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08-03-2010, 06:33 AM
| | Registered User | | Join Date: May 2009 Location: Detroit | | | 225 watts into 2/4/8 ohms...why is the wattage not cut when the ohms go up?
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Just a general question here.
My SVT450 was 225 watts into 8 ohms and 450 watts into 4 ohms.
My new VB-2 is 225 watts regardless of the ohms.
I know one is SS and one is tube, which probably explains something...but can someone explain this to me?
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2005 Fender AV '62 Precision - Peavey VB-2
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08-03-2010, 06:38 AM
| | Registered User
Owner, Bill Fitzmaurice Loudspeaker Design | | Join Date: Sep 2004 Location: New Hampshire | | Quote:
Originally Posted by panama  I know one is SS and one is tube, which probably explains something... | It explains everything. The current delivery of a SS amp depends on the load impedance; not so with tubes. But to work well into different loads tubes have different impedance taps. |
08-03-2010, 06:42 AM
| | Registered User | | Join Date: May 2009 Location: Detroit | | | By impedance tap do you mean the 2/4/8 switch on the back of the amp?
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2005 Fender AV '62 Precision - Peavey VB-2
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08-03-2010, 07:03 AM
| | Registered User | | Join Date: Sep 2008 Location: Sioux Falls, SD | | | Yes exactly. The SS amps feed the power directly to the speakers as they are low impedance current devices. Tubes on the otherhand are voltage devices and have extrememly high impedance on the order of as high as 8000 ohms. You must match impedances to transfer power. It's like a transmission in a car, you need to select the right gear.
An output transformer in a tube amplifier has windings in a primary and secondary wound over each other. A transformer that can match 8000 ohms to 4 ohms will have a ratio between the number of turns between the primary and secondary of 2000:1.
When you select 8 ohms, the secondary has an additional section of windings the size of the 4 ohm section doubling the size of the coil and that makes the ratio 1000:1. Or if it is easier to follow 2000:2.
The actual number of turns in the windings is not this ratio, but a mutliple of the ratio. Meaning the 4 ohm secondary might have 10 turns on it... If it had 10 turns the primary will have 20,000 turns. It really is that simple.
BOB
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Last edited by rbonner : 08-03-2010 at 07:05 AM.
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08-03-2010, 07:08 AM
|  | Supporting Member | | Join Date: Feb 2009 Location: Houston,Tx | | | As a general rule, tube amps have a output transformer with several taps for the output "2-4-8 ohms", they will put out, the full rated power at any of the selected taps, |
08-03-2010, 07:13 AM
| | Registered User | | Join Date: May 2009 Location: Detroit | | | Very cool. Thanks guys!
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2005 Fender AV '62 Precision - Peavey VB-2
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08-03-2010, 08:25 AM
| | Registered User | | Join Date: Oct 2009 Location: Baltimore, MD | | | Panama, this is probably a better thread to ask in...
Which tap do you use for a 2.67 ohm load? |
08-03-2010, 08:27 AM
| | Registered User | | Join Date: May 2009 Location: Detroit | | | Thanks!
Which tap do you use for a 2.67 ohm load?
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2005 Fender AV '62 Precision - Peavey VB-2
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08-03-2010, 09:18 AM
|  | Registered User | | Join Date: Aug 2007 Location: Toronto Ontario Canada | | | Ideally none of them. It's always best to match impedances exactly for a tube amp.
Paul |
08-03-2010, 09:25 AM
| | Registered User | | Join Date: May 2009 Location: Detroit | | | Well, I guess my idea of a 4ohm 212 + 8ohm 15 is out then. Oh well
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2005 Fender AV '62 Precision - Peavey VB-2
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08-03-2010, 10:07 AM
| | Registered User
Owner, Bill Fitzmaurice Loudspeaker Design | | Join Date: Sep 2004 Location: New Hampshire | | Quote:
Originally Posted by BassmanPaul It's always best to match impedances exactly for a tube amp.
Paul | Not really, impedance only being an approximation at best anyway. What transformers don't like is a higher impedance load, so when you don't have a tap that's the same as the load use the next higher rated tap. With a 2.67 ohm load I'd use the 4 ohm tap rather than the 2 ohm. Quote: |
Well, I guess my idea of a 4ohm 212 + 8ohm 15 is out then. Oh well
| A pair of 212 will work better. |
08-03-2010, 10:35 AM
| | Registered User | | Join Date: Aug 2007 Location: Toronto Ontario Canada | | | All the transformer does is convert the load impedance into a very much higher impedance suitable for the plates of the output tube(s). Running a tube into a less than ideal impedance will shorten tube life until one fails. At that time all hell can break loose.
Paul |
08-03-2010, 11:26 AM
|  | Less Ebay, more Mel Bay | | Join Date: Jul 2007 Location: Phoenix, AZ | | | Most tube amp companies I've talked to have said to use 2 ohm taps for 2.67 ohms.
Myself, I'd steer clear of needing that impedance and use matched cabs.
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08-03-2010, 11:32 AM
| | Registered User | | Join Date: May 2009 Location: Detroit | | | Yeah I think the best bet for right now is to keep my 15e and 410he and wait for someone who ants to trade a 410 for my 15..plus cash probably.
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2005 Fender AV '62 Precision - Peavey VB-2
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08-03-2010, 12:20 PM
| | Registered User | | Join Date: Sep 2008 Location: Sioux Falls, SD | | | You'd be really amazed at what the impedance of your cab looks like as you swing the frequency across the range. Just look at any of the charts on the eminence site.
BOB
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08-03-2010, 12:38 PM
| | Registered User
Owner, Bill Fitzmaurice Loudspeaker Design | | Join Date: Sep 2004 Location: New Hampshire | | Quote:
Originally Posted by BassmanPaul Running a tube into a less than ideal impedance will shorten tube life until one fails.
Paul | Care to venture what the impedance is shown here? 
The cab shown is an SVT. Impedance varies from roughly 3.5 to 28 ohms, and that's only to 500 Hz. At the right side of the chart impedance continues to climb to well over 100 ohms in the high end. |
08-03-2010, 01:03 PM
|  | Working on successful. Got the first syllable... | | Join Date: Oct 2006 Location: Huddinge, Sweden | | Quote:
Originally Posted by rbonner An output transformer in a tube amplifier has windings in a primary and secondary wound over each other. A transformer that can match 8000 ohms to 4 ohms will have a ratio between the number of turns between the primary and secondary of 2000:1.
When you select 8 ohms, the secondary has an additional section of windings the size of the 4 ohm section doubling the size of the coil and that makes the ratio 1000:1. Or if it is easier to follow 2000:2.
The actual number of turns in the windings is not this ratio, but a mutliple of the ratio. Meaning the 4 ohm secondary might have 10 turns on it... If it had 10 turns the primary will have 20,000 turns. It really is that simple.
BOB | Almost correct. The actual ratio is the square root of the impedance ratio. The output voltage is proportional to the ratio, and when the voltage drops so does the current, which drops the output power. To maintain the power the current needs to increase with the same ratio. If the voltage drops to half (assuming a 2:1 ratio), the impedance needs to be reduced by a factor 4 in order for the current to rise to the double.
Working backwards in your example means if you want to go from 8000 to 4 you'll need a ratio of SQRT(8000/4), which is 44:1.
Not that it matters to the question at hand. The impedance selector picks the output with the required ratio, keeping the available power constant.
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08-03-2010, 09:52 PM
| | Registered User | | Join Date: Sep 2008 Location: Sioux Falls, SD | | Quote:
Originally Posted by Rune Bivrin Almost correct. The actual ratio is the square root of the impedance ratio. The output voltage is proportional to the ratio, and when the voltage drops so does the current, which drops the output power. To maintain the power the current needs to increase with the same ratio. If the voltage drops to half (assuming a 2:1 ratio), the impedance needs to be reduced by a factor 4 in order for the current to rise to the double.
Working backwards in your example means if you want to go from 8000 to 4 you'll need a ratio of SQRT(8000/4), which is 44:1.
Not that it matters to the question at hand. The impedance selector picks the output with the required ratio, keeping the available power constant. | Well we're lucky I'm not winding output transformers, I've wound a couple large filament transformers and hit the voltage and current on the head but it was 30 years ago. Thanks for the correction.
BOB
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