channels, you need to try different settings and compromise on what works for you and realize that Mesa has addressed this concern with the MK IV. Very Happy

 

[stokes]

Think of it this way.If you have an 8ohm jack on the amp,ideally the tubes wants to see an 8ohm load.I fyou connect a 4ohm load your power tubes are seeing closer to a dead short therefore they will run hotter.If you put a 16ohm load on the 8ohm tap your tubes are working further from a short and will run cooler.If in doubt,call Mesa they will tell you the same.

 

[pmb]

I really believe you
I just don't understand what the other guy was saying actually

 

[Stu Gutz]

Think of it this way.If you have an 8ohm jack on the amp,ideally the tubes wants to see an 8ohm load.I fyou connect a 4ohm load your power tubes are seeing closer to a dead short therefore they will run hotter.If you put a 16ohm load on the 8ohm tap your tubes are working further from a short and will run cooler.If in doubt,call Mesa they will tell you the same.

According to Kevin O'Connor (he wrote several books on tube guitar amps), this is not true. Mismatches of loads the output transformer sees as compared to nominal load reduces the output power and bandwidth only. He says: "Tubes are self regulating power deliverers. They will not try to push more than a set amount of power into any connected load".

(I think) what you said applies to transistor output sections, not tube output sections.

 

[pmb]

At first I also thought it was different with tubeamps and solidestatemps
But in the user manual of the mark III they also say the same as stokes

 

[stokes]

So if you connect to say a 2ohm load you think the tubes are gonna last as long?Every body has opinions.Gerald Weber,who also wrote a couple books thinks otherwise.I spoke to someone at Mesa back in the 1980's trying to lower the volume on the amp and was told this was the case.I have a friend who designed tube circuits for RCA back in the '40's & 50's and he told me the same.Bottom line is that it is safe for your amp,it will change the tone and power output somewhat,try it, if you like it,go for it.Have had my 8ohm speaker in the 4ohm tap for over 20yrs and it is fine.Right now I have a pair of nos 7027A's in there for 2 years,both my sons gig with this amp regularly and the tubes are still strong.I compared them against a NOS pair of Genelex KT66's I have been holding for that amp till the 7027's die and they still sound great.

 

[stokes]

I dont touch SS amps but my understanding is that transistors cannot tolerate an impedence mismatch at all.

 

[Stu Gutz]

When there is a load mismatch some power will be reflected back ( I had electronics training about 25 years ago but I only remember bits and pieces). I'm not sure if the output transformer is the device that absorbs the heat or if it's the output tubes; or maybe both.

With respect to solid state output sections not being able to tolerate a load mismatch; I think that depends on the design. A friend of mine claims he had a Bryston amp that had a speaker cable develop a dead short; apparently the amp was OK after having this short for over a week, but in a general sense you are right ... if a transistor is asked to drive more current than it can handle it will open up.

 

[RR]

Why not rewire your peavey 5150-cabinet to 4 ohms.

I get the impression your peavey 5150-cabinet has 4x12" (or is it a 2x12"?) and its using a series / parallel wiring configuration. Wire all the speakers in parallel and you will get a 4 ohm impdance. Plug into your 4 ohm speaker jack.

 

[pmb]

I'll probably get it rewired after a little while
It's just that I can't do it myself and I don't drive a car yet so it's not
that simple to get my cabinet to someone who can rewire it
Thanks for the answers and I think I believe stokes
Thanks guys

 

[4Draw]

I'll probably get it rewired after a little while
It's just that I can't do it myself and I don't drive a car yet so it's not
that simple to get my cabinet to someone who can rewire it
Thanks for the answers and I think I believe stokes
Thanks guys

Great news Neuze. 8)

 

[kylek]

the cabinet is already wired in series parallel as mentioned above

it will be FINE to play the 16 ohm cabinet plugged into the 8 ohm jack.

the other way around, an 8 ohm cab into a 16 ohm jack will fry your amp.

There should always be a GREATER resistance (i.e. higher ohm number) load on the cabinet than on the jack when using an ohm mis match

Think like this... with it being plugged into the 8 ohm jack, because the cabinet has more resistance, less current will flow to it... If you mis match the ohms and have the cabinet with less resistance than the jack, the amp will send too much current into the cabinet and fry the amp much like a short as mentioned above

to find resistance

wired parallel = ohm of speakers squared/sum of speaker ohms... i.e.

16 ohm speaker.. (16x16)/(16+16) = 256/32 = 8

series = ohm of speaker 1 + ohm of speaker 2

16 ohm speaker = 16+16 = 32 ohms of resistance

parallel = (-'s) of all speakers wired together and (+'s) all wired together
series = (-) to (+) wiring

 

[RR]

 

[pmb]

I got a mail from mesa/boogie and they told me it would be okay to plug it in the 8-ohmoutput
But they also said it would give a little difference in sound
So for the moment I can already start playing my new amp
I can always rewire it afterwards
By the way I went to by the amp today
I can't wait for our band rehearsal this saturday
I'll probably blow away the other guitarist
The amp has got six poweramp-lamps
Does this mean it's what the call a colliseum