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  • home brew = mystery amp

    Someone brought me a home made tube amp to fix. No tubes in it . Needs a power tube socket . Its got 2 octals..pin 1 and 8 are tied then go through a 1 ohm to ground ....1K screen resistors...would this be for a EL34 then ?

    if i remember pin 1 wouldnt be wired on a 6L6 type?

  • #2
    Pin 1 doesn't NEED to be wired on a 6L6, but that need not stop one from wiring it up anyway, then most all the tube types can plug right in, as long as bias needs are met.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Agree and add= measure and post bias voltage.
      Juan Manuel Fahey

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      • #4
        Originally posted by Valvehead View Post
        Someone brought me a home made tube amp to fix. No tubes in it . Needs a power tube socket . Its got 2 octals..pin 1 and 8 are tied then go through a 1 ohm to ground ....1K screen resistors...would this be for a EL34 then ?

        if i remember pin 1 wouldnt be wired on a 6L6 type?
        Instead of guessing how about this. No tubes , power it up and show what voltages are on what pins. There are many octals out there.
        nosaj
        soldering stuff that's broken, breaking stuff that works, Yeah!

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        • #5
          i will after i replace the socket----cheers

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          • #6
            Ok---new socket --all wired up.

            the 5y3 rect is putting out 566vdc

            measuring the power tube sockets with no tubes =
            pin 1 0v

            2 and 7 have 6.9vac heater volts

            3 has 566vdc

            4 has 566vdc

            5 has -14vdc
            6 -14vdc (the bias pot can adjust it to - 64vdc )

            8 ov

            this thing has some juice !

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            • #7
              How about a nice clean, well lit photo of the underside of the chassis?
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                pics
                Attached Files

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                • #9
                  Originally posted by Valvehead View Post
                  pics
                  Whole time I thought we were talking about a 2 tube amp based on original post.

                  nosaj
                  soldering stuff that's broken, breaking stuff that works, Yeah!

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                  • #10
                    At 566V unloaded there's no way that's for a pair of small bottles. So, that 5y3 rectifier is unsuitable. I don't know how much spec you can decipher from the power transformer, but if it "looks" like it could handle a pair of big bottles then the amp is probably for a pair of big bottles. Regarding the unloaded filament voltage, at 6.9V I wouldn't expect a pair of el34's. By my experience most unloaded filament voltages would need to start higher than that to manage the current requirements of el34's. That isn't to say the builder didn't make some errors in component choices so anythings possible. Still, I'm going to guess a pair of 6l6 types and a 5u4 rectifier.
                    "Take two placebos, works twice as well." Enzo

                    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                    "If you're not interested in opinions and the experience of others, why even start a thread?
                    You can't just expect consent." Helmholtz

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                    • #11
                      If pin 1 and 8 are tied this is the EL34 wiring.
                      1 ohm to ground is for cathode current measurement (bias adjustment)
                      It's All Over Now

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                      • #12
                        Originally posted by Chuck H View Post
                        At 566V unloaded there's no way that's for a pair of small bottles. So, that 5y3 rectifier is unsuitable. I don't know how much spec you can decipher from the power transformer, but if it "looks" like it could handle a pair of big bottles then the amp is probably for a pair of big bottles. Regarding the unloaded filament voltage, at 6.9V I wouldn't expect a pair of el34's. By my experience most unloaded filament voltages would need to start higher than that to manage the current requirements of el34's. That isn't to say the builder didn't make some errors in component choices so anythings possible. Still, I'm going to guess a pair of 6l6 types and a 5u4 rectifier.



                        ive got a mesa 5U4 here somewhere


                        what about 6550s / kt88 in this thing?

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                        • #13
                          Originally posted by Valvehead View Post
                          ive got a mesa 5U4 here somewhere


                          what about 6550s / kt88 in this thing?
                          The 5Y3 will drop something like 60V so knocking the B+ down quite a bit when loaded. Agree it seems like a poor choice for the expected DC current.

                          Another couple of clues would be the physical size of the output transformer and it's turns ratio. Does it look like a 25W or a 50W OPT?

                          You can figure the turns ratio by connecting the heater AC supply to the secondary (disconnect both wires to do the test) and measuring the heater supply VAC and the plate to plate VAC and taking the ratio.
                          Experience is something you get, just after you really needed it.

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                          • #14
                            Another rectifier alternative if you want to keep the high voltage drop is a 5R4. Takes higher AC voltage than a 5Y3 but still drops 50V when loaded properly. Has 2A 5V heaters, too so less stress on that winding.

                            Justin
                            "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                            "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                            "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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                            • #15
                              the power trans is Huge...OT looks the size of a 50 or 100w . Both have solid black covers...no brand on them.

                              part number on PT is 54-13 138839

                              OT is 51-14 138826

                              edit... looks like they are Heathkit

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