Ohms and Explosions

  • Thread starter Thread starter kdawg
  • Start date Start date
K

kdawg

New member
I just blew up my Traynor mkII twice this month pluging it into different cabinets. I KNOW I'M STUPID. what's the rule with heads and cabinets? can I plug a 4 ohm head into an 8 ohm cabinet? or can I plug a 8 ohm head into a 4 ohm cabinet? or neither. I guess thats the problem with buying used gear (no manuals)

-thanx
 
4 ohm head into 8 ohm cabinet should be OK.

8 ohm head into 4 ohm cabinet is a recipe for toast.
 
MegaOhms!!

AN ohm is a unit of electrical resistance (or impedence) the more ohms something is rated, the more it resists electrical current.

If you plug an 8 ohm head into a 4 ohm cabinet, the cab will suck out more power than usual, about 2x more!! So your 30 watt amp becomes a 60 Watt amp, only it not made to handle that much power, so all your electrical components dissapate tons of heat and eventually you fry your head.

There are ways you can double up two cabs to create the rating your are looking for - two 4ohm cabinets could create an 8ohm cab, and even two 8ohm cabs could become a 4ohm cab, or you could do some rewiring in a multiple speaker cab to create the impedence level you will need but that gets kinda technical. You could also add a high-power 4ohm resistor to your cab and effectively make it an 8ohm cab- see a guitar tech for help on any of that, though.
 
http://www.colomar.com/Shavano/spkr_wiring.html

Some amps will handle impedence mismatches better than others (Fender). It also depends on how hard you're driving the output stage of the amp. This is why Marshalls (and many other amps) have an impedence selector switch, or separate jacks for different loads (Boogie).

It's best to run the correct load. Otherwise you can cook your amp, and possibly even the iron (output transformer). That's an expensive repair.
 
Re: MegaOhms!!

gusfinley said:
If you plug an 8 ohm head into a 4 ohm cabinet, the cab will suck out more power than usual, about 2x more!! So your 30 watt amp becomes a 60 Watt amp....
Click to expand...
While a 4 ohm cab will draw more current than an 8 ohm cab, you will actually get more power from the speakers if the impedances of the cab and the amplifier match.

Because of the amplifier's resistance, there is power being used up by the amplifier as well as the speaker... So when you use the 4 ohm cab, the current increases, but the voltage being dropped across the speakers decreases. And since power = current x voltage, if the voltage decreases more than the current increases, then you are going to get "less" power from the speaker cab.

Hope that helps. :)

-tkr
 
....Sorry about that sloppy reply, but we were getting ready to watch a movie, so I had to wrap that post up rather quickly. :D

But here's an example that may help explain it better, just using some random numbers...


Say the amp puts out 10V and the amplifier's resistance is 8 ohms...

With an 8 ohm cab attatched:
Current = 10V/(8ohm + 8ohm) = 625mA
Voltage drop across cab = 625mA * 8ohm = 5V
Cab power = 625mA * 5V = 3.13watts

With a 4 ohm cab attatched:
Current = 10V/(8ohm + 4ohm) = 833.33mA
Voltage drop across cab = 833.33mA * 4ohm = 3.33V
Cab power = 833.33mA * 3.33V = 2.78watts

With a 16 ohm cab attatched:
Current = 10V/(8ohm + 16ohm) = 416.67mA
Voltage drop across cab = 416.67mA * 16ohm = 6.67V
Cab power = 416.67mA * 6.67V = 2.78watts


So as you can see, you have the most power at the speakers when the cab and the amp are matched.

Hope that helps a little more. ;)

-tkr
 
The specss for most amps will state what they can and cannot handle in load. You can usually find specs online for most gear.

Most amps will do 4 and 8 ohm loads. Many amps will not handle 2 ohm loads. As others have already said, matching default output load to actual load usually produces the best results.

It's also the case that some amps are a little generous with their specs. If you drive them too hard, even with correct loads, they'll quit. Over the years I've had more problems with speakers than amps, but your mileage may vary.

Most of the gear I have came from EBay. Most came with no manual, but I found manuals online for most at no cost. Google is a great tool.

Ed
 
Tube amps, and solid state are two different animals when it comes to output loads.

Solid state amps don't really care what the actual load is as long as it's within the amp's design parameters i.e not too high or too low.

Tube amps are much more sensitive. The function of the output transformer in a tube amp is much like a transmission in a vehicle. The tubes have a very narrow range of operation, and the transformer's job is to match that to the speaker load.

Drive your car at 70mph in first gear for a while and see what happens. :D
 
You must log in or register to reply here.
Back
Top