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4 Tube Hi Fi Amp Conversion Project

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  • 4 Tube Hi Fi Amp Conversion Project

    Hi All,

    A couple of weeks ago I got an old tube amp chassis and speaker from a friend.
    Looks like a hifi amp that someone pieced together an used as a guitar amp.
    5Y3, 1-12ax7 2-6V6s.I'm thinking 40+ years old at least.
    The speaker (I'm pretty sure) is a 50+something P-12R

    I took things apart and sketched out the layout. From that sketched a schematic.
    It's similar to the 5E3 I built, so I hacked up a 5E3 schematic to use for reference.
    Double, triple, quadruple checked against chassis wiring so I think it's correct.
    Pulled the tubes and started checking things.

    PT has 12518-2 printed on it. OT has 145A23 printed on the paper and 12519-4 stamped on the frame.
    Looks the OT is cooked. 5k one one half of the primary, 815 ohms on the other side.
    The 6.3v, 5v seem ok with all tubes pulled. Got 600+vac feeding the 5Y3.
    Tubes tested good but I'm not going to try to power it up yet. There are other
    issues. I want a fuse inline, and better wiring. And maybe a new 4x20uF 450v can cap.
    What's in there looks pretty new but I'm not sure.

    Speaker seems to work... measures 4.2ohms with DVM. This might be a little low.. or not.
    Hooked it up to a receiver and it sounded real good at low to moderate volumes.

    Now I'm trying to decide what to turn this thing into. I found Gibsonette GA-8 PP version that's
    really similar to this layout. Don't know if it's one versions that are talked about on the forums. That would simplify the build, but there might be something else worth trying.
    Since the OT is likely shot, I'm wondering if maybe trying the parallel 6V6 Gibsonette GA-8 that
    some folks are saying good things about.

    Here's the schematic I hacked together. There's no fuse, no jacks, and no bypass caps.
    Has just a volume control... a double stacked pot with .0022 caps and 22k..
    It's gonna need some work!

    Any advice suggestions etc would be very much appreciated.
    (I'm well aware of the safety concerns...)
    (they drilled that into us back in my tech school days)

    Mike
    Attached Files
    a sign on the desk of Suzanne Schroeder, collector of bureaucratic gobbledygook, AP wire story, Sarasota Herald-Tribune, July 3, 1973
    “I know that you believe you understand what you think I said, but I’m not sure you realize that what you heard is not what I meant.”

  • #2
    With just a single 12AX7 gain stage it probably won't be a very satisfying guitar amp. The global negative feedback won't help matters there either. You have a couple of options, the first being what you've already posted, change it to parallel SE operation and use the 2nd triode for an additional gain stage. Or, you might want to try staying with puhs-pull operation and try a self split power amp. I've never done that, but it might be fun to experiment with and try it out. Or, you could stick with what you have and swap out the 12AX& for something like a 7199 that combines a sharp cut off pentode with a triode, using the pentode section as the preamp and the triode as the PI. And one more thought is to incoporate a mosfet gain stage or two to boost the gain up to something more like a 5E3 style amp would have.

    I don't understand the benefits that dual volume control arrangement provides, I don't see the need for that at all. Maybe someone could explain it.

    Comment


    • #3
      Originally posted by Zephyr View Post
      Hi All,

      A couple of weeks ago I got an old tube amp chassis and speaker from a friend.
      Looks like a hifi amp.
      Mike
      Does it have a big multiple connector or a place where one used to be? A lot of old hifi stuff tapped other power supplies off the power amp to run an external preamplifier, tuner, etc. If this is true in your case you have plenty of heater current to run another couple of 12ax7s. Does the PT look beefy? Also, don't write off the OT because it looks cooked. I've seen quite a few old nasty brown and curly looking PTs work just fine. You can also consider a diode rectifier if you want a little more B+. If the PT is beefy, I would just use the iron, the tubes, and start over with a new chassis and new layout, ceramic tube sockets, etc.

      Comment


      • #4
        Thanks hasserl,

        Think I'm looking to build something a little bit different than the 5E3 this time.

        Your reply got me digging around online and looking at those combination tubes
        I found 3 6CH8 in an old radio/tv tube caddy I've had for a lotta years.
        Almost a direct replacement for the 6AN8 and 6U8 that might sub for the 7199
        They aren't quite right for this one but that sure sounds interesting.

        This chassis has been wired directly to a guitar cable on the input and I'm not sure
        where the volume pot came from. I've read about folks using a single stacked vol/tone pot
        with caps, resistors etc for tone shaping. This may or may not be the case.

        From what I see the chassis originally had an rca jack and was maybe a standalone
        amp being driven from a separate preamp. The rca jack is still connected,
        the guitar cable has been soldered directly to the tube socket. Sorta like something I did I Jr High
        when I wired my friend's suitcase stereo (yeah, back in the mid 60's) ceramic pickup wiring to
        a jack so he'd have an amp for the electric guitar he got for Christmas. It functioned,
        but it really didn't work all that great.
        a sign on the desk of Suzanne Schroeder, collector of bureaucratic gobbledygook, AP wire story, Sarasota Herald-Tribune, July 3, 1973
        “I know that you believe you understand what you think I said, but I’m not sure you realize that what you heard is not what I meant.”

        Comment


        • #5
          Hi olddawg,

          Thanks.

          The transformer looks to be about the same size as the 5E3's transformer.
          Looks real similar.

          The 12518-2 looks like some of the Hammond (I think) PT numbers.
          So far I've haven't had any luck googling for info on the transformers.
          Wonder if there's any way to decipher them to see what the current specs
          might be for the filaments and the B+

          Hadn't thought of diodes. Found a 5AQ5 in the old tube caddy. I wonder if it would be possible to
          use a 5v tube for the input to get around what hasserl pointed out about the lack of input gain as it is now.

          Anyone know of any issues when using two different windings to power filaments in the signal chain?

          I agree with you about the chassis. I'd thought of modding it as is , it would be a bear to do.
          This chassis measures 4 1/4"wide x 9 3/4"long. There is no turret board and only a 3 point terminal strip
          at the input. And very little in the way of wiring. Very compact point to point connections.
          I could probably make the GA-8 out of it as it sits because the circuits are so similar, but for anything else
          I'll need a different chassis. I've never made one. This might be good time to try to do one.
          a sign on the desk of Suzanne Schroeder, collector of bureaucratic gobbledygook, AP wire story, Sarasota Herald-Tribune, July 3, 1973
          “I know that you believe you understand what you think I said, but I’m not sure you realize that what you heard is not what I meant.”

          Comment

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