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looking to build 6l6 based amp from leslie 51c

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  • looking to build 6l6 based amp from leslie 51c

    I happened upon a 51C Leslie power section that has some lovely components in it, really oversized PT, and a nice big OT. I was hoping to put some kind of fender bassman type circuit in it.
    The OT is based around 2 6550's and has a single 16r tap. There is a 125r resistor with a 200uF cap from the centertap to ground. Is this some type of dampening feature or is this to create a better balance with the tubes used?
    There is @ 400v on the plates, so according to the 6550 data the transformer has around a 3500Ra, with a turns ratio of around 15:1.
    If i kept a 16r speaker load, would that mismatch be way out of line for the 6L6's or is it worth pursuing?
    I'm not a big fan of the 6550's for guitars..
    Thanks
    Ian

  • #2
    Notice the 51C has cathode series feedback from the OPT secondary. This is the same as the Leslie 145. There is quite a bit of similarity between these two schem's and I suspect that the OPT is the same as the regular 122 or 145 style Leslie amp.

    In the 122, which has no secondary feedback I have replaced these 6550 tubes with EL34s with a very minor modification (see below). I suppose it could be done to the 145 configured amp. The modification gives slightly less power but a much better overdrive characteristic. Overdrive is blasphemy for some organ players. They like it clean, clean, clean. OTOH, some like a bit of grind. For these players the EL34 gives a nice easily controllable overdrive. IMO the stock 6550s sound like crap when overdriven, giving a gravely crackly noise that corresponds to the periodic signal peaks induced by the Hammond's chorus/vibrato.
    Attached Files

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    • #3
      Rudeboy,
      ah yes. I hadn't looked at the 122 schematic. Definitely something worth looking into. I have far too many EL34's laying around.
      How exactly does tying the secondary of the transformer to the cathode change the circuit? What purpose does it serve?
      I try to gently prod further in my understanding of circuits, and tranformers still are a bit mysterious to me in that way. I was trying to figure out the nature of this particular one in terms of turns ratio, etc.
      For instance, what kind of tube could i use if i had an 8r load instead of 16?
      Because of the low Ra of the 6550 it seems that it would be difficult to make an amp with 2 output tubes if the load was any smaller than 16.
      I would love any edumacation you wanna throw my way. All ears as they say.
      Thanks for the tips and schematic by the way. I will definitely experiment with it when time permits. For now off to play for the next couple of weeks
      Ian

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      • #4
        Originally posted by imo View Post
        How exactly does tying the secondary of the transformer to the cathode change the circuit? What purpose does it serve?
        We are talking about the 145 Leslie amp here. The 122 doesn't have it. It is series voltage feedback. Follow the signal. As the grid voltage of one tube increases so does the voltage across the secondary and thus the tube's cathode voltage increases giving negative feedback. If you wire it up wrong the voltage would drop giving positive feedback so you have to get the phase right.

        I was trying to figure out the nature of this particular one in terms of turns ratio, etc.
        I have one lying about and have been meaning to measure it. I'll get back on that and post in this thread.

        Since that OPT is center-tapped, it should be possible to run either a 16 ohm load across the full secondary or a 4 ohm load from one side to the center tap. I'd be reluctant to try it though because for the same output power the current would go up and who knows whether the OPT was designed to handle it.

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        • #5
          of course! that makes sense. I tend to look a little one dimensionally at amp schematics. from what i've read that would tend to not be so desirable in a guitar amp(the negative, cathode based, feedback), but then again i've never built anything of that type. I know the MArshalls do a somewhat similar thing into the 5k presence pot, coming in at the PI.
          I guess it would probably be safest then to just stick with the 16r load and keep the centertap grounded.
          BTW, when you are testing the OT what method are you using? I have tried taking low level signals and doing the math, but i know that transformers are not linear and would imagine this could distort the results
          Ian

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          • #6
            The Leslie output transformer measured out at about 15.5 to 16:1 turns ratio depending on frequency, measured from the primary to full secondary (skipping the center tap). With a 16 ohm load that gives about a 4k ohms primary reflected impedance. This was a simple secondary to primary voltage test. In one case the primary was open circuited, in a another it had a 22k load. Both test had approximately the same result.

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            • #7
              Rudeboy,
              Thanks for making the test. Looks like that could work well with a EL34 style amp/ I think i will go that way with it. I used to have a clubman 35 that was a pretty great amp. I might use that as a template. for it. Thanks again
              Ian

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              • #8
                I am finally digging into this project. I have decided to use this chassis to build an amp loosely based on the Matchless clubman 35. This is a cathode biased 2 EL34 based amp. I had one for many years, and feel like it is a great amp. I am not worried about the power transformer, as the 51c powers the 2 KT88, as well as 2 6973's.
                I am wondering about the output transformer however. It specs out at @ 4000:16, though i think that Matchless tended to use bigger wattage transformers, and punishes the EL34's by putting 430v on the plates, running them super hot(25v cathode, 380 s.grid). Also, the OT has a centertap, which i would probably use as the 8r tap(as i have a few 8r cabs) Is there any problem with using the centertap in terms of wattage in a design like this?
                I will be running the EL34's with slightly larger cathode resistors(300 instead of 270) and may be running the plates slightly lower, depending on what voltage i have coming out when i get the whole thing going..

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                • #9
                  The amp is coming together nicely! I am messing with different rectifier tubes to get where i need to be. The clubman runs the tubes really hot, but it is a phenomenal sounding amp. I have mine up and running and it sounds fantastic. My tone stack is a little funky, but i can't get a rvs log pot for the bass. I also have linear pots in right now, so i will get a better throw once i change them to log
                  I am going to try a one tube reverb. If that doesn't work, i will go the more traditional AT7 driver route, but i have heard good things with using a single 12ax7, and my chassis is getting really crowded as is.
                  Anyway, kind of talking into the void, but posting the info in case anyone else wants to walk down this road.
                  Best
                  Ian

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