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post #1 of 8 (permalink) Old 07-28-2005, 03:24 PM Thread Starter
 
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Question about ohms and speaker cabs.

The rythm guitar player in my band just bought a really nice new amp. It's a Traynor Custom Valve 50BLUE - 50w / 12 inch - All Tube amp.
http://www.yorkville.com/products.as...&cat=18&id=318
I have an old super reverb and it is 45 watts at 2 ohms. 4 x 10" 8 ohm speakers comes out at 2 ohms I guess.
So his new amp is 50 watts, it sounds great but it seems much less powerfull than my old super. I noticed today on the Traynor website it says..

Cabinet Impedance (Ohms) 8
Power @ min. impedance (Watts) 50
Minimum Impedance (Ohms) 4.

Now they make a matching extension cabinate that is also 8 ohms. (http://www.yorkville.com/products.as...&cat=18&id=334) Will adding the second cab then bring the load to 4 ohms, and therefore make the amp "louder" than it is now? Is it only running at 25 watts with the one cabinate?
I really don't understand the ohm thing and was wondering if someone could clear things up.

Here is the amp with the ext cab. It sounds realy realy sweet.
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post #2 of 8 (permalink) Old 07-28-2005, 04:16 PM
rgr
 
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Re: Question about ohms and speaker cabs.

It'll move more "air", so it will probably seem louder by filling more sonic space. Yes, with the ext cab it will be a 4 ohm load.

Your cab is probably wired with 2 pairs of 8 ohm speakers in series, then the 2 pairs are in parallel to get an 8 ohm load, not many (none that I know of) tube guitar amp output trannies can handle a 2 ohm load.

Roger
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post #3 of 8 (permalink) Old 07-29-2005, 02:47 AM
 
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Re: Question about ohms and speaker cabs.

Impedance: the resistance to the flow of current in a circuit measured in ohms. Impedance ratings of a speaker represent how much resistance to current the speaker produces. Nominal impedance is the average impedance present in a speaker. Minimum impedance is the lowest impedance presented by a speaker. A speaker's impedance changes at different frequencies so that it may have a low measure of 3 ohms and a high of 35 ohms.

The lower the number of ohms (lower impedance), the more power flows into a speaker. Power is measured in watts and is the result of multiplying current by voltage. As more current is able to flow with lowering impedance (less resistance to current), the more power flows into a speaker. Speakers that exhibit low impedance ratings of one or two ohms can be difficult to drive since they require so much power from an amplifier, thus speakers with very low impedances require robust amplifiers capable of producing power into those low ratings.

Most speakers exhibit nominal impedance ratings of four to eight ohms. Speakers with four ohm nominal impedances should be mated with good quality amplifiers and typically should not be used with most receivers. Eight ohm speakers are generally easy to drive and thus can be mated with a broader range of receivers and amplifiers.
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post #4 of 8 (permalink) Old 07-29-2005, 12:03 PM
rgr
 
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Re: Question about ohms and speaker cabs.

Thanks for the cut and paste definition, it is a good general explanation, but not as relevant here. From the 2nd paragraph one might get the idea that a lower impedance = more power from an amp. On most solid state amps, the answer is yes, if you notice, most solid state power amps are rated something like, "75W per side @ 4 ohms, 55W per side at 8 ohms, 150W into 8 ohms in bridged mono". SS power amps feed the current directly into the speaker, some straight from the power transistor or IC, others through a transformer, but, it doesn't matter what load you plug in, it connects to the same place. So, a lower impedance = more power (unless you hook up too little impedance and it = smoke).

Tube power amps are different, they can't deal with different impedances, hence the need to set the impedance on the back of the amp. What this does is taps the output transformer at different spots so the power tubes always see the same load, and, consequently, are always putting out the same amount of power. A 50W tube amp puts out 50W into 16, 8, or 4 ohms, provided the impedance selector is set correctly. What is bad is to run a 4 ohm load with 8 or 16 selected. You can go the other way, i.e. a 16 ohm load with the selector on 4 or 8 and it shouldn't hurt the amp (try at your own risk, I won't be responsible) and theoretically it will make the amp work harder but reduce the output, although you probably won't be able to tell the difference though. The transformer taps are also why you have to reduce the selector by half when pulling 2 tubes out of a 4 tube amp, i.e. if you pull 2 tubes out of a 100W Marshall to run it at 50W and have an 8 ohm cabinet, the amp should be set to 4 ohms.

Hope this helps,
Roger
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post #5 of 8 (permalink) Old 08-01-2005, 01:29 PM Thread Starter
 
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Re: Question about ohms and speaker cabs.

Quote:
Your cab is probably wired with 2 pairs of 8 ohm speakers in series, then the 2 pairs are in parallel to get an 8 ohm load, not many (none that I know of) tube guitar amp output trannies can handle a 2 ohm load.
My fender Super Reverb is definately a 2 ohm load. http://www.fender.com/products/searc...tno=0217600000
Mine is a '75 but basicly same specs.

Quote:
It'll move more "air", so it will probably seem louder by filling more sonic space.
I'm prety sure the differnce in power betweern my 45watt Super and his 50watt Traynor is a lot more than just the amount of air that is moving. I could be wrong but I would bet my amp is twice as loud as his.

So hooking up the ext cab, and changing the ohm load from 8 to 4 ohms will only move more air, and not actually make the amp put out more power? Even the way they word it on the Traynor website "Power @ min. impedance (Watts) 50" would seem to suggest that at minimum impedance (4ohms) it is 50 watts and perhaps at less that minimum impedance (one cab, 8ohms) it is pushing less than 50 watts.

I think I'll email Traynor and see what they say.
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post #6 of 8 (permalink) Old 08-02-2005, 04:50 PM Thread Starter
 
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Re: Question about ohms and speaker cabs.

Well I emailed Traynor. I asked them why my 45 watt Fender amp is way louder than thier 50 watt Tranor.

Quote:
Those two amps put out roughly the same amount of power, but as with all different amplifiers, the control settings are going to be very different from one to the other as are the gain factors and a whole range of other parameters. Impedance is not a factor, nor is power. They are simply 2 very different amplifiers.
Thank you for your enquiry.
Mike
So I guess impedance has no effect on overal volume. And it looks like wattage is not a meter by which you can judge how loud an amp will be either.
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post #7 of 8 (permalink) Old 08-03-2005, 12:21 PM
rgr
 
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Re: Question about ohms and speaker cabs.

Quote:
Originally Posted by Rotti
So I guess impedance has no effect on overal volume. And it looks like wattage is not a meter by which you can judge how loud an amp will be either.
An extreme oversimplification, but yeah, I guess you can say that. 1W = 1W, just like 1 inch = 1 inch and 1cm = 1cm. It is a measuring standard, can't change. Unfortunately, measuring it isn't as standard as pulling out a ruler. The rating is probably a theoretical number based on the circuit design. >It is also an rms number, often called "program power" or "average power" on PA type power amps. This is the average power that the amp can put out, the peak power is around 1.414 X rms power.<(>OT theory<). But again, variations in components, transformers, tubes, etc... can cause variations from one amp to the next. This is why many Marshalls from the 60s and early 70s varied so much, there was alot of variation in components, lots of times if they ran out of one part, they would just use the next closest thing.

Another major factor you are forgettting is the speakers, 50W into high efficiency speakers cause more dB out than 50W into lower efficiency speakers. And no, all guitar speakers are not the same. Also, your Fender has 10" speakers, the Traynor has 12", yours may not actually be louder, but it may have more "cutting" power because of more high frequency content from the smaller speakers.

Not trying to confuse things, but amps and speakers are "systems", you have to take the whole thing into consideration.

Roger
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post #8 of 8 (permalink) Old 08-03-2005, 02:25 PM Thread Starter
 
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Re: Question about ohms and speaker cabs.

Well thats just it there is a lot of variables to consider. Wattage being just one.
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extension cab , fender super reverb , ohm cab , ohm cabinet , ohm load , ohm speaker , ohm speakers , power amp , power amps , solid state amp , solid state amps , solid state power amp , tube amp , tube power amp

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