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  • Dummy Loads: Possible solutions

    I have a small 8W homebrew amp that has an 8 ohm output. My problem is that I have 2 8 ohm Vintage 30's (I know the amp barely drives the 60w speakers, but it sounds good). I would like to drive them both and have concocted a few solutions and I wonder which is best for the amp, so here goes:

    1) I have a 16 ohm and could wire the 2x8 ohms in series (16 ohm) then in parallel with with the single 16 ohm to get myself 8 ohm. Problem is I don't really want another speaker in the mix, but it would work, right?

    2) Might I use an 16 ohm / 15W or 25W power capacitor in place of the 16 ohm speaker in the last setup as a dummy load? Something like this:
    http://www.arcol.co.uk/uploads/products/hsseries-10.pdf

    3)Last resort is to go into the amp (it has an OT with multiple taps, 4,8,16) and convert to 16 ohm or install 2 separate jacks, 8 and 16 ohm or add a switchable output load. I'm confident to do the first, the second I have a few questions but could do, the third I'm not so confident.

    4)My last question is if I go with a 16 ohm dummy load should I be using something like this:
    http://www.duncanamps.com/technical/dummyload.html
    is it important in this application to have that dummy load react like the speakers it is running in parallel with?

    I know I've asked a lot of questions and Thank you to any who would take time to solve even one small piece of this puzzle.
    Patrick

  • #2
    If you have other taps on your O.T. that you're not using, surely that should be your first option? How hard can it be to drill a hole and fit a switch, compared to all the other complicated hookups you suggested?

    Failing that, just wire your two speakers in parallel, hook them up and hope for the best. An impedance mismatch one step downwards shouldn't harm anything.

    I have a 50W homebrew that only has a 16 ohm output, and I often use it as a bass amp with an 8 ohm 2x10". I replaced the power tubes with 6550s and it now makes about 60W into either 8 or 16 ohms.

    It's not as good as a perfect match, but it still goes more than loud enough for the band I play bass in.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

    Comment


    • #3
      Multiple taps

      Yeah, this really is the best way - but - let's say I wanted to do this today... I'm not sure I know what switch to use - could I get away with a nice honking DPDT toggle from another amp or one of my tiny blue Mountain DPDT's that I use for building effects... I guess I'm saying does the switch need to be rated at X value... my gut instinct was to add a second jack - I went ahead and bought an Alpha 12a that grounds the tip when the jack is removed. The cabinet with 2x 8 ohm 12's was being shared by two amps in a stereo config - I have since gained a second cab... Ya know... I think that's what I'm going to do - install a second jack - like Steve says, drill a hole, solder up a few wires, bang, done!

      I'm still certainly willing to listen to other opinions....
      Many Thanks and Happy Chrismakwaanzachannukah............

      Comment


      • #4
        I thought running a lower impedance cabinet/speaker than the amp would damage the output transformer???

        Just a couple of links pointing this out, the first couple of a quick search...

        Here and 14 and 15 Here

        I always stick to the impedance printed on back of tha amp, everything I have is 4 ohm anyway so all speakers and cabinets are 4 ohm. The worst that happened so far was many years ago when I didn't even know what impedance was, I wired 2 PA cabinets and the internal 12" speaker to a new Kustom SS amp, probably a 2 or 3 ohm load on an 8 ohm amp, it blew a diode somewhere, fortunately they repaired it as a warranty job. I got lucky...
        Why do I drive way out here to view the wildlife when all the animals live in town?

        My Photography - http://billy-griffis-jr.artistwebsites.com/

        Comment


        • #5
          Tube OTs vary a lot in the impedance they reflect to output stages, and tbes have a wide range of tolerable primary impedances, which together produces a wide ballpark for possible speaker impedance. Problem is, where are you on the ballpark?

          The reason you need to match the impedance on the secondary is so that the output tubes see the impedance on the OT primary that they want. But this does vary a great deal. The range in the impedance that output tubes can tolerate is wide one figure I have heard is that a pair in push-pull want to see from 10% to 20% of the overall resistance of one tube. Different manufacturers' OTs might be anywhere in that ball park or, often, outside it.

          Any given tube amp is likely to be quite happy operating at any two given standard speaker impedances - say, 4 or 8 ohm, or 8 or 16 ohm, given the width of the ballpark discussed above. Trouble is, you don't know whereabouts in the ballpark your amp is until you know your OT better. But it's not too hard to test an OT for turns ratio - once you have that, you can work out the rest.

          Or you could just connect up, see if it sounds ok, and watch for redplating. If it works, it works.

          I'd be interested to hear about whether all SS amps are openminded about output impedances - I know that many are but don't understand the electronics.

          Comment


          • #6
            Originally posted by Alex R View Post
            I'd be interested to hear about whether all SS amps are openminded about output impedances - I know that many are but don't understand the electronics.
            Generally SS amps don't care as long as the load is greater than a a specified minimum load. You don't have output transformers. It is determined by how much current can be drawn from the output transistors. Most SS amps have a 4 ohm minimum but many have a 2 ohm minimum load. You can run an SS amp with no load at all all day. Just don't short the output. For SS amps you can always add a 2 ohm or so resistor in series with the load to bring it up to operational parameters. It's done all of the time.

            Comment


            • #7
              Thanks for that olddawg.

              Comment


              • #8
                OT impedance

                I always stick to the impedance printed on back of tha amp...
                Me too..

                The reason you need to match the impedance on the secondary is so that the output tubes see the impedance on the OT primary that they want.
                How do I determine this - I'm running 2X 6V6's (technically 5992's) in parallel.
                When I built the kit the supplier stated to use the 2500 ohm figure as primary impedance. In this situation the 125CSE the last tap is used and only capable of 8 ohms max (if it were 5000 I could get 16). How do I know / how can I measure the impedance the OT sees from the circuit/tubes (a slightly modified 5F1 Champ) to know for sure?

                Any given tube amp is likely to be quite happy operating at any two given standard speaker impedances - say, 4 or 8 ohm, or 8 or 16 ohm, given the width of the ballpark discussed above. Trouble is, you don't know whereabouts in the ballpark your amp is until you know your OT better. But it's not too hard to test an OT for turns ratio - once you have that, you can work out the rest.
                How can I determine the turns ratio and how is this info applied to determining my ballpark?, do I need the specs from Hammond? Will I have to do math???!!!!

                I suppose another solution is to wire the speakers in parallel and switch to the
                next tap at 4 ohms, duh... I'm still interested in all the tech stuff above....

                Comment


                • #9
                  OK so you know your Primary Z is 2.5K and your secondary is wanting to see 8ohms.

                  2500/8 = 312.5
                  Sq root of 312.5 = 17.7
                  Your turns ratio is 17.7:1

                  Your OT doesn't know that it is 2.5K:8ohms...it could just as easily be 5K:16ohms, or 10K:32ohms...it only knows the ratio between primary & secondary.

                  Having said that 2.5K is at the lower end of the ball park for a pair of 6V6s (most SE 6V6 amps have OT's with a primary Z of 5-8K per tube, halve this for 2 tubes = 2.5K to 4K), running the current tap into a 4ohm load would mean 1.25K:4ohms...IF your 6V6s go pop (they may not) you'll know why they did.

                  Connect up the 2.5K to 4ohm tap.

                  Comment


                  • #10
                    If you're scared of math, then just wire the speakers in parallel and use the 4 ohm tap, like you said: problem solved.

                    As other posters pointed out, solid state amps can take any impedance from their rated minimum (4 or 2 ohms) up to an open circuit. A well designed SS amp shouldn't be damaged by any impedance at all, down to a complete short. Protection circuitry should shut it off before any harm gets done.

                    Tube amps are happiest when the speaker impedance is within the "ballpark" that Alex R. suggested. The most dangerous condition is an impedance higher than the rating, eg. a 16 ohm speaker plugged into a 4 ohm amp. This causes excessive voltages that can arc over the output transformer, doing expensive damage. An extreme case of this is forgetting to hook up a speaker at all, which is an impedance of infinity.

                    Connecting an impedance lower than the rating just makes the power tubes work harder and less efficiently, which may wear them out prematurely.

                    In all discussions of speaker impedance, you should bear in mind that the actual impedance of a speaker varies by about a factor of 20 over its frequency range. An "8 ohm" speaker might be 6 ohms at some frequencies, and 100 ohms at its resonant frequency.

                    If you ponder this, you can see that Alex R's "ballpark" must be rather wide for tube amps to survive in the field, which they have been doing for maybe 70 years. If playing a string of notes at your speaker's resonant frequency was enough to blow the O.T., something would have been done about it by now!

                    So mismatching by a tap in either direction probably isn't much cause for concern.
                    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

                    Comment


                    • #11
                      No, just kidding.. I'm not really scared of the math as long as I have the right formula - statistics in college was a breeze, just plug in the numbers and do the math... sort of. But, it doesn't fry your expensive NOS tubes (even though these things are the toughest out there) - and having said that, I'll be popping in my $20 tubes for this modification! I guess I'm just trying to understand my amp a little better in this whole process.

                      MWJB
                      Thanks for the "math". I understand it a bit better now... the spec on the OT states the yellow tap is 8 ohm @ 5k primary / 4 ohm @ 2.5K primary - so if I connect an 8 ohm speaker the primary sees 5k / if I use a 4 ohm speaker(2 8 ohm in parallel) the primary sees 2.5k ohms.... the green tap 8 ohm @ 10k primary / 4 ohm @ 5K then it's a matter of will my tubes be happy with one arrangement over another? What spec in the tubes am I looking for to determine this? My guess is the tubes won't work as hard with the yellow with 4 ohm speaker arrangement, thus would work harder and could "pop/fry" with the green. Am I close, in the right ballpark (ta dum dum chshhh ? And where does the OT ratio fit into all this figuring?

                      I will be hooking up the 2.5K/4 ohm tap... still interested in the theory...

                      Thanks so much guys....
                      Last edited by captntasty; 12-17-2007, 03:53 PM. Reason: grammar

                      Comment


                      • #12
                        As you already know primary & secondary Z, you don't really need to be concerned with the turns ratio...for unknown values you would apply a voltage to the OT to asscertain the turns ratio & therefore Z.

                        For a 4ohm load try both yellow (2.5K) & green (5K) taps & see which you prefer, either option will work, as Steve says, the impedance of your speaker floats with frequency response. Neither of these taps will result in any damage.

                        Comment


                        • #13
                          Originally posted by Paleo Pete View Post
                          I thought running a lower impedance cabinet/speaker than the amp would damage the output transformer???

                          Just a couple of links pointing this out, the first couple of a quick search...

                          Here and 14 and 15 Here

                          I always stick to the impedance printed on back of tha amp, everything I have is 4 ohm anyway so all speakers and cabinets are 4 ohm. The worst that happened so far was many years ago when I didn't even know what impedance was, I wired 2 PA cabinets and the internal 12" speaker to a new Kustom SS amp, probably a 2 or 3 ohm load on an 8 ohm amp, it blew a diode somewhere, fortunately they repaired it as a warranty job. I got lucky...
                          The first link is wrong, the guys don't know what they are talking about and are confusing problems associated with running a solid state amp with too low impedance load with a tube amp doing the same. While this can harm a solid state amp and cause a failure it is not really harmful to a tube amp, other than making the tubes work a bit harder. The second link agrees, maybe you misread it, go back and read it again; they both tell you that the danger comes from running too high an impedance load, not from too low; and if you have to run a mismatch the safest mismatch is with a lower impedance than the amp is designed for. I.e. an 4 ohm load into a 16 ohm amp is safer than a 16 ohm load into a 4 ohm amp.

                          Comment


                          • #14
                            "I.e. an 4 ohm load into a 16 ohm amp is safer than a 16 ohm load into a 4 ohm amp."...running more than 100% mismatch either way is asking for trouble, don't do it, especially if you don't know what your primary Z is (e.g. if your 6L6 amp already has a 2.5K/tube primary Z OT you're already on the back foot) run an amp expecting an 8ohm load into 2 ohms, or an amp expecting 4ohms into 1 ohm, and your tubes may only last minutes.

                            Your amp will run most efficiently at a perfect match (well as near as you can get with nominal values anyway).

                            In Captntasty's case, neither his green or yellow taps into a 4 ohm load exceed a 100% mismatch. Both are a viable option.

                            Comment


                            • #15
                              Originally posted by MWJB View Post
                              "I.e. an 4 ohm load into a 16 ohm amp is safer than a 16 ohm load into a 4 ohm amp."...running more than 100% mismatch either way is asking for trouble, don't do it, especially if you don't know what your primary Z is (e.g. if your 6L6 amp already has a 2.5K/tube primary Z OT you're already on the back foot) run an amp expecting an 8ohm load into 2 ohms, or an amp expecting 4ohms into 1 ohm, and your tubes may only last minutes.

                              Your amp will run most efficiently at a perfect match (well as near as you can get with nominal values anyway).

                              In Captntasty's case, neither his green or yellow taps into a 4 ohm load exceed a 100% mismatch. Both are a viable option.
                              Of course, it's best to match the impedance. I used an example of the extreme to make the point that the lower impedance load is the safer of the two. BTW, a 4 ohm load on a 16 ohm amp isn't more than 100% mismatch. 100% of 16 is 16.

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