Tube amp ohm mismatch with speaker info & question

[MadAsAHatter]

I know the safety concerns with mismatching but was having a brain fart on the power reduction and tonal shifts.
My quick question first since I can't remember for sure & couldn't find a direct answer: A 2:1 mismatch will effectively cut the power output in half, correct? As in 100 watts, 8 ohm amp to a 16 ohm speaker will drop the amps power to 50 watts? Which will reduce volume and headroom some?

In my search for an answer I came across 2 articles that I though explained the whole mismatch thing and what is/isn't safe pretty well. Some of y'all may have seen them before but since mismatch questions come up enough I figured I'd share to those who haven't.

https://bossarea.com/impedance-matching/

http://blog.hughes-and-kettner.com/ohm-cooking-101-understanding-amps-speakers-and-impedance/

 

[SQUAREHEAD]

Hey brother, I think there is a mathematical equation that works it out exactly but essentially, youre very close to being correct…
While a lot of people say mismatches are dangerous, I think they are much more dangerous if you have the amplifier at double what the cabinet is.
Don’t run your Amplifier at 16 ohms, into an 8 ohm speaker cabinet.

For instance, I know a lot of guys who run their Marshalls at half the impedance of the cabinet, to achieve a softer, smoother, less sharp tone, and many run Mesa Mark series Amplifiers at 8ohms, into 16ohm cabinets.
I’m sure someone will pipe up here and say how dangerous it is and that you will blow your amplifier up in three minutes doing this, but I’ve played around with this for over three decades and never blown up anything.

 

[mixn4him]

Changing Taps is NOT going to cut the output of the amps power, pulling tubes will. But as it has been said before the difference between a 100w and 50w is only 3db...

 

[Racerxrated]

Hey brother, I think there is a mathematical equation that works it out exactly but essentially, youre very close to being correct…
While a lot of people say mismatches are dangerous, I think they are much more dangerous if you have the amplifier at double what the cabinet is.
Don’t run your Amplifier at 16 ohms, into an 8 ohm speaker cabinet.

For instance, I know a lot of guys who run their Marshalls at half the impedance of the cabinet, to achieve a softer, smoother, less sharp tone, and many run Mesa Mark series Amplifiers at 8ohms, into 16ohm cabinets.
I’m sure someone will pipe up here and say how dangerous it is and that you will blow your amplifier up in three minutes doing this, but I’ve played around with this for over three decades and never blown up anything.

Exactly. You can run 8 ohm or 4 ohm out, on the amp to a 16 ohm cab. Darker, smoother and with a slight increase in lows when you do this; and lower perceived volume but I'm not sure that it's exactly half. It is quieter though.

 

[Racerxrated]

Changing Taps is NOT going to cut the output of the amps power, pulling tubes will. But as it has been said before the difference between a 100w and 50w is only 3db...

Right..the amp is still putting out 100w but the cab does get noticeably quieter...

 

[MetalHeadMike]

Like tube swaps, I've only ever experienced extremely subtle to non-discernable tone or feel changes when playing with mismatches. 8ohm amp setting into 16 ohm cab, 16 ohm amp into 16 ohm cab, 4 ohm amp into 8 ohm cab, blah, blah, blah...all sounds and feels the same to me. Not hearing any benefit to mismatching, I just stick with a direct match.

 

[scottosan]

That absolute safest way to reduce power is to turn down the volume.

Your welcome!

 

[TheRealDonnie]

I run an 8 ohm amp out into a 16 ohm speaker just for the slight attenuation.

Don't see it mentioned above but DO NOT run a lower ohm cab than the amp.

8 ohm amp into 16 ohm speaker = OK
16 ohm amp into 8 ohm speaker = BAD and can kill your OT

 

[psychodave]

I'll add... most speakers do not meter to their ratings. For example, most 16 ohm speakers rate around 13 ohms (now add in multiple speakers and it really gets interesting depending on how its wired). Make your own conclusion as to why you can or cant do certain things.

 

[TheRealDonnie]

I'll add... most speakers do not meter to their ratings. For example, most 16 ohm speakers rate around 13 ohms

Thisistrue!

Because transducer impedance fluctuates during actual use the number published (8, 16, etc) is
the 'nominal' impedance of the driver during use.

Stick an ohm meter on a static speaker and it will measure closer to what Dave mentions for a 16
and usually down around 6.5 for an 8 ohm rating.

 

[scottosan]

Thisistrue!

Because transducer impedance fluctuates during actual use the number published (8, 16, etc) is
the 'nominal' impedance of the driver during use.

Stick an ohm meter on a static speaker and it will measure closer to what Dave mentions for a 16
and usually down around 6.5 for an 8 ohm rating.

Because speakers operate on AC and are rated by “impedance”, which is frequency dependent, not DC resistance, which is not frequency dependent.

 

[SQUAREHEAD]

I'll add... most speakers do not meter to their ratings. For example, most 16 ohm speakers rate around 13 ohms (now add in multiple speakers and it really gets interesting depending on how its wired). Make your own conclusion as to why you can or cant do certain things.

Yep, I have a couple real bright ole Marshalls that sound better (to me) at 1/2 ohm to cab.

 

[MadAsAHatter]

I had to dust of some physics knowledge and pull out Ohms law, but I figured out my answer - The power does go down by about half.

Using the the formulas Power=Voltage*Current and Impedance=Voltage/Current and the practical knowledge with regards to tube amps on this subject that when going to a higher ohm speaker the current will drop by about half (or the opposite going to a lower ohm cab the current will marginally increase) it can be demonstrated mathematically.

The following will be hypothetical numbers to start but will demonstrate how it works out.
Let's say a 100 Watts amp at 8 Ohms going into an 8 Ohm speaker will output 5 amps of current. From there we can calculate the voltage

8 ohms = x-volts /5 amps
x=20 volts
We can also prove the wattage with this info by:
x-watts = 20 volts * 5 amps
x=100 watts

So if the impedance is doubled we know the current will go down by about half (for simplicity sake we'll call it exactly half)
@ 16 ohms the current is 5 amps/2 which equals 2.5 amps
in calculation the voltage will stay the same
16 ohms = x-volts/2.5amps
x = 20 volts

Going the opposite way if the impedance is halved the current only marginally increases. Again for simplicity we'll say it stays the same.
and again we can calculate voltage to show it drops by half
@ 4 ohms current = 5 amps
So..
4 ohms = x-volts/5amps
x=10 volts

Knowing the volts and amps produced at each impedance we can calculate the wattage for the given impedance.
We already know that into 8 ohms we get 100 watts
@8 ohms: 20 volts * 5 amps = 100 Watts
@16 ohms: 20 volts * 2.5 amps = 50 Watts
@ 4 ohms: 10 volts * 5 amps = 50 Watts

So it is shown that changing the impedance affects the amount of current or voltage produced depending on if it's higher or lower impedance drops by half; therefore, the output wattage drops by half.

 

[TheRealDonnie]

Because speakers operate on AC and are rated by “impedance”, which is frequency dependent, not DC resistance, which is not frequency dependent.

Also why static loads like the original Marshall Power Brake suck sonically as attenuators.

 

[glpg80]

I had to dust of some physics knowledge and pull out Ohms law, but I figured out my answer - The power does go down by about half.

Using the the formulas Power=Voltage*Current and Impedance=Voltage/Current and the practical knowledge with regards to tube amps on this subject that when going to a higher ohm speaker the current will drop by about half (or the opposite going to a lower ohm cab the current will marginally increase) it can be demonstrated mathematically.

The following will be hypothetical numbers to start but will demonstrate how it works out.
Let's say a 100 Watts amp at 8 Ohms going into an 8 Ohm speaker will output 5 amps of current. From there we can calculate the voltage

8 ohms = x-volts /5 amps
x=20 volts
We can also prove the wattage with this info by:
x-watts = 20 volts * 5 amps
x=100 watts

So if the impedance is doubled we know the current will go down by about half (for simplicity sake we'll call it exactly half)
@ 16 ohms the current is 5 amps/2 which equals 2.5 amps
in calculation the voltage will stay the same
16 ohms = x-volts/2.5amps
x = 20 volts

Going the opposite way if the impedance is halved the current only marginally increases. Again for simplicity we'll say it stays the same.
and again we can calculate voltage to show it drops by half
@ 4 ohms current = 5 amps
So..
4 ohms = x-volts/5amps
x=10 volts

Knowing the volts and amps produced at each impedance we can calculate the wattage for the given impedance.
We already know that into 8 ohms we get 100 watts
@8 ohms: 20 volts * 5 amps = 100 Watts
@16 ohms: 20 volts * 2.5 amps = 50 Watts
@ 4 ohms: 10 volts * 5 amps = 50 Watts

So it is shown that changing the impedance affects the amount of current or voltage produced depending on if it's higher or lower impedance drops by half; therefore, the output wattage drops by half.

This is all wrong.

You’re assuming RMS values and that cannot be done if the load is not purely resistive. Because the source is a frequency dependent inductor and the load is a frequency dependent transducer, you have to go much, much deeper into the math with power factor calculations and frequency dependent Z parameter plots if you’re going to prove it mathematically. It would all be done in VA (voltamps).

 

[glpg80]

I run an 8 ohm amp out into a 16 ohm speaker just for the slight attenuation.

Don't see it mentioned above but DO NOT run a lower ohm cab than the amp.

8 ohm amp into 16 ohm speaker = OK
16 ohm amp into 8 ohm speaker = BAD and can kill your OT

This is sound knowledge.

Just know impedance mismatching will put additional stress on your power tubes, especially if you’re also cranking on them volume wise while mismatched.

Is it ideal? No. Can it be done? Yes.

 

[TheRealDonnie]

Celestion Greenback 16 ohm rating

 

[psychodave]

Because speakers operate on AC and are rated by “impedance”, which is frequency dependent, not DC resistance, which is not frequency dependent.

I always understood that the DC resistance was always close (or close) to the AC impedance when simply measuring as I do?

If not, is an amp seeing the speakers actually rating of 16 ohms or 13ish ohms using AC?

 

[TheRealDonnie]

I always understood that the DC resistance was always close (or close) to the AC impedance when simply measuring as I do?

If not, is an amp seeing the speakers actually rating of 16 ohms or 13ish ohms using AC?

~13 until you turn the amp on and it starts funneling varied frequency content - then it becomes a ying and yang between the
amp and load.

 

[psychodave]

~13 until you turn the amp on and it starts funneling varied frequency content - then it becomes a ying and yang between the
amp and load.

I get that, but there has to be something relative otherwise the fluctuations would be too much?