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Dirty-South Blues Harp forum: wail on! >
Speaker impedance....again....
Speaker impedance....again....
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HawkeyeKane
988 posts
Jun 15, 2012
9:35 AM
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This topic is a moldy oldie. But I'm trying to get a little clarification. These are two posts from another thread.
toddlgreene: "On another thread I mentioned I just acquired a Silvertone 1482 amp head. Thanks to Hawkeye Kane, I learned I've got an 8 ohm output. Here's the question: Can I BRIEFLY hook up to my only cabinet, which is 16 ohms, without causing damage? At what point would doing something like this cause damage, and what exactly would happen? Enquiring minds want to know."
MP: "nothing bad will happen. it's just not right. no big deal. you are just useing double the specified impedence. your head may have to work harder but not much harder. some folks would use 4ohm to really make it grind. that is more dangerous."
Okay, dangerous how? To the amp? To the speaker? I ask because today I'm trading my Marshall MG10CD for a Univox UB250 bass amp speaker cab, which, being a bass speaker, is rated at 4 ohms. Now, my Vypyr is 4 ohms, so using that with the UB250 is no problem. But if I were to plug an 8 ohm amp into it, even briefly, what might occur?
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Hawkeye Kane
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Jim Rumbaugh
734 posts
Jun 15, 2012
7:20 PM
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Here's my unproffesional opinion.
Ya feel lucky?? Just do it.
I would not worry about brief use. Don't run the amp on max.
I haven't owned a tube amp in years, but here's some ramblings from my training in electronics from 1967 to 1970. Any impedance mismatch means the maximum powere will not be transfered from the amp to the speaker. In the case of tube amps, or amps with an output transformer, that energy has got to go somewhere. So over a period of time, you could overheat the amp (tubes and/or transformer). Or you may feed too much power into the speaker. I can see enough variables to argue both ways.
But if you like that overdriven sound of an amp turned up, then you may run into problems over a period of time.
If you wanna be safe, buy a huge 4 watt resistor to put in series with the speaker.
Now let me be creative for a minute. A typical 1500 watt toaster should have a resitance of about 9.6 ohms ( per the formula watts = Volts squared divided by watts). So 2 toaster wired in parallel would be about 4.8 ohms. Then wire those in series with the speaker. You could then have music and toast at the same time ..... maybe.
:)
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theharmonicaclub.com (of Huntington, WV)
Last Edited by on Jun 15, 2012 7:21 PM
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SuperBee
318 posts
Jun 15, 2012
9:42 PM
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What could happen: if the impedance is too high it's like running with no speaker connected. Like, no speaker is the ultimate in high impedance. High impedance, no current, high voltage exceeding design tolerance. Pop! Something gives, shorts out.
If the impedance is too low, you get more current and exceed component tolerance. Maybe the tubes burn out, or maybe the OT.
But there are tolerances. When you exceed the tolerances you will have trouble. Maybe you can run within tolerance for a long time. But I think it would be generally true to expect a mismatch to result in shorter lifespan for some components. I forgot what sort of amp you are running. Some are more robust than others. I've heard many fenders are well able to cope with a 2:1, 1:2 mismatch, also Mesa boogie seem to cope well. Oh, of course it will also change the sound somewhat
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5F6H
1235 posts
Jun 16, 2012
3:50 AM
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The big problem is that not all amps that use a certain type of power tube, use an output transformer with the same primary impedance. Ideally you want to establish this before trying anything (Power off, remove tubes, apply a small AC voltage to the OT secondary wires, then measure the voltage developed accross the primary. Then divide primary voltage by the secondary, square the answer, multiply by speaker load. This can also be roughly worked out from manufacturer's schematics IF they contain AC voltages (Fender schems at Fender.com do, but most just show dc voltages).
6V6s typically use 4,000 to 10,000ohms, 6L6s use 2,500 to 7,000ohms. You can see from these figures that the range of "safe" impedances is larger than halving the larger end, or doubling the smaller end of the scales, BUT YOU MUST KNOW the impedance, DO NOT ASSUME that doubling or halving will work.
You can also see that if your impedance is at one, or other, end of the scale, that you may not have ANY room for mismatch whatsoever.
BF/SF Fenders with the parallel speaker "Ext." jacks have a middle of the road primary impedance, to allow safe operation with half the rated speaker load...these are safe to assume halving/doubling is acceptable EXCEPT 4x10" Concerts & Super Reverbs! (Running any amp into 1ohm load is asking for trouble).
Nevertheless, there is also one universal truth, or caveat...speaker impedance "floats". A speaker's minimum impedance at it's resonant frequency, is how we ascertain its rated "impedance". So an 8ohm speaker is 8ohms at perhaps 90Hz (low frequency) as the frequency of the notes you put through it rise, so does the impedance. That speaker's impedance NEVER falls below the nominal rating. So, every day, in normal operation we are constantly using speakers at higher than rated loads...this suggests to me that doubling the load, at the very most, is safe for any amp.
As you mismatch down to a lower impedance, you reduce AC voltage accross the OT primary, but you increase current, which increases heat, which kills insulation & burns out wire.
As you mismatch up, you increase voltage but reduce current. Eventually voltage spikes will reach a point where they can punch through insulation. But doubling speaker load only increases the AC by around half (not double)...should be safe enough.
Just in case, people skipped over this before ...WORK OUT or REQUEST A REFERENCE as to your OUTPUT TRANSFORMER PRIMARY IMPEDANCE & work from there. DO NOT ASSUME that just because your pal with a Fender 'gets away' with mismatched speaker cabs, that your amp will too. Transformers are the most expensive components in a tube amp chassis.
Apologies for the use of caps, but it is important that assumptions are not made in these areas.
SS amps are less fussy about impedances, just be sure to NEVER short the output.
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Last Edited by on Jun 16, 2012 5:07 AM
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Jim Rumbaugh
735 posts
Jun 16, 2012
5:04 AM
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@Superbee & 6F6H
So what's your opinions to his oringinal question?
8 ohm amp
4 ohn speaker?
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theharmonicaclub.com (of Huntington, WV)
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5F6H
1236 posts
Jun 16, 2012
5:11 AM
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"So what's your opinions to his oringinal question?
8 ohm amp
4 ohn speaker?"
Hi Jim, it's not as safe as 4ohm amp & 8ohm speaker. I wouldn't do it without fuller information.
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Greg Heumann
1647 posts
Jun 16, 2012
7:23 AM
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5F6H - what a great answer. People often just want a simple answer but those can be dangerous. People also sometimes recite old wive's tales as answers - and that can be dangerous too. Your answer says (and I agree 1000%): If you're going to mess with this stuff:
1) Do the math
2) Here's how
3) Here's why
You're a great resource!
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/Greg
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5F6H
1237 posts
Jun 16, 2012
7:42 AM
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Thanks Greg (blushing...), right back at ya! :-)
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HawkeyeKane
989 posts
Jun 16, 2012
12:39 PM
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Okay...here's the deal. Last night I ran the Vypyr into it and it worked wonderfully. However...
My bandleader is dying for me to use a tuber tonight. My 'Zoo is still out of commission, and the only other tube amp I have at my disposal is our bassist's Marshall C5 head. It can only run at 8 or 16 ohms. Tonight's a big jam fest that our band is hosting. I'm basically only gonna be playing the first 40 minutes, and the last hour of the show. Too long to be running an 8 ohm amp into a 4 ohm 15" speaker?
And just out of curiosity...does anyone make like an attenuator for speaker impedance? LIke the VHT Special 6 has three ext speaker jacks on it for 4, 8, and 16 ohms. Is there any kind of adapter or unit that one can run the speaker outputs into and select what impedance they wanna run out of it?
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Hawkeye Kane
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5F6H
1238 posts
Jun 16, 2012
1:48 PM
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"Tonight's a big jam fest that our band is hosting. I'm basically only gonna be playing the first 40 minutes, and the last hour of the show. Too long to be running an 8 ohm amp into a 4 ohm 15" speaker?"
You can hook up the amp to a 4ohm speaker for a decade if you want...you just better not play anything through it! ;-) It's not the time that is critical, it's the AC signal voltage & current...5W amp and "jam" suggest that simply trying not to push it, to keep the demands on the OT down, is not really sensible advice.
"And just out of curiosity...does anyone make like an attenuator for speaker impedance? LIke the VHT Special 6 has three ext speaker jacks on it for 4, 8, and 16 ohms. Is there any kind of adapter or unit that one can run the speaker outputs into and select what impedance they wanna run out of it?"
Weber VST make a "Z matcher". But Jim touched on this issue earlier when he said, "If you wanna be safe, buy a huge 4 watt resistor to put in series with the speaker." I suspect he meant to say "4ohm" not "Watt". But if securely mounted (if it becomes disconnected your amp dies), you can run a suitably rated resistor in series with the speaker. For a 5W 8ohm output amp, a 10W or better resistor would work in this instance. A 5W amp into 8ohms makes 6.3VAC at the speaker...6.3x6.3 = 40, 40/4ohms = 10W dissipation, the resistor only has to do half the dissipating (5W), the speaker is doing the rest.
The VHT has 3 different, discrete & separate OT taps, as long as you just use one at a time, with the right load...it is always matched, there is no "attenuating/switching" per se, going on.
However, for tonight, I'd just use the Vypyr until you have a more practical/longterm answer...it's nice that your bandleader takes an interest in what you are doing, but he doesn't have to pay your repair bills! :-)
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Last Edited by on Jun 16, 2012 1:51 PM
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Jim Rumbaugh
736 posts
Jun 16, 2012
3:31 PM
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@5H6F Thanks for the correction. I did mean 4 ohms.
(or 2 1500 watt 120 volt toasters in parallel would be close enough!! {grin})
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theharmonicaclub.com (of Huntington, WV)
Last Edited by on Jun 16, 2012 3:32 PM
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HawkeyeKane
991 posts
Jun 18, 2012
7:22 AM
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Thanks, Mark, for the advice. I'm definitely gonna look into investing in that Z-Matcher. Would it be prudent to splurge the extra 20 bucks and get the 100W rated one?
I wound up using the Vypyr/Univox rig again Saturday night to much success.
Basically, the gig was a 6 band outdoor revue. Everything from blues to country, from power pop to hard rock, and of course, Springfield's very own ZZ Top tribute band, ZZ Tripp. They were the last act to take the stage before the rain began to blow in. Sound guy shut down the PA rig. Didn't stop me though.
Me, a guitarist from the hard rock band, the Billy Gibbons impersonator from Tripp, and a drummer attendee got up onthe pavilion stage without any lights or PA, switched on our amps and went to town on a slow blues shuffle in G, which suddenly morphed into what sounded like a bastard child of Tush and Whammer Jammer, but it still rocked. Had the women attendees dancing on the picnic tabels in the rain.
It really surprised the hell outta me that my little 15W SS modelling amp was keeping the pace with a pair of Marshall 50W tube heads. I still have to figure out what kind of wattage I can pump into that thing though. 4 ohm 15" alnico bass speaker that came with a 25W solid state bass amp head, and no power rating sticker anywhere on the cabinet, inside or out. Not looking to promising for higher wattage if you ask me....
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Hawkeye Kane
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5F6H
1241 posts
Jun 18, 2012
7:49 AM
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"Would it be prudent to splurge the extra 20 bucks and get the 100W rated one?" I guess so, you never know what might come your way, extra headroom won't hurt.
"It really surprised the hell outta me that my little 15W SS modelling amp was keeping the pace with a pair of Marshall 50W tube heads. I still have to figure out what kind of wattage I can pump into that thing though. 4 ohm 15" alnico bass speaker that came with a 25W solid state bass amp head, and no power rating sticker anywhere on the cabinet, inside or out. Not looking to promising for higher wattage if you ask me...."
Well the Univox speaker must be at least 25W, your "15W" SS amp won't make much, if anything over 15W (unlike a tube amp, which often make 50% over the clean W RMS rating...hence speakers often have a dual rating like "200W SS/100W tube".
This is a discussion I seem to be having on a weekly basis..."Watts" are most relevant when discussing similar amps (like a fixed bias 50W 2x6L6 amp with 2x12" and 100W 4x6L6 amp with 2x12", both running similar plate voltage). Watts on their own are a poor indicator of "loudness", if I said my car made 150BHP and asked how fast it went, I'd expect people to ask how big/heavy/aerodynamic it is, in order to guage a ball-park estimate...otherwise the answer could be anywhere from 100-170MPH. Top speed is measured in MPH/KPH, not BHP, similarly loudness is measured in dB, not W.
Most people determine Wattage by counting the number & type of power tubes...which is a bit like determining how fast a car goes, by measuring how big the fuel tank is. You are only guaging the potential, without balancing against efficiency...
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Last Edited by on Jun 18, 2012 8:01 AM
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