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  • PT current question

    Hi all,
    I have a salvaged PT from a 1947 Sparton model 10-76-PA radio/hi-fi. It looks like it's got the standard HV, 6.3v & 5v windings. I'm considering using it for a 5e7 build.
    The original radio this came from was powering a full dozen tubes, mostly octals in the 6s*7 family, a few radio-specific receiving tubes, a single 6V6, a 5y3 rectifier and even a 6e5 "Magic Eye".

    I'm wondering if the main HV output of the transformer can handle a pair of output tubes. I have the schematic for the original radio, but unfortunately the transformer specs aren't outlined in it - only a part number. It's a fair sized chunk of iron, but not like something you'd find in a VT40 or anything.

    Any way to test it or am I stuck doing a whole build around it and if it doesn't handle it buy a new one?

    Cheers,
    - JJG
    My Momma always said, Stultus est sicut stultus facit

  • #2
    Here's my opinion - typically, a 5Y3 is a 125mA tube, so I'd use that number as a guesstimate.

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    • #3
      Out of a tube datasheet, 2 x 6L6 draw up to 134mA of plate current and up to 11mA of screen current with no signal on the grid - (i.e. 145mA idle). Allow 50% for max signal and 12mA current through 3 x 12AX7 tubes, and 220mA should get you there.

      Svetlana 6L6GC data (Oops it says 116mA at idle for a pair)

      http://www.drtube.com/datasheets/6l6gcsvet1996.pdf

      The other sheet I was looking at for the above numbers was a 6L6GB:

      http://www.pentodepress.com/tubes/6L6GB.pdf





      You'll need a 3A recto winding for the 5U4GB, and I reckon about 4A for the 6V heater winding.
      Last edited by tubeswell; 04-18-2008, 01:50 PM.
      Building a better world (one tube amp at a time)

      "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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      • #4
        For a pair of 6L6s, EL34s or similar, I usually count on 90mA at idle and 300mA on full crank.

        So, the PT might be better suited to something with a pair of 6V6s in the output.

        Do you know what B+ voltage the transformer gives? The 5e7 schematic calls for 415v, but a tube radio would probably have run off 250-300v. If it's 300, there's another reason to use 6V6s, because 6L6s will barely get out of first gear, so to speak.

        If it was 250, it's probably best used as a doorstop.
        "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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