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questions on Ohm Ratings

 
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orange72468



Joined: 02 Jan 2010
Posts: 503
Location: Twin Falls, Idaho

PostPosted: Mon Aug 30, 2010 8:06 pm    Post subject: questions on Ohm Ratings Reply with quote

I have a question on how to check the ohm rating on a speaker. I was wondering if I am doing this correctly, I took my ohm meter and put it on the speaker leads and got the reading of 8.0 so I would guess that is an 8 ohm speaker? I also checked another speaker it said 6.5 so would that be a 6 ohm speaker? This makes sense to me but I didn't know if you have to take anything else into consideration, I bought one of those Vox ac4tv8 amps and was thinking about using an extension cab. The vox uses a 16 ohm for the ext speaker.

Also does anyone know what the ohm rating is on the ext speaker on a Twin Reverb?
Right now I am running a Leslie out of it, and do not know what the ohm rating is on the Leslie either.
My Dad used this setup for years.
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74 Strat



Joined: 23 Feb 2008
Posts: 5639

PostPosted: Mon Aug 30, 2010 8:34 pm    Post subject: Reply with quote

You are metering correctly. I just checked my vintage Twin Reverb and the ext speaker is 4 Ohm...Dean
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Structo



Joined: 21 Nov 2004
Posts: 26415
Location: Salem, OR

PostPosted: Wed Sep 01, 2010 9:37 am    Post subject: Reply with quote

Just about every time I have ever measured the DC resistance of a speaker, it has given me a reading that is below what the speakers rated impedance is.
Impedance is a dynamic property of a speaker and it changes with the frequencies that are fed into them.
But they rate them for a kind of middle of the road load type thing.

An 8 ohm speaker usually measures around 6-6.5 ohms.

A 16 around 13 ohms and a 4 ohm speaker around 3.

On the Twin Reverb, most of those used a 4 ohm output transformer.
So that would be two 8 ohm speakers connected in parallel.
So if the combo already has the two internal speakers connected, the amp sees it as a 4 ohm load.

If you were to plug another 8 ohm speaker into the external jack, you would have a 2.66 ohm total load.

If you plugged in a 16 ohm speaker then the total the amp sees is 3.2 ohms which would be better on the OT.

Most of the old Fender amps can handle a one step mismatch either way from the spec'd one without damage but that is not to say it won't harm the output transformer.
That is, the + on each speaker go together and the two - go together then the two pairs of wires connect to a jack or plug with the tip of the plug or jack usually being the + side.
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orange72468



Joined: 02 Jan 2010
Posts: 503
Location: Twin Falls, Idaho

PostPosted: Fri Sep 03, 2010 4:18 pm    Post subject: Reply with quote

Thanks for the help guys.
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Les



Joined: 06 Jan 2005
Posts: 1961
Location: Kansas City

PostPosted: Wed Sep 29, 2010 10:30 am    Post subject: Reply with quote

Structo wrote:
Most of the old Fender amps can handle a one step mismatch either way from the spec'd one without damage but that is not to say it won't harm the output transformer.

Tom is right about the mismatch... but - as long as you follow the rule of thumb - don't worry about the transformer as that is the component to be concerned about.

At any rate, if the amp expects to see a 4-ohm load, hooking up a 2-ohm load or an 8-ohm load will be ok (not preferable but ok)... similarly, if the amp expects to see an 8-ohm load, hooking up a 4-ohm load or a 16-ohm load will be ok.

Keep in mind, this conversation does not apply to all amps however.
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tangelolemon



Joined: 26 Jan 2009
Posts: 2691
Location: Brooklyn, NY

PostPosted: Wed Sep 29, 2010 10:43 am    Post subject: Reply with quote

Impedance has its own name because it's different than simple resistance.

Impedance of a speaker is resistance + inductive reactance.

Resistance is the resistance of the voice coil's magnet wire by itself. Inductive reactance is an impediment to current flow that results from the fact that it is wound into a coil and therefore has inductance.

I have a slightly different opinion than Tom (structo) re: the ideal impedance of the extension cab.

If you use a 16 ohm extension cab on your Twin Reverb the total impedance load will indeed be closer to the ideal 4 ohm load, but since the extension cab will be higher impedance, it will only get half the signal that the onboard speakers will. Electricity always takes the path of least resistance. Since the onboard speakers would have half the impedance of the ext. cab, the onboard speakers would get twice the power and seem much louder than the ext cab, which in many cases kind of defeats the purpose.

The only way to have the same amount of power sent to both onboard and extension speakers would be to use an ext. speaker cab with an impedance of 4 ohms. This will actually give you a total load of 2 ohms, but the Twin can take it. The old Fender amps with Ext. Speaker jacks were spec'd with transformers that could take the abuse.

Remember also that impedance in speakers is an inexact science. For one thing, as Tom alluded, impedance changes with frequency. Also, the ratings on speakers can be fudged sometimes. JBL for one has many times throughout their history used one driver of 8-10 ohm impedance and sold it with 4, 8, and 16 ohm labels-- nothing different other than the label.

Use it as a guideline and understand that there can be risks to mismatching, but also understand that in a TUBE amp, lower than recommended impedance is safer than going higher. The OPPOSITE is the case with solid-state amps (and therefore most conventional wisdom), which is why you see a lot of conflicting information. Especially on the internet.

Fender has a shorting jack on the speaker out so that in the event a cab isn't plugged in, the amp operates into a short rather than an open. This is because the short (zero impedance) is actually safer for the amp than an open (infinite impedance).

The tweed 3x10 Fender Bandmaster (one of my favorite amps ever) used the same 4 ohm OT as the tweed Super and Pro-- and used 3 speakers for a 2.667 ohm load. Stock, from the factory. It performs fine and sounds great, and I've never broken the one I built.
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