Jump to content
Hamer Fan Club Message Center
  • 0

Impedance Mismatch


Steve Haynie

Question

22 answers to this question

Recommended Posts

Posted

loss in volume, headroom, make it a lil richer

you can go down in ohm load, ex.~ 8ohm into 16ohm cab

this is the no no~ 16ohm load into a 8ohm cab

Posted

It'll sound more like a Marshall. :blink: Fender amps are more flexible with the impedance matching, especially with Bassman heads. They all have extension speaker jacks which is just proof that they don't require strict impedance matching. (I could be wrong though). I've played my Bandmaster head through 4x12, 2x12, and 1x15 cabinets and have had no problems whatsoever with doing so. The internal resistance is changed a bit, but the sound changes from the tubes are quite are quite pleasant. You'd be surprised how much a sealed 4x12 cabinet can make a Blackface or Silverface Bassman head sound like a Plexi. You absolutely will get a loss in volume, but that will only allow you to crank them tube voltages up and rock out even harder.

Posted

I agree with what's been said so far.

But:

Depending on the # of speakers and the impedance, you may be able to wire the cab. at either 4 or 8 ohms.

One thing never to do is to run the amp WITHOUT a cabinet connected.

That would damage the amp.

Posted

Probably be alright. I won't personally use or recommend more than a 2 to 1 mismatch though. Overheating of transformers and arcing due to flyback voltages are possibilities.

Posted

Solid state power amps that don't use an output transformer are fine when connected to higher impedance loads than what they are rated for- they just produce less power. A load with too little impedance can cause the amp to overheat and either shutdown or burn up.

Tube power amps with output transformers are different. A higher than rated impedance load could damage the transformer. Too low an impedance could damage the tubes. Fenders will usually tolerate a 2:1 mismatch, Marshalls might not.

Or so I've been told.

Posted

Solid state power amps that don't use an output transformer are fine when connected to higher impedance loads than what they are rated for- they just produce less power. A load with too little impedance can cause the amp to overheat and either shutdown or burn up.

Tube power amps with output transformers are different. A higher than rated impedance load could damage the transformer. Too low an impedance could damage the tubes. Fenders will usually tolerate a 2:1 mismatch, Marshalls might not.

Or so I've been told.

The correct answer.

Solid state heads can go higher, but tube amps (because of the transformer) can go lower.

Posted

Bassman output transformers can deal with an 8 ohm load no problem. I have played a bandmaster head through a 16 ohm cabinet with great success for years, but that is within the 2:1 mismatch ratio that has been discussed previously. You could always wire the cabinet in parallel instead of in series to provide the amp with a much more friendly 8 ohm load.

4x12a.gif

4x12wiring1.gif

The dangers are what GregC had to say:

Overheating of transformers and arcing due to flyback voltages are possibilities.

The pleasant effects are that an impedance mismatch can smooth out the razor sharp high end of some Fenders and soften the single note presence a bit. I personally like the sound.

My 1967 Super Reverb has a single JBLD130F speaker in it instead of the (4) 6k ohm 10 inch speakers wired in parallel that it comes with originally. The power transformer wants 2 ohms and I give it 8 ohms with absolutely NO loss in volume or bandwith. The sound is FEROCIOUS. I've played other Super Reverbs with the original 4x10 configuration and they sound thin and brittle compared to mine. The impedance mismatch mod in my SR has taken none of the single note presence away strangely.

But I have heard my Bandmaster through a 2x12 cabinet and through a 4x12 and the sound does change quite a bit. It makes the Bandmaster sound much more metal!

Posted

To those who have answered and obviously understand this topic better than I: When you go from say a 4 ohm output into an 8 or 16 ohm cabinet or speaker, is the amp working a lot harder and therefore more susceptible to damage or overheating?

Posted

Is that a 212 cab? I'll make this real simple for you.

Is not the point of what could happen. What is happening is you are NOT matching your head ohms to your cab ohms. Your head is rated ot 4-ohms. your cab should be rated at 4-ohms.

As mentioned above your cab needs to be rewired to decrese the ohms. BTW what none of us no for sure is if you have a 212 [i assume you do] or is it a 412? This is why your not getting the exact help you need.

But the 412 cab shown in Series and Parallel wiring shows you how to reduce your Ohms load and this could be applied to a 212 cab also "if" thats what you have. gregc points are valid. I guess most of us don't do this because there will be consequences somewhere someway at some point. For sure you are reducing the life of your Circuit. Ohms Law is a basic of electricty. When you change your R or Ohms your equation of E=IxR no longer solves itself mathmatically. In essense you now have a problematic circuit.

Posted

not to hijack the thread, but... I've just bought second hand Peavey Delta Blues 2x10. It says in the manual "30W into 8 or 16 Ohms" and then on the backplate next to External speaker output jack they wrote "minimum load 16 Ohms". Does that mean I can safely connect 8 ohm cabinet? What about 4 ohm cabinet?

Thanks to all experts who bothered to enlighten B)

edited to add: in the combo they have 2x16 Ohms speakers, so I guess they give total load of 8 Ohms?

Posted

I think they mean 16 ohms for the extension, only. If you're using the 8 ohms from the internal pair, and add a 16 ohm cab in parallel, you're just under 6 ohms. Probably safe.

But don't use a 4 ohm cab on the extension jack, or you'll start getting into dangerous territory.

make sense?

Posted

thanks, but the issue I have all my cabs are 8 ohms... I guess they cut off internal speakers and use only external one. Still safe if minimum load has to be 16Ohms?

edited to add: I am idiot, took 2x8 ohms cabs for 2x16, so I guess 2x8 in series is 4 Ohms load then?

Posted

2X8 in Series is 16 ohms.

2x8 in Parallel is 4 Ohms.

Are you sure the internal speakers cut off if you plug something into the external jack? That makes a big difference, here.

If the internal speakers DO cut off when you use the external jack, 8 ohms will be fine. If not? Assuming the external speaker jack is in parallel, an 8 ohm external cab will bring you down to 5.33 ohms.

Posted

On a lot of the Peavey tube combos, your total load should not be less than 8 ohms. Typically the EL-84 powered Classic and Delta Blues have an internal speaker impedance of 16 ohms and the extension speaker jack is wired in parallel. Therefore you shouldn't hook up anything less than a 16 ohm cabinet to it.

Come to think of it, every EL-84 powered amp I have wants to see no less than 8 ohms total. I wonder if this is due to the nature of the tubes or maybe the O/T ?

Posted

sorry to sound dumb, but does that mean that my 8 ohm cab is ok to connect then? B)

Nope :P Assuming a 16 ohm internal speaker load (your 2 x 10's in the amp) adding an 8 ohm cab would bring your total impedance down to 5.33 ohms.

Posted

Come to think of it, every EL-84 powered amp I have wants to see no less than 8 ohms total. I wonder if this is due to the nature of the tubes or maybe the O/T ?

That's entirely the way the OT is wired.

My EL84 amps all have 4/8/16 ohm outs because that's how I spec the transformer.

Tubes (any tubes) want to see a certain impedance. The transformer is designed to transform that impedance to something else. So, if the tubes want to see 4000 ohms, the transformer can have 4000 ohms on the tube side and 4, 8, 16, or whatever you want on the speaker side.

For the peavy combo -- it's just a matter of practicality. They design it for 16 ohms, make sure it's safe for 8 ohms, and go.

Posted

Interesting -- not germane to the current topic, so folks worried about extension cabs please don't get confused!

Not a very good Wiki article, though. Looks like it was written by a fan, full of un-sourced comments, and no solid information. Wouldn't mind seeing a schematic, just for fun.

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...