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4s Universal Tube Preamp build

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  • 4s Universal Tube Preamp build

    With lots of help from tom_s I am attempting to build the 4sUniversal tube preamp found here; He also thought it best to do a writeup. I have a couple free days not dictated by overtime or homework and hope to get this thing up and running in that time. Fingers crossed.

    Keep in mind, I hate electricity and electricity hates me. It's a relationship that goes waaaaay back to childhood.

    The wiring in our home was done by your's truely, but there's really not much figuring required for that, just basic stuff. Something learned about electricity was to not trust my memory on whether the breaker to the circuit being worked on was indeed off and never, UNDER ANY CIRCUMSTANCES, p on an electric fence. Trust me on that one. Seriously, never ever do it. Don't even dare a drunk buddy. Other than that, I know about as much about building electronics as my dog does about rockets.

    There is likely to be some smoke, maybe fire, but hopefully no electrocutions.

    Basic sheet of aluminum which just so happens to be the same thickness as a tablesaw blade. You'll see why later.

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    Drilling holes for the transformer, wires, tube sockets, phono plugs, etc.

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    And the reason for the aluminum sheet being the same thickness as a saw blade. For the bottom and sides I cut a few pieces of scrap to ~3" x whatever length it took to fit around the metal sheet then cut a groove in the board for the sheet to slide in. Figuring this project may be dissembled many times during the build, and facilitating my clumsy hands and soldering ability, the ends are simply held in place by screws. Not pretty, but unscrew one end and the metal sheet holding everything slides out making things easier to work on.

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  • #2

    The parts are in, just waiting on the wood to dry.

    Now what Tom???

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    Bottom side, most of the stuff that needs bolted down is. Used Loctite on all the bolts/nuts.

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    • #3
      I sent Kornbread a PM answering some questions. Thought I would post this here in case someone else wants to take the tube plunge at some point.

      On the EZ81 (rectifier tube), red wire goes to pin 1, other red to pin 7 on that socket. The Red & Yellow lead will go to your ground buss, starting at the 33uf cap. The EZ81 and 12AX7 don't have the same pinout, but you can Google the and see the connections on the data sheets.

      On the Hammond transformers, they give you the option of wiring the primary side to run off 115 or 125 volts. I know in our neighborhood the voltage coming in from the pole is high (124+), so I would use the White & Black, then just heat shrink the gray and tie it off somewhere.

      The little > symbol represents a heater coil for a tube. You'll see two for the 12AX7 tube. Those tubes have two 6.3v segments in the heater. You'll be running them in parallel, so one lead goes to pin 9, the other goes to both pins 4 & 5. You could tie 4 & 5 together with a small piece of wire and just wrap your heater lead around that. You can use the red & white twisted wire I sent to go between a terminal strip near the transformer and the tube sockets. You can do separate runs to each tube socket that way. That might also make it easier to wire those 100 ohm resistors to the ground buss.

      The caps should have a designation for the negative terminal. Usually a stripe or a black dot on the terminals. Don't know why they do it that way, but that's just the way it's always been for most brands.

      I'll be on the road heading to my brother's for a while tomorrow. I'll check in before we leave and when we get to the hotel. Bruce has a nice picture of his wiring in the article - that might help answer some questions, but you can't quite see everything. If anyone else can help with answers, please feel free to chime in. I know the basics, but I'm not what you could call a tube guru.
      Co-conspirator in the development of the "CR Gnarly Fidelity Reduction Unit" - Registered Trademark, Patent Pending.


      • #4
        I cut a rabbet and made the plate to fit inside it exactly. Basically press fit, so that I could remove the plate without using tools.

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        • #5
          That is gorgeous! Love the stripe in the base. What tubes are those?
          Co-conspirator in the development of the "CR Gnarly Fidelity Reduction Unit" - Registered Trademark, Patent Pending.


          • #6
            Here's some great comments/advice from the MAC forum from PWRRYD (aka Craig)

            I have two comments:

            1) Make sure the aluminum top plate is properly connected to earth ground for safety. Then your B+ power supply's ground must be correctly tied to this point as well. This is crucial for safety! Tom or I can go into more detail if needed.

            2) That Alps volume pot is nice but its metal case (the threaded mounting shaft and front plate) are electrically tied to the ground lugs. You need to electrically insulate it from the aluminum top plate. Otherwise you create a ground loop and will get some crazy sounding noises. Especially since this preamp has some decent gain. On my personal projects I usually mount the pot through the front wooden case panel, so it is floating. On a rebuild I did for a friend's preamp he had the Alps pot mounted just like you do. There were these really strange swooshy noises. Took me a while to hunt down that issue. I ended up drilling the through hole over sized and used an insulating shoulder washer set, like your insulated RCA connectors use.
            Co-conspirator in the development of the "CR Gnarly Fidelity Reduction Unit" - Registered Trademark, Patent Pending.


            • #7
              AEIOU That's sweet. Mine ain't gonna look nuttin like yours.

              Used heatshrink on the volume pot shaft and a tiny piece on the tab that holds it from turning, then made a gasket from an old motorcycle innertube to insulate it from the sheet metal. Checked ok.

              To the pic.

              Used a terminal strip at the -33uf (#1) so all of this needs grounded, that's why you called it a ground buss? The other terminal strip (#2) does not get grounded?

              These resistors (#3) look different (these are smaller and brown while those on the website are blue and larger) than those on the website. The package was labeled as 100ohm. This is where those 100ohm resistors on the power supply schematic should be? Are they directional?

              Since so much stuff needed pin 4&5 on ez81 (not enough room) I ran a short jumper from those pins to a terminal strip and connected everything else there (as Tom suggested). The heater feeds from pins 4&5 go directly to the 12ax7 heaters pin 9 and 4+5 with nothing in between? I see a couple 100ohm in the schematic but those are the pair tying pins 4&5 (ez81) together and then feed back to the -side of the 33uf cap?

              Should the multimeter show continuity between pins 4&5?

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              • #8
                Originally posted by tom_s View Post
                That is gorgeous! Love the stripe in the base. What tubes are those?
                I built it from a kit purchased from S-5 Electronics. 11MS8 tubes.

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                Last edited by AEIOU; 12-24-2018, 05:12 PM.


                • #9
                  Very cool! And your work so far looks great. After checking out the link to the original design, I must say I am a little puzzled as to why the designer didn't incorporate a 2nd cathode follower stage per side. It seems like it would confer quite a bit of sonic advantage at very little added complexity or cost.


                  • #10
                    Have to give a shoutout to PWR RYD and a gigantic shoutout to tom_s without whom I would have already abandoned this project.

                    Important safety warning first. The linked preamp has a ground lift switch, I was warned not to due this in fear a short on the HT (high tension ~250volts) B+ line could energize the sheet metal with potentially lethal voltages.

                    I found this website helpful and easy to understand. It helped in understanding what went where and why.

                    Google is your friend when searching for tube pinouts. There are many tubes, many tube sockets, and many different pinouts. Even a little knowledge helps immensely in getting everything wired.

                    Check out the classification chart about halfway down the page. Does anyone else see faces in those tubes?

                    Axial caps have a - and + side. It matters which side goes where.

                    Used about half a dozen 2/3 lug terminal strips in this small preamp. If several wires go to one or more lugs on the tube socket it get crowded in a hurry. Place a terminal strip near the tube. Run single leads from the tube socket to the terminal and use the terminal for all the other connections. Thanks tom_s.

                    The stem and metal face of the ALPS volume pot RK27 250kohm PE part#023-202 needs to be electrically separated from the metal chassis. Thanks PWRRYD.

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                    More wires.
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                    Other than power in and its sides, it's pretty close to slapping some tubes in it and making smoke. You can see how the preamp section grounds all go back to the same spot. That then travels back to the - sided of the 33uf cap on the power supply side where it grounds to chassis and where the ground to earth will also be. I hope only one ground point to chassis will help with hum.

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                    50 watt head I have no idea why the designer did it this way, nor do I have any idea what I'm doing ... but I'm learning and I find it exciting.



                    • #11
                      Originally posted by 50 watt head View Post
                      Very cool! And your work so far looks great. After checking out the link to the original design, I must say I am a little puzzled as to why the designer didn't incorporate a 2nd cathode follower stage per side. It seems like it would confer quite a bit of sonic advantage at very little added complexity or cost.
                      I think he (the designer) just wanted to highlight a very simple single-stage design. He said it came from a discussion on their forum. I totally agree that adding a cathode follower would be a good choice - definitely lowers the output impedance. But the added complexity might look too daunting for first time builders and this will give DIYers a taste of the tube sound. Then they are hooked, eager for their next tube fix...searching the web for those hyped Euro smooth plates! I have a few SRPP linestages that might actually have fewer parts under the hood, but they probably seem more complex for a newbie staring at a schematic. I look at this like building a single opamp headphone amp in an Altoids tin - a great way to jump in and learn something new.
                      Co-conspirator in the development of the "CR Gnarly Fidelity Reduction Unit" - Registered Trademark, Patent Pending.


                      • #12
                        Here is a transformer that would be ideal for tube hifi (and guitar) preamps:


                        It allows the use of a 12VAC wall wart, to keep all of the big AC safely at the wall and out of the chassis.


                        • #13
                          After some though to Tom's post it seems that discussion on the other diy site was geared towards being able to row a lot of different tubes and being able to hear their sonic flavor. Maybe that had something to do with it?

                          Anyhow ...

                          tom_s alerted me to the cathode grounds I missed, all from a simple pic. Good eye Tom. Those nasty cathodes are now grounded with 1.2kohm resistors.

                          Sides/base on. Didn't have those gaps in the corners before, guess the wood shrunk. Oh well, first try at this and I am a bit giddy to get er' done. The wood is just scrap Pine, if she works and sounds good I'll use some better wood and rework the base. The second switch in the middle was the ground lift switch which we decided not to do for safety reasons. It's just there to fill the hole.

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                          W/tubes. Note to self, look at the description when ordering stuff. The volume knob is tiny and really throws the look off. It may take a few beers before getting enough ba**s to actually smoke test the thing. Tom, which one of those tubes you sent were cheapest, don't want to cook a good one.

                          Time to get caught up on homework then maybe a couple mommy/daddy beverages to calm the nerves before plugging it in.

                          What goes better with tubes and high voltage; rum, whiskey, or beer?

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                          • #14
                            Replaced that tiny volume knob with this. Better? Thanks for the idea PWR RYD.

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                            • #15
                              And now some pics of those glowy glass things lit. To my son's dismay, no smoke, no sparks, no one got electrocuted. Yep, I know, I'm surprised too.

                              Plate voltage was a bit high on the output tube at 280Vdc. May have to do something with that.

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