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Old Tube Amp Without Power Transformer


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  • Old Tube Amp Without Power Transformer

    I got an old tube amp in a trade today, and it sounds pretty good. It needs some new filter caps to get rid of the hum, but other than that it's all good. It's got one of those tube radio configurations with a 12au6, 50C5, and a 35W4. It's very similar to, if not the same as a Harmony 400a:

    I see a couple transformers in there, and I know there's an output transformer with no power transformer, but is the other one in there an isolation transformer, or do I need to buy an external one?
    I'd just like to make this thing as safe as I can. Also, if it doesn't have a PT, how do I go about replacing the 2-prong power cable with a 3-prong? Can the neutral wire still go to the chassis?

    Awesome Transactions with: boonestunes

  • #2
    These are a trip, I think Gerald Weber wrote about these in one of his books (available at Google books) they use odd ball tubes to use wall power and avoid the (expensive) PT.

    The other tranny may be a choke to filter all that nasty AC, 3 wire hookup should be straight forward, maybe run it through a lightbulb (also in GWs book) to see if its ok with the new wiring; the ground should always go to chassis but it may never sound very good...at least it won't kill you!


    • #3
      Am I best off buying a power transformer and building a clone of another amp? I'd like to just keep it as it is, but if I can't do that safely, I guess I'll just have to switch some things up....

      If I can find what power transformer I need, I could probably clone an earlier Harmony amp that has the same tubes:
      Awesome Transactions with: boonestunes


      • #4
        Is it possible to get a new type plug with one blade fatter than the other & just not hook up ground? At least polarity would be correct. And most newer consruction uses gfci at least in the wet areas of the house. Also aint hard to gfci any old 120v circuit any where in a modern (built in the last 25 yrs.) house. Just need a gfci outlet.
        There can never be a perfect plan. What's perfect for one human bean ain't perfect for the next one. Cuz he's a lentil.


        • #5
          Please, at least to avoid get injuried, use an line isolation transformer.


          • #6
            Interesting schematic. Reminds me of the Knight-Kit Ocean Hopper short wave radio I got when I was a kid. Same tubes, except at 12AT7 instead of a 12AU6. Close enough The radio had a chassis ground, and an unpolarized wall plug, so we marked one side of the plug, and one side of the wall socket with fingernail polish, so that I wouldn't get shocks from it being plugged in the "wrong" way. But I digress.

            In my radio, the tube heaters were wired in series along with a power resistor, so that the total voltage in the heater circuit would be close to the 110 volts from the wall socket (50 volts for the 50C5, 35 volts for the 35W4, 12 volts for the 12AT7, and a 200 ohm 10 watt resistor).

            The H-400A has a 100 ohm resistor in the heater line, and the 2nd transformer (T2) that you noticed. I guess it is used in place of the other 100 ohms in my radio to drop the voltage and supplies the 12 volts for the 12AU6. It's definitely not an isolation transformer. And you are right about the T1 transformer, that's the output transformer.

            Assuming you could get the exact same transformer that is in the H303A (and I doubt you'd find one), you would have to rewire the heater circuit for all the tubes. That power transformer in the H303A, has a tap to supply a lower voltage to the tube heaters. Looks like around 50 volts since the 12AU6 and the 35W4 are in series (a total of 47 volts), and the 50C5 is in parallel for 50 volts on it's own.

            I think your best bet for maximum safety is to get an isolation transformer.

            BTW, a good place to ask questions like this is the ax84.com board.
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