Safe to run an 8ohm head into 16ohm cab?

[Gasp100]

I never did get a really good answer on this one. If I get an 8ohm head I don't really feel like rewiring my 16ohm cab so is it safe to run it this way (pretty tame volumes all of the time)?
If I had to rewire I could just change it from series (2 - 8 ohm speakers in series = 16ohm) to paralell (2 - 8 ohm speakers = 8ohm), right?

[doublescale1]

The short answer to your question is yes - But this longer piece will answer your questions in depth - I refer to it often, since I don't retain this stuff in my active memory - a good reference piece: All About Ohms...

Ohms:
A value of electrical resistance. The lower the number, the less resistance. The higher the number, the greater the resistance.

Speakers are rated at certain ohms (or impedance), most commonly, 4, 8, and 16.

Your amplifier is also rated at a certain impedance, or has multiple taps for speaker configurations. It is important that you match
these numbers up.


EXAMPLE --- 8 ohm cabinet--->8 ohm jack on amp
EXAMPLE --- 4 ohm cabinet--->4 ohm jack on amp
EXAMPLE --- 16 ohm cabinet--->16 ohm jack on amp

It is typically safe to mismatch one step higher


EXAMPLE --- 8 ohm cabinet--->4 ohm jack on amp.
EXAMPLE --- 16 ohm cabinet--->8 ohm jack on amp.


Now for the do-not do's.

NEVER EVER hook up a cabinet with a lower ohm rating than the head.

EXAMPLE ---4 ohm cabinet--->8 ohm jack on amp...THIS IS A NO-NO!!!

You will cause your output transformer to overheat, and eventually, it will blow.

NEVER - Turn on a tube amp without a speaker load attached to it. This can cause an output transformer to blow in a short time.

The only instance where it would be safe to turn on a tube amp without a load would be on amps that have a standby switch (Class AB typically) This would only be for visual diagnostics (tube heater check, bulb check, etc) BUT, DO NOT take it off of standby. While it's in standby, only the tube heaters are working, and will not cause damage to the output transformer.

Also, NEVER use instrument cable to hook up a speaker cabinet. The speaker jack can put out a high enough voltage to burn the small wire in instrument cable, thus leaving the amp without a load, and frying the output transformer. ALWAYS use a good heavy speaker cable.



General Speaker Info

Sensitivity - This number indicates how efficient the speaker works. Generally, this number will be between 89 and 105. If you replace a speaker with one of a higher sensitivity rating, you will also increase the apparent volume.

Generally speaking, an increase of +3dB will double the overall loudness of the amp.

Wattage - How much the speaker can handle. Smaller, combo amps usually have a speaker rating close to the rating of the amp. If when cranked, you don't like the sound of the speaker breaking up, then you might look into changing it out with one of a higher wattage. Some like the way a speaker sounds while being pushed to it's limits, and some don't. It's all in your personal taste.

Wiring up multiple speakers.

When wiring speakers, there are three ways to do it

SERIES: one wire from the input jack to the "+" terminal of one speaker, from the "-" terminal of the same speaker to the "+" of the next speaker, last wire from the "-" terminal to the input jack. When wiring speakers in series, multiply the ohms by the number of speakers used.
Also, when one speaker blows, it will cut power to the other speaker as well, leaving both speakers not working.

EXAMPLE: jack------"+ '8ohm' -" ----"+ '8ohm' -" ---jack=16ohms

8x2 = 16

PARALLEL: One wire from the input jack to the "+" terminals of both speakers. Other wire from the input jack to the "-" terminals of both speakers. When wiring speakers in parallel, divide the ohms by the number of speakers used.
In this case, when a speaker blows, the remaining speaker will still work.

EXAMPLE:
jack----"+ '8ohm'"------------ "+'8ohm'"
jack----" - '8ohm'"-------------"- '8ohm'" = 4 ohms
8/2 = 4

This is also the way you will want to hook up multiple cabinets to an amp.

EXAMPLE: (2) 8 ohm cabinets ---->4 ohm jack on amp.
EXAMPLE: (2) 16 ohm cabinets--->8 ohm jack on amp.



SERIES-PARALLEL:

This is the most confusing. The total speaker load will equal the ohms of one speaker. Most commonly used on speaker cabinets that contain 4 speakers. This theory only works with 4 speakers of the same (ohm) rating.
In this case, if a speaker blows, it will take out it's series counterpart, leaving two speakers working, and two not working, hence doubling the impedance.

Starting at the upper left speaker, you will run one wire from the input jack to the "+" of that speaker, then from the same terminal to the "+" of the speaker below it.
Then run a wire from the input jack to the "-" of the upper right speaker, then from the same terminal to the "-" of the speaker below it.
Finally, connect the "-" of the upper left speaker to the "+" of the upper right speaker, and the "-" of the lower left speaker to the "+" of the lower right speaker.

Wiring Multiple Cabinets

Sometimes, it's inevitable that we need to run multiple cabs of different impedances. Well, here's how you figure out how to solve that problem.

EXAMPLE:

The amp has 4, 8, and 16 ohm taps.

Cabinet A = 8 ohms
Cabinet B = 16 ohms

In this case,

1/8 +1/16 = 3/16

Divide the back number by the front number to get the impedance.

16/3 = 5.33

You would want to run both cabinets from the 4 ohm taps, or daisy chain them if (2) 4 ohms taps aren't available. Remember, it's safe to run a slightly higher mismatch.

...And just as a reference

(2) 8 ohm cabs = 4 ohms
(2) 4 ohms cabs = 2 ohms - *typically unsafe*
(2) 16 ohms cabs = 8 ohms

(1) 8 ohm cab + (1) 4 ohm cab = 2.66 ohms - *typically unsafe*
(1) 4 ohm cab + (1) 16 ohm cab = 3.2 ohms - *somewhat unsafe*

The two above examples would typically be unsafe to run at, although, the very bottom example would be better to run, since it's closer to 4 ohms. This would get by in a pinch on some occasions.



The information provided above is a general reference. In some circumstances, it may not be appropriate to mismatch impedances. Use common sense, and consult the manufacturer if possible. If you are in doubt, the safest bet is to find a cab that has a nominal rating of that of your amp. I am not responsible for damages caused from mismatching impedances.

[rockon1]

NEVER EVER hook up a cabinet with a lower ohm rating than the head.

EXAMPLE ---4 ohm cabinet--->8 ohm jack on amp...THIS IS A NO-NO!!!

You will cause your output transformer to overheat, and eventually, it will blow.



Hmmmm...yet a higher ohm load (16 ohm cab with 8 ohm amp)will cause more flyback voltages to hit the tranny. Ive read from respected sources a lower ohm mismatch is actually safer. Bob

[VaughnC]

I never did get a really good answer on this one. If I get an 8ohm head I don't really feel like rewiring my 16ohm cab so is it safe to run it this way (pretty tame volumes all of the time)?
If I had to rewire I could just change it from series (2 - 8 ohm speakers in series = 16ohm) to paralell (2 - 8 ohm speakers = 8ohm), right?

I have to disagree with "doublescale1" on some points:

Yes, with a tube amp it is generally safe to go as high as 2X the amp impedance setting. Beyond that, there's a risk of flyback voltage...which can damage the output transformer and/or tube sockets.

Running a tube amp with a lower impedance cab than the amp impedance setting will stress the output tubes but there is less risk to the output transformer & tube sockets because there is no flyback voltage created in this scenario. And the tubes are self limiting so they can't produce enough current to damage the output transformer windings.

If you have 2, 8 ohm speakers in series the total impedance is 16 ohms. If you rewire 2, 8 ohm speakers in parallel, the total impedance is 4 ohms. If I had a choice, I'd rather run a 4 ohm cab on my 8 ohm amp rather than 16 ohms because there's less chance for flyback voltage...but 16 ohms should be safe as you're in the 2X window.

Just turning a tube amp off standby, without a speaker attached, will not do any damage. Modern tube amps typically have shorting type speaker jacks so if you forget to plug in a speaker cable, the jack shorts the output transformers secondary winding to prevent flyback voltage from damaging the output transformer and/or tube sockets. However, if you plug a speaker cable into a tube amp and don't plug the other end into a speaker cab, then there's no flyback protection and a risk of damage. So, yes, to be on the safe side, it's probably not a good idea to turn a tube amp off standby until you make sure of the speaker connections...but it's not instant doom either. In order for flyback to occur, there has to be a signal present in the input to an amp. Without an input signal, a normally functioning tube amp, without a load attached, will just sit there and do nothing.

[m3g]

From the Germino site in regards to the Classic 45 . . .

Try the 16 ohm setting with 8 ohm cabs for a more dynamic response/feel from the amp.


Now for the do-not do's.

NEVER EVER hook up a cabinet with a lower ohm rating than the head.

EXAMPLE ---4 ohm cabinet--->8 ohm jack on amp...THIS IS A NO-NO!!!

You will cause your output transformer to overheat, and eventually, it will blow.

[baron55]

Hmmmm...yet a higher ohm load (16 ohm cab with 8 ohm amp)will cause more flyback voltages to hit the tranny. Ive read from respected sources a lower ohm mismatch is actually safer. Bob

Your sources are correct!
The big confusion here is that solid state principals seem to always be incorrectly applied to tube amps.

In solid state amps, running a lower speaker load will cause the output transistors to try and produce too much power causing transistor failure.

Since most of today's electronics techs are more knowledgeable in Solid State than tube amps, they assume the same is true.

In tube amps the impedance is determined buy the plate resistance on the power tubes which is determined by plate voltage, screen voltage and class of operation.

On tube data sheets the plate resistance is shown for the most power at lowest harmonic distortion.

Output transformers are designed to take the high plate resistance high voltage signal and convert it to a low resistance high current signal. (kind of like a transmission to power tubes).

So when you run a impedance mismatch, it causes the reflected impedance on the transformer to the power tubes to change. This always reduces power output whether or not you have a lower load or higher load to the amp.

For example: We will be running EL34's at about 400volts which have a plate resistance of 3.2k to 3.4k a pair. A typical EL34 50 watt Output transformer has a primary impedance of 3.2 to 3.4K (We will use 3.4K) The outputs are your standard 4,8,16 ohms.

The EL34's want to see 3.4K reflected to them from the output transformer. So when you use the correct Ohms settings to the speakers, the output transformer has the correct number of ratio winds to achieve this. So when you are using the correct speaker impedance to amp impedance. The reflected impedance is close to 3.4K.

The ratio of the output iron for 4 ohms = 850, for 8 ohms = 425, and 16 ohms = 212.

So lets say you run a 8 ohm cab at the 16 ohm jack. So multiply the current speaker impedance of 8 ohms by the ratio at the 16 ohm jack of 212. 8 X212. This will give you a reflected impedance of 1696 or 1.7k.

Do the same with a 8 ohm cab into the 4 ohm jack.

8 X 850 = 6800 or 6.8K

So running a 8 ohm load into the 16 ohm jack will drop the plate impedance reflected to 1.7K, this will reduce power output but the power tubes will work harder. Unless the output transformer is not very good it should be fine. Many Marshals had crappy output iron, so they were not good at handling this mismatch. If you do this, bias the amp a little cooler.

Running the 8 ohm cab into the 4 ohm tap will give a reflected impedance of 6.8K. This will also reduce power output but also increase harmonic distortion. The Trainwreck amps run output iron at 5.0k to 6.6K for a pair of EL34's at 400 to 440 volts which means those amps are technically always running at a impedance mismatch, thus the 35 watt output designation for those amps.

So going to a lower load will not increase power output and generally fry iron, the same is true for going higher. As long as your are only about one step off either way.

[Sleep Terror]

I have a quick question regarding this. I have a Bugera 6262, and an Ampeg V-412 200Watt Cabinet. 16 ohms. What would i set my Bugera to? i really don't wanna blow out anything in it and im pretty new to these things.

Never mind this, i've recently called a place and its obvious that 16 - 16...

[LPMojoGL]

I never did get a really good answer on this one. If I get an 8ohm head I don't really feel like rewiring my 16ohm cab so is it safe to run it this way (pretty tame volumes all of the time)?
If I had to rewire I could just change it from series (2 - 8 ohm speakers in series = 16ohm) to paralell (2 - 8 ohm speakers = 8ohm), right?

Yes, it is safe.
2x8ohms in parallel = 4 ohms