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  1. #1
    Medlem Album Eier
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    300b SET by J.C. Morrison


    My first diy 300b SET - found the schematic online and started to order parts...funny project and fantastic outcome.



    5u4GB as rectifier, 6SN7 as driver and 300BXLS from emissions LAB - tested output with my HP8903 and managed to get 15W pure tube craziness

    Gå til albumet her

  2. #2
    Overivrig entusiast
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    Well done! What is the operating point of your 300BXLS'es? I've made something similar many years ago, but that was with standard 300B tubes.

  3. #3
    Medlem Album Eier
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    Hi VB,
    Thanks a lot !!!
    The 300BXLS'es can handle max 550V...I run them at 466V @ 80mA...as far as I remember was the plate curve quite linear around that voltage

  4. #4
    Overivrig entusiast
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    And B+ or bias voltage if I may ask? (sorry for this "interrogation". Just trying to learn as much as possible).

  5. #5
    Medlem Album Eier
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    I think the grid voltage is about -100VDC, B+ @ about 460VDC and the plate current is about 80mA...if you like I can send you a copy of the schematics

  6. #6
    Hifi Freak Dyret's Avatar
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    Nice! Can you just post the schematics here?

  7. #7
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    If the operating point of the output tubes is 466V/80mA and bias is about 100V, I would guess that B+ is 566V with so called "auto" bias.

  8. #8
    Medlem Album Eier
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    sure Navn:      300B-SET-Amplifier-Schematic.jpg
Visninger: 289
Størrelse: 75.0 Kb

    I made a few modifications on the way and changed R8 and C7....R8 to reach higher voltages C7 for the eyes. Thought they would look better so they are just for the optics
    The large silver cylinders on the PSU are the C7 caps. Solen 650VDC 100uf. Had to polish them for a week or two.

  9. #9
    Medlem Album Eier
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    Sitat Sitat fra VB Se Innlegg
    If the operating point of the output tubes is 466V/80mA and bias is about 100V, I would guess that B+ is 566V with so called "auto" bias.
    You know what I'm kinda keen to find that out....have vacation now and try to find time to hook up a handful of multimeters and measure once more...its been a while that I completed the build. So I could be wrong...better save than sorry

  10. #10
    Overivrig entusiast
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    Yes, get back to us. But be careful and don't electrocute yourself in the process. I hate to be the one responsible for that.

  11. #11
    Medlem Album Eier
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    Sitat Sitat fra VB Se Innlegg
    Yes, get back to us. But be careful and don't electrocute yourself in the process. I hate to be the one responsible for that.
    I make sure that I have one hand behind my back while measuring

  12. #12
    Bransjeaktør jane's Avatar
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    The 300B needs a good driver that delivers enough clean voltage into the grid. But you will pay a price when the driver DC connects to the input stage. The price is that it burns away more than 70 volts in the cathode resistor (R3), voltage that the driver cannot exploit. If you look at the schematic, the max positive voltage swing from the driver stage before it runs into cutoff is equal to the voltage drop over R4 which is 290v-215v = 75v. That can limit the ability to drive the 300B into full tilt, due to the design of the driver.
    And I love to live so pleasantly - Live this life of luxury
    Lazing on a sunny afternoon

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