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Ebay LDMOS amplifiers

Discussion in 'Amplifiers' started by TIN_CAN, Jul 28, 2020.

  1. TIN_CAN

    TIN_CAN Well-Known Member

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    Just wondering if anyone else have seen these things on ebay, the guy is offering 3 different amps 1000w 1500w and 3000wRMS all running blf188xr these would be fun to try out, prices dont seem too bad, then again how is the quality.



    https://www.ebay.com/itm/3000-watts...409020?hash=item3b4b00b8fc:g:Vh0AAOSw0D5fF3uN

     
    Handy Andy likes this.

  2. Shockwave

    Shockwave Sr. Member

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    This almost looks like an exact copy of the one "RF Man" was demonstrating on YouTube a few months back. Like most built before it using these 50 volt pallets, they work great when properly driven. Over drive it with just one watt too many and you'll have instant failure placing about $1,000 worth of transistors at risk. If you're going to go this route with no protection, the only way to achieve some reliability is to select an input attenuator that will make any type of over drive condition, completely impossible to achieve with the rig exciting it. Unfortunately, that also reduces maximum output power.

    By the way, the knob marked "gain" is not a gain control. If it were a gain control it could be set in any position without a change in signal quality, other than strength. It's a bias control that should be used to control class of operation and not the gain of the amplifier since doing so, will introduce distortion. The power being measured at the same level for peak and "RMS" is also not so accurate. What he's using as an "RMS" rating is really a flat out CW rating since there is no capacity to modulate and peak anything above that point, like would be required with an "average power rating".
     
    #2 Shockwave, Jul 28, 2020
    Last edited: Jul 28, 2020
  3. TIN_CAN

    TIN_CAN Well-Known Member

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    Thanks for the info on these, was looking for something different then the 2879 pill boxes which I will probably just stick with for now.
     
  4. nomadradio

    nomadradio Analog Retentive

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    Not one pic of the inside. For two grand-plus I would need to see inside the thing.

    73
     
  5. Handy Andy

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

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    Because of the pellet size and mounting - not exactly a drop in, but to place scale on this, the older Pill (Read Bipolar) and its' "conjugate" needs of "interface layer" from the outsides' RF dynamics and power levels - the "Waveguide" approach is still used. So the Bias is now changed from Current thru a simple diode as a "Fix it all" is now Voltage-regulation based - and if any feedback is needed - Thermal to help with the transition.
    upload_2020-8-3_11-45-31.png

    Everyone thinks 50Volts is the new Baseline - well - they said similar with Tube designs back in the 30's and right on up to the Day they stopped making Hybrid design radios' like the T-240D - found it easier, and safer, to run single ended supplies - so can't really say the 50 Volts is a necessary requirement.
    upload_2020-8-3_11-49-40.png
    Just remember you're dealing with charts using a much higher frequency range spectrum.
    Gain INCREASES when you use lower Frequencies...
    Refer to Maximum Useable Frequency or
    HFE as your transition gain factor.​

    But if you have access to that voltage, you can use it with Envelope Tracking designs as your ability to "boost" your power envelope from your standard fare regulation (AM or Modulation) - just be careful with the outputs for small trickle currents at those voltages placed on the right parts of the body can cause Arrhythmia.

    It's not that hard to transpose what was done with Tube and Solid State - they did it before.

    You just have to reengineer the design to obtain the higher voltage - if that is a necessary requirement for you.

    But if you own a Tesla or Prius, well, let us know how it goes. Be sure to carry a Parachute, for the batteries in those vehicles do emit gasses that is not vented quickly enough, can cause injury and some considerable concussions if you don't have a sunroof or upper exit strategy in place.

    Our current (SIC) dilemma is limits, as well as the supply, not the voltage breakdowns. Heck the MOSFET is made rugged due to this. Bipolar has the voltage constraint (Direction Junction) Reverse breakdown issues. MOSFETs' give you that extra layer of protection like Cottonelle provides their fans - we're just now getting back into the fun part.
     
    #5 Handy Andy, Aug 3, 2020
    Last edited: Aug 3, 2020
  6. Trailblazer

    Trailblazer Supporting Member

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    Hello all,
    Believe me when I say I dont have a dog in this fight. I did however come across in my wanderings info and sales of this palette design which piqued my interest. I dont have the hands on experience that builders have but a general understanding of the concepts being discussed. I just found this video, right, wrong or indifferent to be quite informative
    Cheers
    Rob
     
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  7. Handy Andy

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

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    That's some great stuff right there...

    Fun to watch - well - at least I enjoyed it.

    Sorry, it's not Star Wars, it's better, much better, than that.

    There will be a Quiz on this later.
     
    Tokin likes this.
  8. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Welcome!
     
    Trailblazer likes this.
  9. kb1sel

    kb1sel Active Member

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    You'll learn more from that video with the volume turned down because what he does makes more sense than what he says. BBI mentions all types of problems from uneven trace lengths between transistors and transformers, to Rob using the wrong type of 50 ohm coax on the output transformer. He then shows us "the correct 12.5 ohm white coax”. If you look at the end result, most of what he says on audio, does not match what he did on video.


    None of the trace lengths have been made symmetrical. The 12.5 ohm coax was never used and just replaced with the same turns ratio in heavy black Teflon wire. The biggest change here was adding some capacitive padding across the primary and secondary of the output transformer. What that did was match the transistors output impedance to deliver more PEP power into the 50 ohm load at this frequency and power level.


    Make no mistake, this design is still broadband push pull even though he says otherwise. Having learned about spectrum analyzers, the biggest problem I have with this video is what looks like the intentional overdrive of the fundamental frequency done on the first test, to make the results appear horrible. BBI runs the stock test with the fundamental frequency well above the correct attenuation point, where it is now driven right off the top of the screen!


    Using too little attenuation so that the fundamental frequency is driven off the screen is the fastest way to make harmonics appear much worse than they are, to the untrained eye. Luke is careful not to make the same mistake when he makes his second improved test. Then, you will see the fundamental has been attenuated down to about 10db below full scale. This is absolutely critical when determining harmonic content since you must be able to compare the size of the harmonics, against the full size of the fundamental frequency. You can't even start this process when part of that signal is hidden off the top of the screen.


    The last issue is, you see how hot that output transformer was running at 1,400 watts? Improving the match made the RF power through that transformer double, at about the same DC current. The thermal gun never gets pointed at that output transformer after the power increases to 2,800 watts for a reason. 25 years ago "Dave" designed that style and size core as a heavy duty replacement for a pair of 2SC2879 transistors having a total dissipation of 500 watts.


    The idea that this transformer core is suitable to handle four sections of transistors with 3000 watts of total dissipation, is insane. I would avoid any amplifier that tries to stuff four 1,500 watt transistors through one push pull output transformer. It's hard enough to design a transformer that can efficiently match two of these transistors without core saturation. Doubling that RF while still passing twice the DC current through it, guarantees core over heating.


    Things like running the amplifier output right up against the power supplies current limit in order to provide amplifier protection, has other problems. If you get this at a correct point where it can protect the amplifier, the power supply can easily cause all of your positive peaks to flat top without you ever noticing it on the watt meter because the voltage must start to drop at or around the level of these word peaks. You've given yourself no headroom in the power supply because it's now being run at 100%, with no more current available. That increases the likelihood of power supply failure too.


    The video also reveals some lack of technical understanding with things like "Flyback resistors" that are found in older CRT based televisions and confusing them with "Negative feedback resistors" used in amplifiers. Then, a "Crowbar" over voltage protection circuit is mistaken as an over current protection circuit. How big of a deal are these mistakes? Well, that depends on some other things like results. While I'm quick to point out mistakes, the same can be said for pointing out the constructive things.


    Luke has indeed addressed a well known problem with this particular pallet that has driven the people that purchased it, absolutely nuts because nothing they can do, gets it to make more than half of rated power. The fact that Luke was able to identify which section of the amplifier was causing this problem and one way to fix most of it (other than transformer heat), says more about his technical abilities, than his words in the video. His clean construction skills have to be in the list of top builders in this field too. I'd just like to see competition, not cause any temptation to "fudge" test results on the SA.
     
    #9 kb1sel, Aug 6, 2020
    Last edited: Aug 6, 2020
  10. nomadradio

    nomadradio Analog Retentive

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    It appears that the DC supply current to each transistor drain passes through the brass-sleeve primary conductor on each side of the push-pull output. An alternative to this is to use the output transformer for RF current only. The DC is supplied through a push-pull 'common mode' choke with simple insulated conductors passed through separate ferrite cores. DC feeds in one end, and out two wires that attach directly to each drain.

    Seems to me that 38 Amps of DC through that output transformer would get it warm all by itself, without any RF current piled on top of that.

    Gotta wonder how much of that transformer's temperature rise is from RF losses alone, and how much it would be reduced using a separate DC choke to carry the supply current. The RF losses from core saturation would be reduced by reducing the magnetic field of the DC-supply current, too.

    And old idea of Helge Granberg's, pretty sure. You'd think Luke would know about that one.

    73
     
  11. Trailblazer

    Trailblazer Supporting Member

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    Hello all
    In for a penny, then in for a pound. Unfortunately I can scrape together the former and only dream of the latter. I would be lying if I said this surprised me as a controversial topic. The convoluted "truth" in absolute matters has yet to be determined. I am however confident when sharp minds, without hidden agendas, can speak freely amongst their peers, debate the relevant topics in a meaningful way, and yes even agree to disagree, the result is most people are ABLE to choose how to proceed. I trust people to make choices based on information and necessary personal research.
    Wow sometimes I even amaze myself. For a brief moment I thought we were talking about palette design.
    Be safe, be free
    Rob
     

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