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Post by billyguitar on Oct 18, 2006 10:15:25 GMT -7
I, and probably others on this forum, receive a free magazine in the mail each month called Musicians Hotline. It's okay, considering what we pay for it. There is a technical question and answer column about amplifiers, currently the expert is Jeff Bober of Budda Amplification. This has come up twice now and his advice is not what I've come to believe to be true from both experience and what I've read in several books. For instance in the latest issue, someone writes in asking about running a Silvertone 1484 head into an 8 ohm cab. Jeff tells them that the Silvertone is a 4 ohm head, which is probably true since it was always sold as a mate to a 2x12 cab, and that running his 4 ohm head into an 8 ohm cab will cut his power in half. That ain't right, is it? He's thinking solid state where that is the rule, surely. I can't imagine the loss due to this mismatch could be more than 20%. The first time he said this a few months ago I thought it odd but now he says it again and it's got me wondering if I'm wrong. What is the real story, Myles?
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Post by myles on Oct 23, 2006 9:38:05 GMT -7
I, and probably others on this forum, receive a free magazine in the mail each month called Musicians Hotline. It's okay, considering what we pay for it. There is a technical question and answer column about amplifiers, currently the expert is Jeff Bober of Budda Amplification. This has come up twice now and his advice is not what I've come to believe to be true from both experience and what I've read in several books. For instance in the latest issue, someone writes in asking about running a Silvertone 1484 head into an 8 ohm cab. Jeff tells them that the Silvertone is a 4 ohm head, which is probably true since it was always sold as a mate to a 2x12 cab, and that running his 4 ohm head into an 8 ohm cab will cut his power in half. That ain't right, is it? He's thinking solid state where that is the rule, surely. I can't imagine the loss due to this mismatch could be more than 20%. The first time he said this a few months ago I thought it odd but now he says it again and it's got me wondering if I'm wrong. What is the real story, Myles? It will reduce the output a bit but not cut it in half. You have to remember than an 8 ohm speaker is not really always 8 ohms. It will only be 8 ohms in one very narrow area of it's frequency band. So ... it is always generally above 8 ohms as one sees here on a generic impedance chart with ohms vs frequency. Most of the time we are between 60 and 200 ohms for much of the operating range. You can always use a higher impedance speaker at a lower amp impedance setting but not the other way around.
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Post by billyguitar on Oct 23, 2006 12:45:35 GMT -7
Thanks, Myles. I just wanted someone with technical knowledge to back up what I've learned over the years. It's odd to me that a tube amp designer doesn't understand this.
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Post by benttop (Steve) on Oct 23, 2006 17:30:31 GMT -7
Now I'm confused. Virtually every tube amp I've ever owned had a shorting bar on the output jack to protect the transformer in the event the speaker cable was yanked out while the amp was playing. A shorting bar would be infinitely smaller impedance than the rated output impedance. If the transformer could tolerate a higher impedance, it seems an open circuit, which is infinitely larger, would be better. But they don't make them that way, do they? What am I missing Myles?
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Post by myles on Oct 24, 2006 10:17:19 GMT -7
Now I'm confused. Virtually every tube amp I've ever owned had a shorting bar on the output jack to protect the transformer in the event the speaker cable was yanked out while the amp was playing. A shorting bar would be infinitely smaller impedance than the rated output impedance. If the transformer could tolerate a higher impedance, it seems an open circuit, which is infinitely larger, would be better. But they don't make them that way, do they? What am I missing Myles? I am used to some amps, such as Rivera, that when a speaker is not plugged in the B+ is shut off. You can plug a higher speaker inpedance in a set speaker amp jack but not the other way around under load.
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Post by benttop (Steve) on Oct 24, 2006 17:00:06 GMT -7
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Post by hdahs143 on Oct 25, 2006 5:09:27 GMT -7
I'm pretty sure that Jeff is not purposely spreading misinformation, although I can see how his choice of words could be misleading.
His response to the Silvertone question reminds me of the way you turn a 100w Marshall into a 50w. First you pull the 2 outside El34's. But since the power tubes connect the amp to the output Tranny, you have effectively cut the impedance in half. In other words, after you remove the 2 tubes, and you are connected to say a 16 ohm cab, you must set the amp's impedance selector to 8 ohms. And it's been stated here, and on the GAB website that a 100w Marshall is not twice as loud as a 50w Marshall.
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SG123
Full Member
Posts: 221
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Post by SG123 on Oct 25, 2006 7:43:34 GMT -7
I'd be careful about using a higher than rated speaker load, especially at high volumes. There is a flyback voltage effect from the reflected load that can arc your output tube sockets. The arcing can also occur between internal parts of the output tubes or inside the output transformer. The real world impedance data Myles presented above becomes even more critical as the "starting point" is increased.
That being said, most amps will tolerate a 2:1 impedance mismatch either way. If the amp is rated 8 ohms, between 4 and 16 is generally O.K.
If forced to choose, I always go lower.
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Post by billyguitar on Oct 25, 2006 7:49:23 GMT -7
No, that's not what he said. He said "What you do have to realize however is due to the impedance mismatch, you have now cut the amount of output power transferred to the speakers by half." So you see he's wrong.
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Post by myles on Oct 25, 2006 11:04:15 GMT -7
I do not like the method of pulling two tubes out of a quad as the power supply that was designed for 100 watts as an example is never taxed at all at 50 so whatever feel the amp had is gone. You find that in many cases at moderate levels the amp has more clean headroom than before.
Cutting the power in half only drops your sound level by 3dB.
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Post by hdahs143 on Oct 25, 2006 11:16:35 GMT -7
I do not like the method of pulling two tubes out of a quad as the power supply that was designed for 100 watts as an example is never taxed at all at 50 so whatever feel the amp had is gone. You find that in many cases at moderate levels the amp has more clean headroom than before. Cutting the power in half only drops your sound level by 3dB. I agree wholeheartedly. I'm a big fan of using things the way they were designed to be used. I don't own a Marshall and if I did, it would be a 50watt, non master volume, as to me, that is a great sound. I only mentioned the tube pulling scenario because I had read about it in Groove Tubes Tube Amp Book, and I felt it was an example that applied to this threads topic. Great book by the way! That and Gerald Webers book! ;D Harold
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Post by billyguitar on Oct 25, 2006 12:01:18 GMT -7
SG123: Your post wasn't up in when I made my comment. I wouldn't dispute what you've said anyway. The "He" in my post refers to Jeff Bober.
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SG123
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Post by SG123 on Oct 25, 2006 15:24:12 GMT -7
So we were typing at the same time! I was wondering who you were talking to !
You talkin' to me ? ;D
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