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Elimination of 15db Gain Limit on Amateur Amplifiers

Discussion in 'Ham Radio Discussions' started by KI4AX, Aug 2, 2017.

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  1. NL7W

    NL7W Premium Subscriber QRZ Page

    An SB-200 or SB-220, and most of the components in my old Alpha will resist transient pulses to a greater degree as compared to a solid-state amp. By their inherent high-voltage design, tubes are not nearly as susceptible to damage. It won't take a direct strike, only a nearby induced one coming in via commercial power or via the antenna system, to take out a solid-state PA without a ccommercial-grade grounding and bonding system.

    If you cannot see this, I cannot help you.

    This is not to say that I will never own an SSPA. It only means that my station will meet Motorola's R56 -- Standards and Guidelines for Communication Sites, before I go down that road. My station will far exceed NEC Code...

    BTW, I work as an 800 MHz trunking system manager, and have had direct strike damage at a new comm site TWICE this summer because of an improperly installed new site grounding system (the site has a lightning-attractive 250-foot freestanding tower). The site's ground, measured at the bonding conductor exiting the ground from the building's ground ring and bonding 4 feet away to the building's AC entrance panel Neutral leg, was 8.9 ohms. This is NOT low enough, given the damage to equipment inside the building -- both times. This is being corrected!

    And yes, the separate tower and building grounding rings are below-grade bonded by 2 each, 2 AWG cables, to include bonding to the site's generator, fencing, etc.
     
    Last edited: Aug 13, 2017
    WD4IGX likes this.
  2. N2RJ

    N2RJ XML Subscriber QRZ Page

    The LDMOS devices are pretty rugged. These aren't your fragile amps of yesteryear.

    I also know people who have had tube amps damaged by a nearby strike, mostly diodes and other semiconductors. Since Alpha amps tend to also have solid state circuitry, they can be damaged too.

    My station meets R56. The grounding field at the antenna field is extensive and cadwelded with a ring around the house.
     
  3. NL7W

    NL7W Premium Subscriber QRZ Page

    LDMOS are rugged operationally speaking -- yes. But I contend they will not stand up to induced HV transients. I'm fairly certain of that.
    All transients had better be effectively sunk to ground before reaching the shack.
    Good luck to you. :)
     
    Last edited: Aug 13, 2017
  4. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    I am fully aware of the capability of FET amplifiers and the potential circuit gain. I also know that as the gain of the stage increases so does the need for careful design or it can become an oscillator very quickly. Current MOS amps use DC voltages between 48 and 70 volts and have better linearity than a comparable tube type amp. They are not operating at more than 15db total circuit gain in fact, most are slightly less. The Elecraft KPA-500 will "output" 500 watts with just 40 watts of drive, that's pretty good.

    Anyway, there is plenty of design data available for power MOS FET and LD power MOS FET's in linear amplifier service, so what's stopping you from building one. Don't forget that Lateral diffusion FET's have a substantial Miller effect that effects performance and stability in the HF range of frequencies. Also, the power supply must not only supply the necessary current without voltage sag but also must be able to maintain a highly stable idling current or the devices will become unstable, linearity goes out the window. With the devices that you mentioned the idling current tends to drift with temperature and higher gain means higher temperature.

    So your assessment that these devices can provide lots of gain is true. Unfortunately there are some issues that need to be dealt with when building a high gain amplifier that can't be ignored. Such amplifiers can be built but at tremendous cost compared to the current amateur service amplifiers.

    What I just said is, there is no money in such expensive designs for manufacturers since only a very small few would buy them. Until they become economically feasible either build your own or be happy with what's available.

    Again whiners whining.
     
  5. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    I am only surprised by your lack of understanding of MOS amplifiers and SDR. I am also surprised that you don't understand that pre-distortion is only necessary with dirty exciters that have a lot of harmonic energy in the envelope. I have heard lots of analog radios driving common tube amplifiers that sound wonderful. The most important thing to having a clean signal is understanding how to set up the radio and amplifier to produce a clean signal. I assure you it can be done, if you know what you are doing.

    AFWIW, I am one of a great many people who have helped to propel this hobby into the future. Solid state amplifiers will eventually be cost effective enough to replace tubes, until then, get over yourself, you don't know nearly as much as you think.

    They have a breakdown threshold voltage of around 75 to 100 volts depending on the device so you are 100% correct. Tubes can take a lot more abuse than solid state devices.
     
    NL7W likes this.
  6. WA9SVD

    WA9SVD Ham Member QRZ Page


    I know a LOT more now than I knew when I took my Amateur Extra Class test @ 94%. (DUH!) One of the reasons and purposes is that we (should) continue to learn!
     
  7. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    Absolutely.
     
  8. W0AAT

    W0AAT XML Subscriber QRZ Page


    I disconnect when lightning is in the area, why take a chance! And I do have a good ground system with polyphasers in a box outside the house. 7 ground rods all bonded to each other with 2/0 copper in a star pattern from the coax entrance box.
     
    KA4DPO, N2EY and NL7W like this.
  9. W0AAT

    W0AAT XML Subscriber QRZ Page


    Why reinvent the wheel? http://w6pql.com/1_kw_sspa_for_1_8-54_mhz.htm assembled pallet for $800, complete pallet kit ~$550. Add control circuits, filtering circuits(he sells those as kits too!) and you have a low cost 160m-6m amplifier package. And might learn something from assembling it. 2 watts drive BTW! 1kw out.

    I have 2 of his 150w 1296 blocks I am currently packaging into a transverter/amplifier system. Very well built and solid.
     
    Last edited: Aug 16, 2017 at 7:07 AM
    KA4DPO likes this.
  10. WD4IGX

    WD4IGX Premium Subscriber QRZ Page

    Mind you, many of us like and prefer antiques.
     
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