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  • A question about power ratings for speakers and amps

    I'm getting ready to load a couple of 10" speakers into a cabinet to run my amp head through. The amp is an old Bassman modded to run on a pair of EL34s. I don't know how many watts that is, but well over thirty, I'm sure.

    I have a thirty-watt Jensen and a twenty-watt Weber that I'm thinking of using.

    How likely am I to live to regret it? I checked with my amp whisperer, and he said: "You might as well try it. The worst you can do is blow a speaker."

    But...I have a line on a used 75-watt Eminence Copperhead that I could get for $45 shipped - and maybe sell the unblown Weber. What would you do?

    Thanks!

    Del

  • #2
    The tubes can put out between 50~65 watts RMS which can be more then the 6L6 tubes the original head used depending on how the head was modded and the tubes biased. I'm guessing its going to be around 45~50W based on the transformers and voltages the power transformer produces. No way of knowing 100% without testing it but the mod is common enough so we have to assume it was a mod for tonal differences, not power changes.

    The speaker power ratings are additive in series or parallel with tube amps. If the head puts out 50 watts, and you have a 35 and 20W speaker in series or parallel, the total wattage of the two is 55W. If this is a Silverface or blackface Bassman the head is likely rated for 4 ohms. It should say on the back near the speaker jacks. If these two speakers are 8 ohms you should run them in parallel.

    Running them in series would give you 16 ohms which is too high for those heads and will blow the grid resistors and cook the tubes. I had this happen several times with my Bassman head as a kid before I learned what I was doing wrong. I was buying new power tubes once a month and took the grid resistors out several times before I realized you could only run the head at 4 or 8 ohms safely. I'm not sure if the EL34's are any more durable, but there's no sense tempting the devil by abusing it.

    Your two speakers shouldn't blow connected in parallel, but they may not sound as good as having a matching pair. Normally you can match different speakers if they have similar SPL and wattage ratings. This way both will be equally loud and consume equal amounts of power. Their sensitivity/saturation curves should be similar as well. The frequency responses would likely be different and the total sound quality of the cab would be a blend of the two.

    With two different speaker wattages, the loudness of the two may be uneven depending on the volume settings. This may or may not be a cool thing. With two matching wattages the speakers should reach maximum tone quality running at around 50~77% or maximum power. With two unequal wattage speakers this power response curve is going to be different for each speaker so you'll just have to try it to see if its to your liking.

    Chances are one speaker is going to sound farty and the other clean as you crank it up, or maybe one has a higher SPL then the other and one speaker will be loud and the other barely doing anything. Unless you compare the specs on each, there's no way of knowing. For all I know these can be Hi Fi speakers with rubber surrounds and can sound terrible for guitar so you need to provide more details on the speaker types. Or you can just try it and see what results you get.
    Last edited by WRGKMC; 06-12-2014, 06:51 AM.

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    • #3
      Maximum Rating of the EL34 is 25 Watt per Tube or 50 Watts P-P from Svetlana Spec Sheet. I would use a more Robust Bass speaker like the 75-watt Eminence Copperhead. I am assuming this is a speaker designed for Bass Amps which are specially designed for low frequency response and continuous music power or RMS.

      Comment


      • #4
        I disagree with WR's analysis. Assuming that the drivers share power equally, each would get half of the available power, so the maximum power the pair could handle is 40 watts (twice the lower rating), not 55 watts. However, in practice, drivers can usually handle considerably more power from an amp than they're rated for, simply because there are a lot of times when the amp isn't running at full power, and the driver has a chance to cool down. On the third hand, if you're running a lot of distortion, then the amp will be putting out more power than the rated power, up to twice if it's a full square wave. So there's a lot to think about.
        "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
        -- Bob Parks

        "A thing is not necessarily true because a man dies for it."
        -- Oscar Wilde

        "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
        -- Oscar Wilde

        "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
        -- Theodore White

        Comment


        • #5
          ^^^ With the same impedance they should share power equally. I'm used to having matching speakers when figuring that. I tried to find a formula that figured unequal ratings and the only one I could come up with was the formula for Power.

          In series or parallel. P1 + P2 = P total In this case 35W + 20W = 55W

          I think I mixed up source with ratings in this case.

          The is a method to get it to work, even though its not what I'd choose because speakers are cheap and its best to just get the right speakers.

          You can get a couple of 8 ohm, high wattage, wire would ceramic resistors and put one in series with each speaker, then wire both in parallel. This should give you 8 ohms total which the head could handle and the wattage would be dropped to around 12.5W per speaker.

          The problem is, wire wound resistors aren't very good inductors and don't change their resistance anywhere near as much as a speaker coil does so it does tend to brick wall the amps transformer more then speaker coils would. Based on the experience I've had with the blackface I've owned for 47 years it would work OK, but with a head that's been modified I probably wouldn't tempt fate. Just spend $50~100 and get a matching pair that can handle the power.
          Last edited by WRGKMC; 06-13-2014, 08:53 AM.

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          • #6
            WR, are you not aware of the nature of speaker impedance? The rating, 8 ohms for example, is only a nominal value. The DC resistance might be as low as 5 ohms or as high as 7.5 ohms. From DC, the impedance rises to a peak at the resonance frequency (or two lower peaks with a dip in between for a ported cabinet). After that it goes down again to a low not too far above the DC resistance, after which it rises again. Even the same model driver from the same manufacturer can have different DC resistances and different resonant frequencies, though they are usually close enough that load sharing is not adversely affected. In this case, we're talking about different drivers from different manufacturers, which will inevitably have different impedance profiles, and will not share equally.
            "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
            -- Bob Parks

            "A thing is not necessarily true because a man dies for it."
            -- Oscar Wilde

            "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
            -- Oscar Wilde

            "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
            -- Theodore White

            Comment


            • #7
              Originally posted by isaac42 View Post
              WR, are you not aware of the nature of speaker impedance?
              Yes I understand Impedance and how it changes with frequency. The higher the frequency the higher the reluctance.

              Comment


              • #8
                There's more to it than that, as I wrote above. Here's a plot of one 12" driver:
                http://www.jensentone.com/frequency_...850&height=550
                It's a Jensen Mod 12-35, 8 ohms. At 75Hz, the impedance is about 70 ohms. At about 150Hz, it's about 8 ohms. DC resistance (DCR) is 6.7 ohms.

                Here's another Jensen, the P12N:
                http://www.jensentone.com/frequency_...850&height=550
                At 90Hz, the impedance peaks at about 50 ohms. Minimum impedance is also 8 ohms, but at about 330Hz.

                So at 150 Hz, the 12-35 has an impedance of 8 ohms, and the P12N about 12 ohms. If they're in series, then the P12N will dissipate 60% of the power, the 12-35 40%. In parallel, the power sharing would be reversed. At 90Hz, the P12N is 50 ohms, while the 12-35 is about 14 ohms. In series, the P12N will dissipate 78% of the power, the 12-35 only 28%. Again, reversed if they're in parallel.

                At higher frequencies, the sharing is much better, but it's never going to be exactly even.
                "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
                -- Bob Parks

                "A thing is not necessarily true because a man dies for it."
                -- Oscar Wilde

                "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
                -- Oscar Wilde

                "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
                -- Theodore White

                Comment


                • #9
                  I used to use a volt meter when tuning the cabs I built. You'd find the speakers resonant frequency in the specs, then feed the speaker with that frequency. Then you would lengthen or shorten the port till you had the highest AC voltage. The theory in back of it is, as the resistance increases, the greater the voltage drop. (I believe that's how I did it, its been a good 30.years) It might have been done with current, or you could do it with a current meter as well except you'd go for a minimal current reading because current goes up as impedance goes down. Main thing is you'd adjust for the resonant peak so the speaker produced maximum bass response within the cab.

                  From what I remember, the resonant peak is where the speaker is freest to move and act like a DC resistor with the largest peak-to-peak amplitude and velocity. The back EMF generated by coil motion in a magnetic field is also at its maximum.

                  At high frequency extremes the coil acts more like a choke than an electromagnet producing back EMF. In between the resonant peak and the high frequency peak the impedance is more capacitive and can drop to half the DC resistance. .

                  Chances are that higher frequency peak is above speakers maximum Frequency response range. (at least with must musical gear) If a speakers rated for 3K max, that second impedance peak may be above the speakers ability to produce those frequencies so its not like you have to tune a cab for two resonant peaks. This may be something that affects Full Frequency Speakers, but with Musical instrument speakers they either target a speaker for the instruments range or use crossovers with separate drivers.

                  This is unique to tubes an transformers of course. SS has a lower impedance to where the back EMF is less of a problem affecting sound quality.

                  Comment


                  • #10
                    Amazingly, you've all been a big help. Thanks!

                    A few things:

                    - It turns out the speaker my friend gave me isn't a Jensen. I was happy to discover when I took it out of the box that it's a 30-watt Eminence (Fender Design) V 1030.

                    - Yes, I switched to EL34s for tonal reasons. Also for tonal reasons, the amp now also has a tube rectifier, a mids knob, a master volume, and a more useful circuit. A blackface never had it so good.

                    - The speakers are both eight ohms wired in parallel, and yes, it is a four-ohm amp.

                    - The fact that two EL34s can put out up to 65 watts is news to me and good to know. It'll help me make speaker decisions in the future. Likewise, learning that the higher-watt speaker is likely to dominate the tone is something I never thought about but seems reasonable.

                    - The 20w+30w=50w information is also a huge help. I don't turn the amp up all the way and don't go very crazy with overdrive pedals, so it'll probably work.

                    - I took my amp guru's advice and tried it. Very happy - a real upgrade. It has all the headiness and character of a good Eminence-loaded Fender amp, but (thanks, I'm assuming, to the lower-watt Weber) there's a Cream-flavored crunch that lacks (in a good way) the out-of-control quality of a Marshall. Like a good son-in-law, it cleans up nicely.

                    So I'm going to keep it this way for a while and see how it does. If I decide it's not what I want (or if the Weber blows), I'll probably replace the Weber and put back the 75-watt Ragin Cajun that used to be in there or find a 30-watt speaker to share the load with the V 1030.

                    Thanks again. You just taught me a lot!

                    Del

                    www.thefullertons.net
                    Last edited by Delmont; 06-16-2014, 05:19 PM.

                    Comment


                    • #11
                      I'm sure you'll be fine. I love theoretical analysis, but I recognize that the way things work in theory isn't always the way it works in practice.
                      "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
                      -- Bob Parks

                      "A thing is not necessarily true because a man dies for it."
                      -- Oscar Wilde

                      "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
                      -- Oscar Wilde

                      "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
                      -- Theodore White

                      Comment


                      • #12
                        I believe the EL34's need a higher plate voltage to get up in the 65W range. If the amp is using the stock transformer of the Bassman, its unlikely the amp will produce any more then it did running the 6L6's. I know Music Man 65 has a much higher plate voltage of 585V on the plates and puts out 65W with the high power switch on. If its switched to low the Plate voltage drops to around 400V and the wattage drops to around 40~50. (sounds like 40W to me but loudness can be deceiving) I think the Bassman uses around 425V for the HV bias with its stock transformer so I doubt it would produce any more clean watts then the 6L6's provided.

                        My sound City also put out 65W but I don't think they changed HV bias to do that. I's pretty sue it does it by boosting preamp gain and run the tubes hotter all the time. Haven't used that amp in a long time because I blew the transformer in it but I remember the hiss from the power amp remaining the same when you kicked it from high to low power so it probably didn't change the plate voltage to change the power output. Not sure what plate voltage they used on that one but it was a loud amp for sure. Maybe it was only 50W and had all its gain with the first 50% of the amp volume turned up. Should have used a Variac to power it up after changing the power caps. Finding another Partridge power transformer is hard to come by.

                        Comment


                        • #13
                          Glad to know I'm probably not pushing 65 watts. I'd generally rather play a turned-up lower-wattage amp than a turned-down high-wattage amp. Right now, I seem to have plenty of headroom when the master volume is turned up all the way. Haven't taken it out of the house yet, though. I'll let you know after I've been to a rehearsal with it.

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