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Mystery Speaker from 760 Leslie

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  • Mystery Speaker from 760 Leslie

    The badly butchered Leslie 760 I bought came with this 15 inch 4 ohm woofer. I would like to know what it is so that I can sell it and raise funds for a 16 0hm 15 inch since I am driving it with a 147 amp. Thanks
    B. The speaker is in really good shape and sounds good. I just did not want to run it too hard or to long as the 147 is looking for a 16 Ohm load.
    Attached Files

  • #2
    1098 = Pyle

    031070, according to another post on this forum, is the stock speaker number for a 760 woofer. Leslie may have started sourcing woofers from Pyle after Utah closed down. Some early 122A Leslies have Pyle woofers.

    Running a tube amp with a lower than expected load will not harm the amp or speaker. The 4Ω impedance will, however, throw off the crossover frequency if you're using a standard 147 passive crossover. It will drop the effective crossover frequency for the woofer to ~200Hz, so you'll have a hole between 200 and 800Hz. It's the midrange where it won't sound right. That said, the impedance may look too high to the tube amp between 200 and 800Hz.

    You might not get that much for a Pyle woofer since Pyle doesn't have the brand reputation of other speaker companies.
    I'm David. 'Dave' is someone else's name.

    Comment


    • #3
      Originally posted by David Anderson View Post
      Running a tube amp with a lower than expected load will not harm the amp or speaker.
      This is categorically incorrect and dangerous advice!
      Valve amps MUST see the correct load or you run the risk of damaging the output transformer and output valves.
      There is no golden rule such as "It's OK to run a lower load, just don't go higher".
      Current:
      1971 T-202 with Carsten Meyer mods: Remove key click filters, single-trigger percussion, UM 16' drawbar volume correction. Lower Manual bass foldback.
      Korg CX3 (original 1980's analogue model).
      1967 Leslie 122 with custom inbuilt preamp on back panel for 1/4" line-level inputs, bass & treble controls. Horn diffusers intact.
      2009 Marshall 2061x HW Plexi head into Marshall 4x12 cabinet.

      Former:
      1964 C3
      196x M-102
      197x X5
      197x Leslie 825

      Comment


      • #4
        Originally posted by Papus View Post
        This is categorically incorrect and dangerous advice!
        Valve amps MUST see the correct load or you run the risk of damaging the output transformer and output valves.
        There is no golden rule such as "It's OK to run a lower load, just don't go higher".
        My grandfather taught me this, and he was an electrical engineering professor at Clemson and North Carolina State University who was educated during the golden age of tubes. I'm also on multiple professional tube amp tech lists where this is accepted as Tube Amp 101.

        What happens when a load is larger than expected is that the reflected impedance on the output transformer primary is higher than it should be. With a very high reflected load, the primary winding can produce extremely high voltage swings that can punch through the magnet wire insulation in the transformer, short output tubes, or produce arcs between output tube pins 3 and 2. The primary winding acts like an ignition coil.

        I had to replace output tubes and output tube sockets on a Fender Bassman where the owner's son plugged his amp into a subwoofer, not realizing that the subwoofer had a passive filter blocking much of the audio band, making it appear like an open circuit above subwoofer frequencies. The tubes were shorted, and there were carbon tracks burned between plate and heater pins.

        Also note that many guitar amps with 1/4" speaker connectors use shorting jacks such that the output transformer secondary is shorted when no speaker load is present. That's because you can short the output of a tube amp with no problems.

        I hate to be so blunt, but, in this case, you simply don't know what you're talking about. This is not controversial.

        I can explain it. Can you?
        I'm David. 'Dave' is someone else's name.

        Comment


        • #5
          Take it to Van Audio on Guelph St. just south of East Broadway.They'll make up a nice basket with a 16 ohm kit.
          One of my ceramic square back types was rebuilt for 16 ohms and is in a 145 working perfect!
          Might even take your 4 ohm to them for a look as a 'basket' ? Anyway,they'll measure the magnet etc.
          By the time you get a mail order offshore speaker,they'll have one ready to go for you.
          The 15" P15LL I've been using in a '64 251 box they built for me? Nothing like fresh paper!
          Both recording and performing micing up beautifully!
          A100/251 A100/147 A102/222 B2/142 BV/147 BCV/145 M3/145 M102/145 M111/770 L101/760 T222/HL722 M111/770 no B3/C3!

          Comment


          • #6
            Hello,

            When the impedances are matched, the power dissipated in the amp is equal to the power send to the HP and it is a maximum.

            Example:
            Zamp = 8 Ohm; Zspkr = 8 Ohm
            > Pamp = 50W; Pspkr = 50W

            The same (purely theorical) calculation give with an HP of 4 Ohm :
            Zamp = 8 Ohm; Zspkr = 4 Ohm
            > Pamp = 89W (+ 89%); Pspkr = 44W (-10%)
            It sounds fine, but the tubes are overloaded as explained by Papus.

            When the output is open, Both powers = zéro.
            Then, the voltage at the primary of the output transformer is doubled in steady state.
            But can reach very high values on transient as explained by David's grandfather.

            JP
            Last edited by Jyvoipabo; 12-13-2018, 03:20 AM.

            Comment


            • #7
              I think your purely theoretical calculation ignores a several real-world parameters such as output tube plate resistance and power supply limitations. It's telling that you don't post how you arrived at your figures.

              I've been working on tube amps since 1990, longer if you count my education in my grandfather's home laboratory, and to date, I've never seen a tube amp with damage that could be traced to a speaker impedance that was too low. I know of a 1953 Premier Multivox 120 amp that apparently had the wrong speaker installed in the factory, a 4Ω speaker where an 8Ω speaker should be. It was undamaged. It didn't sound as good, but that was the sole observable effect of the speaker impedance mismatch after 60 years. (The amp's owner collected amps and had two of them. The output transformers and speakers were identical except that one speaker seemed to have the wrong voice coil even though it wasn't reconed.) I've also run into quite a few Leslies with 8Ω replacement woofers in them. It's a common mistake. None of the amps showed any signs of damage.

              Also, not every amp designer, historically, has chosen the same plate to plate impedance for a given pair of output tubes, so that alone would throw a monkey wrench into your calculations. Datasheet specs are often different for fixed bias vs. cathode bias.

              In contrast, I've had to fix quite a few tube amps that had tube, tube socket, and output transformer damage from being inadvertently run with no load. Bassmans and Marshall heads are particularly vulnerable to this since speaker cables can get unplugged or damaged.

              Granted, optimal load provides best power transfer. No one is arguing that.

              Still, running a tube amp with a low load isn't going to do much to it, if anything. Running a tube amp with no load can cause major damage in a few seconds.
              I'm David. 'Dave' is someone else's name.

              Comment


              • #8
                Vancouver Audio,great speaker shop! No argument there!
                Veering back to the OP.........hey Bri.......playing Saturday at the Fairview 4:30 sharp!
                I'll be there.
                A100/251 A100/147 A102/222 B2/142 BV/147 BCV/145 M3/145 M102/145 M111/770 L101/760 T222/HL722 M111/770 no B3/C3!

                Comment


                • #9
                  I do not ignore the impedance of the tubes.
                  They are included in the 8Ohm of the output impedance.
                  This is why the output transformer exist.

                  I also explained that the tubes would be overloaded on low impedance load, with respect to the situation where impedances are adapted.
                  Overload does not necessarily involve damage.

                  This is the basic laws of electricty, that i don't want to rewrite.

                  JP

                  Comment


                  • #10
                    Originally posted by Jyvoipabo View Post
                    Overload does not necessarily involve damage.
                    That's the critical distinction. If you read the post I was responding to, the claim was that this would cause damage to the output transformer and/or output tubes.

                    I readily admit that a tube amp usually sounds its best with the optimal impedance load. No one is arguing that.
                    I'm David. 'Dave' is someone else's name.

                    Comment


                    • #11
                      Originally posted by David Anderson View Post
                      ...Running a tube amp with a lower than expected load will not harm the amp or speaker...

                      ...I've never seen a tube amp with damage that could be traced to a speaker impedance that was too low. I know of a 1953 Premier Multivox 120 amp that apparently had the wrong speaker installed in the factory, a 4Ω speaker where an 8Ω speaker should be. It was undamaged...

                      ...In contrast, I've had to fix quite a few tube amps that had tube, tube socket, and output transformer damage from being inadvertently run with no load. Bassmans and Marshall heads are particularly vulnerable to this since speaker cables can get unplugged or damaged...

                      ...Still, running a tube amp with a low load isn't going to do much to it, if anything. Running a tube amp with no load can cause major damage in a few seconds.
                      Umm... aren't you all just talking about the same thing, but with a bit misleading terms?

                      No load = highest possible (infinite) impedance = open circuit -> problems with tube amp (that's why they'll need a dummy load at least), which means:
                      Highest possible load = shorted circuit = lowest possible (0Ω) impedance -> no problem with tube amp (but no-no with a solid state one...)

                      Now, how we should use term "load" then? Is a lower impedance 4Ω speaker higher or lower load to amp than 16Ω?

                      Comment

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