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Radio

Discussion in 'Shock Brother's DIY Amps' started by radiocaster, Jul 8, 2020.

  1. radiocaster

    radiocaster Poster Extraordinaire

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    I got this one from some guy for free. It was in a basement, which had sand on the floor and was kind of a pain to clean up. Unfortunately had to get rid of the wooden parts because they looked like they were heavily munched on by a bunch of wood mites, and also very brittle and broke when I lifted it.

    The speaker looks pretty good, except for that yummy rubber the mites ate. Unfortunately, it measured only 6.5" without the rubber part.
    p1.png
    p2.png
    I had a slight shock when I clipped the leads, so I think it's a field coil speaker.

    Would I be able to use the output transformer for anything? As the coil in the power circuit at least?

    The chassis doesn't look too bad for what it is, there is a little rust though. I might decide to spray paint it. p3.png
    p4.png
    p5.png
    The output transformer is inside the chassis. The rectifier tube mounts horizontally above the power transformer.

    It came with Svetlana tubes dated 1954. I was able to test the 6P6S and it tested as functional. My tester doesn't test the other tube types, but they look good. I accidentally broke the 6B8S when trying to remove it.

    Here is the schematic:
    pionier016a.php.jpeg
    I am thinking of keeping the power section, of course changing the caps. The cloth wire is pretty gnarly too, I think I'll have to change that as well.
     
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  2. dougsta

    dougsta Tele-Meister

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    Hi, it's hard to tell from the photo but the speaker looks like a permanent magnet type, just test if it is magnetic.

    As the 6B8S is broken, you'll need a preamp stage, re-use the octal socket and find a 6SL7 or similar to drive the 6p6s (6V6 equiv). You could use diode rectification too if the tube rectifier B+ is a bit low.
    Don't use the capacitors, the resistors might be ok.

    Should sound good.
    Doug
     
  3. radiocaster

    radiocaster Poster Extraordinaire

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    Interesting. I tried to drain it with a resistor to see if it had any juice in it, and it did indeed attract metal things, so I guess it is magnetic.

    What's the deal with that coil in the power circuit that's in the output transformer? It's pretty weird.
     
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  4. tubegeek

    tubegeek Friend of Leo's

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    OK, that IS weird. I think it's negative feedback applied to BOTH the speaker and the screen of the output tube?

    Here's my reasoning: the center tap of the output primary is an AC ground (the power supply.) So whatever signal is developed in the upper half of the primary will be mirrored (opposite phase) in the bottom half.

    That signal is sent both into the secondary and the screen. The 3K3 resistor and the 30uF cap will filter all the highs out of the nfb signal and so the screen will see an out of phase low frequency feedback. So the result should be low-cut filtering of the output.

    I think.

    That's all I've got and it's a real head scratcher.

    Of all the things I've never seen before, this is one of them!
     
  5. dougsta

    dougsta Tele-Meister

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    I have an output transformer that was in a 1950's radio, it has a tap in the primary and is used as a HT supply choke :)

    See attached pic
     

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  6. dougsta

    dougsta Tele-Meister

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    Here's more from the service sheet, radio is a Cossor Melody Maker 500
     

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  7. radiocaster

    radiocaster Poster Extraordinaire

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    Last edited: Jul 16, 2020
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  8. printer2

    printer2 Poster Extraordinaire

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    They use the OT as a choke to feed the rest of the radio dc. With the current flowing the opposite direction that to the plate it bucks the magnetizing current that you normally would get from running SE.Some hum in the power supply to the output transformer is canceled out with the current going the opposite direction.
     
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  9. tubegeek

    tubegeek Friend of Leo's

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    Super genius!

    So OK, I was right that the LF was cut due to "NFB" (cancellation really), I just didn't realize it was hum and not audio.

    Do I get a half a point?
     
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  10. printer2

    printer2 Poster Extraordinaire

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    And a high five.
     
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  11. dougsta

    dougsta Tele-Meister

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    We got there in the end, high 5's all round

    I'm using that 1950's OT with the extra tap as a regular choke between the 1st and 2nd caps in the power section of a 2x10 watt stereo audio amp project I've currently got running. It works as good as a "real" choke I have.

    Installed the OT/choke at the top left of the chassis in the attached pic
     

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