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Suicide prevention for "competition" base amplifiers.

Discussion in 'Amplifiers' started by nomadradio, Oct 18, 2018.

  1. nomadradio

    nomadradio Analog Retentive

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    It's drop back and punt time.

    I started to write up a "how-to" for reducing the suicidal 20-plus Volt DC supply voltage inside a 'competition'-type solid-state base amplifier.

    Yes, you get more power from the transistors this way. But the "KBK" rating comes way, way down.
    As in "Keys Between Kabooms".

    I like to keep that number as high as I can get it. A 20-Volt power supply only qualifies as assisted suicide for the RF power transistors.

    [​IMG]

    We dropped the 120-Volt input to the transformer in this box with a 20-Volt transformer wired to oppose 20 of the 120 Volts. This way only 100 Volts AC reaches the big, black power transformer. Cuts that 20 Volts down to about 16.

    Of course, it's an UNregulated supply, and sags to about 12 Volts DC under load. The thing still puts out 500-Watt peaks. Enough for this guy, anyway.

    But I got bogged down explaining how a 'buck' winding works in series with another transformer winding.

    I should probably track down a YT vid with a nice, complete visual explanation of how you can turn 120 Volts AC into 100 Volts AC with a 20-Volt transformer secondary.

    Or go back to trying to do it myself.

    I find I didn't really shoot enough step-by-step pics to make this a "do it this way" kind of explanation.

    But I will say that the white doughnut in the picture is a toroid transformer that steps 120 Volts down to 20 Volts. The transformer comes from Antek, and cost 40 bucks with shipping. Suicide prevention doesn't come much cheaper than that.

    The buck winding should always have a current rating equal to the current draw of the big transformer you're dropping the voltage for. This amplifier won't draw more than 10 Amps from the 120-Volt wall outlet, so a 20-Volt 10-Amp transformer should do the job.

    We used an Antek AS-2220. It has two 115-Volt primary windings. We wire them in parallel, to connect where the 120 Volts originally feeds into the big transformer. The two 20-Volt secondary windings get wired in parallel, to get the 10-Amp rating.

    Here's a link to Antek's web page for that part: http://www.antekinc.com/as-2220-200va-20v-transformer/


    And here's where I should have shot more pics.

    And where explaining how to get that winding phased correctly is done. Get it backwards, and you'll end up boosting the line voltage to the big transformer, not reducing it.

    But yeah, this trick works.



    Gotta find a clear explanation to show what's going on that I can borrow and use for that.

    73
     

  2. kopcicle

    kopcicle Sr. Member

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    Well you usually start about the time I do (time zones and all ) What do you expect for 9:00PM ? This is usually about the time I solder my fingers together ...
     
    Robb likes this.
  3. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Is this why ENRON failed?

    Negative Energy from too much RF floating about the shack leaking in from Common Ground currents caused by Aurorae events as well as Flipper-lips on skip?

    (Conspiracy Theory here)
     
  4. kopcicle

    kopcicle Sr. Member

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  5. kopcicle

    kopcicle Sr. Member

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    aliens , it's the aliens. and I'm not talking about molm's gardener.
     
  6. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Ok, I see, connecting the dots...I appreciate you throwing the book at me...

    (It would have been nicer to just hand it to me - don't throw it so hard next time!)
     
  7. Riverman71

    Riverman71 Part-time Member

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    (MOLM) Meyer, Olson, Lowy & Meyer. A family law firm in southern California. I guess they would have a gardener. ;)

    Have to say we don't get many exchanges between the two of you. Definitely entertainment of the highest order. Love it.
     
    #7 Riverman71, Oct 18, 2018
    Last edited: Oct 19, 2018
    Shadetree Mechanic likes this.
  8. Shockwave

    Shockwave Sr. Member

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    There are also a couple of ways to re-tap the original transformer. With a high current secondary you can usually just start pulling up on the end of the winding and peel out one turn at a time, dropping about .5 volts with each turn removed. Since most secondaries are wound over the primary, another option is to remove the outside paper and use a sharp pick to "poke through" the insulation on the winding until you find the wrap that has the lower voltage you want.

    The trick you did also works on older equipment when upgrading from tube rectifiers to solid state. Since you don't need the 5 volt filament winding for the new solid state rectifiers, place that winding in series with the plate transformers primary. That will help compensate for the higher plate voltage the solid state rectifiers supply. Transformers with an unused winding can be rewired 4 different ways to buck or boost either the primary or secondary. Just be sure the added winding can handle the current.
     
    #8 Shockwave, Dec 5, 2018
    Last edited: Dec 5, 2018
    Shadetree Mechanic likes this.
  9. nomadradio

    nomadradio Analog Retentive

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    Now that's a cool idea I hadn't had. Using the now-unneeded rectifier-filament windings to buck the now-higher B+ voltage downwards a bit. But doing this to the primary of a single transformer that also powers parallel tube heaters might turn them down along with the B+.

    One of those "try it and see" techniques, I suppose.

    73
     
  10. Shockwave

    Shockwave Sr. Member

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    This could be a problem if the equipment does not have a separate plate transformer but that older equipment was designed to work with a lower line voltage anyhow. Back when it was made you might have between 110 and 115 volts but most locations in the USA are showing 125 volts today and could use that 5 volt drop on the primary even with the filament winding tied into the loop.
     
  11. nomadradio

    nomadradio Analog Retentive

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    Good point. The older an item is, the lower the line voltage it was built to use.

    73
     
  12. psycho

    psycho Running a special on our rooms!

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    Is it idling at 20 volts or does it bang out 20 volts under load?
    The Worldwide 2x8 power supply that I sold last week floated at 19 volts and dropped to 14.5 when keyed.
     
    543_Dallas likes this.

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