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AA864 Project Question

Discussion in 'Shock Brother's DIY Amps' started by Jeru, Sep 30, 2015.

  1. Jeru

    Jeru Tele-Holic

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    I'm planning to build a single-channel Fender Bassman (AA864) and then sell my vintage one.
    The Bassman is diode rectified, and wants a B+ of about 422vdc. Schematic is below.

    I have a Bogen H-30 that I can very likely use as a donor for OT and choke, not sure about the PT. The Bogen sports a whopping 780v CT, 380-0-380.
    Looking at the Bogen schematic, even with the voltage drop of the 5U4, I'm not sure how how the B+ is so low (435v)..?

    The real question is, what do you think about that circuit with a 5U4 (or other voltage-dropping tube rectifier) instead of the diodes? What changes in
    sound/feel would you anticipate if I did it this way..?

    I could spring for the ~$100 new production PT, but don't really want to unless using the Bogen PT and a tube rectifier would really change what I'd get.

    Any thoughts would be greatly appreciated.

    AA864 Schematic is HERE
    Bogen H-30 Transformer specs are HERE
    Bogen H-30 Schematic (with voltages) is HERE
     
  2. muchxs

    muchxs Doctor of Teleocity

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    The Bogen shows a plate voltage around 420v, the Bassman shows a plate voltage around 420v.

    What's the problem? If the transformer fits the hole... use it!

    Using my tube rectified fudge factor calculator of 1.15x the voltage on each side of the center tap I get 435v, perfectly acceptable for 6L6s.

    The reason the supply voltage in the Bogen seems high is if you're used to SS diodes. Those max out around 1.4x. or 532 volts.

    The idiot math ignores the fact it's all voltage at a particular load. Unload the power supply, the voltage skyrockets. Load it heavily, the voltage goes astonishingly low and the PT gets hot.

    Regulated power supplies are common in just about everything besides guitar amps. The supply voltage in a regulated power supply is a lot more predictable. It stays relatively constant regardless of load.
     
    Last edited: Sep 30, 2015
  3. Jeru

    Jeru Tele-Holic

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    Thank you for the thoughtful response. So, knowing what you know about designing and implementing power supplies,
    how will the AA864 (or whatever mutt circuit I end up building with these parts) sound or feel differently if tube
    rectified?
     
  4. muchxs

    muchxs Doctor of Teleocity

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    Tube rectified isn't the only factor at play. The output transformer and parts selection will contribute to the overall tone. We're talkin' the convergence of multiple variables so who knows?

    My prediction: A new "Bassman" built with old iron and mostly new parts should spank the pants off your old Bassman. :cool:
     
  5. Jeru

    Jeru Tele-Holic

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    This is great. Thank you.

    One last question -- Thinking about chassis layout now. Would you advise against having the filter caps inside the chassis as opposed to outside and covered,
    assuming that the grounding is done properly and decent spacing is observed so things don't get crowded?
     
  6. muchxs

    muchxs Doctor of Teleocity

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    Considering it's a one channel Bassman there should be a lot of room available inside. Having said that some amps pack it all into a relatively tight space, the tweed Champ for example. Those literally have a chassis the size of a postcard.
     
  7. Jeru

    Jeru Tele-Holic

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    With an old RCA 5U4GB and some cheapo 6L6s plugged in (as well as all the preamp tubes), I get 408vac on each side of the center tap,
    and rectified B+ of 480vdc at the first filter cap. SO - voltage is high. I still don't know where their voltage readings are coming from on
    the schematic. I'm not sure I care though -- Looking ahead, it's
    about how to get the voltage that I need out of it.

    I'm thinking about using ~5 zener diodes between the PT's center tap and ground as per Rob's handy diagram. Will these get it done?

    http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_804754_-1
     
  8. muchxs

    muchxs Doctor of Teleocity

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    Take another look at the Bogen schematic. There's a big fat choke in front of everything. If you use the Bogen transformer go with the Bogen power supply including the big ol' choke.

    Most of the numbers are irrelevant until you load the power supply. That means all your tubes should be lit up and functioning as they should. Set the amp (your Bassman clone) up cathode biased temporarily.

    Here is an experiment you may want to try when you get a chance: Try it with any old Fender or Marshall with adjustable fixed bias. Measure the supply voltage then set the bias tube meltin' hot. Observe what happens to the supply voltage.

    Next, adjust the bias ice cold. Observe the supply voltage.


    If you end up with 480v on the plates... no big deal. Bias your 6L6s at 44ma-45ma. You'll likely end up juggling bias because the plate voltage is a moving target.

    Bear in mind that '60s Marshalls are Bassman circuits with elevated supply voltage.
     
  9. Jeru

    Jeru Tele-Holic

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    Excellent, I'll give it a go. Thanks all.
     
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