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Switchable biasing Bob85 style

Discussion in 'Home Brew' started by DIRTYWHITEBOY, Dec 24, 2008.

  1. DIRTYWHITEBOY

    DIRTYWHITEBOY Active Member

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    [​IMG]

    Regulation is great,further testing required(y)


     

  2. Crusher

    Crusher Well-Known Member

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    Looks good. Curious is that made for just a 2 transistor module or is it adaptable later for more? I see that it only has 1 pot so I am assuming this is just for 2pill. Did you use a 723 or a series regulator? Can't really tell, looks like it may be hiding behind relay. Very neat & tidy shop area by the way.
     
    #2 Crusher, Dec 26, 2008
    Last edited: Dec 26, 2008
  3. DIRTYWHITEBOY

    DIRTYWHITEBOY Active Member

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    Crusher it should carry a 4 pill no problem and possibly more.the bias circuit uses a 723 reg. and the pass transistor for the circuit mounts on the amp heatsink.ive load tested it to over 1 amp at .62 volts with regulation.:)
     
  4. NorthStar

    NorthStar Administrator

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    I like the Professional boards bob uses.
     
  5. bob85

    bob85 Supporting Member

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    crusher, the ideal setup is one board per pair of transistors but the board should be capable of higher output with a few parts changes,
    212's load regulation from 100ma to over 1 amp is 7mv,
    heres a better view

    [​IMG]
     
  6. DIRTYWHITEBOY

    DIRTYWHITEBOY Active Member

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    Load testing is complete(y), regulation is 2mv at 1 amp.Installation and rf testing is next.
     
  7. mackmobile43

    mackmobile43 Jock Supporter

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    I have mine set at 250mv idle current and .640v and it seems to have a nice smooth sound on sidebands and the thermal tracking diode mounted in close proximity to the transistors does an excellent job of regulating the voltage as the temperatures rise.

    Disclaimer: This amp is the first amp I ever built about 3 years ago and has seen many,many,many changes as you can see by the empty solder blobs on the board but has been a reliable test bed for all.

    [​IMG]
    [​IMG] [​IMG]

    [​IMG]
    [​IMG]
     
    #7 mackmobile43, Dec 29, 2008
    Last edited: Jan 1, 2009
  8. mackmobile43

    mackmobile43 Jock Supporter

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    So today I plotted the power levels of my 2/2879 amp in AB and C classes in 1 watt increments from 1 watt to 14 watts carrier without mic gain on and no audio input.

    The meter used is a Bird 43 terminated into a model 82A Bird dummyload.

    In AB the power requirements and outputs are: 250ma draw for the board with .645 volt output and load regulation is 2mv with a rise of 7ma from cold to hot on the transistors so the thremal tracking sensitivity is quite good and that's adjustable in varying the value of the load sharing resistor but at the 7 watt 10ohm value installed it's very good for intial tests.

    Here are the radio input watts/amp output watts from the RCI-2950dx in AM mode into the homebrew 2/2879 amp with 3 windings on the input transfromer and 3 windings on the output transformer which I will get around to changing to 4 in and 3 out in the next series of test.

    Radio......Amp AB.......AMP C
    1...........29.............17
    2...........65.............45
    3...........102............80
    4...........122............100
    5...........160............125
    6...........180............148
    7...........199............179
    8...........209............196
    9...........240............209
    10.........245............228
    11.........260............242
    12.........265............260
    13.........280............262
    14.........280............275
     
    #8 mackmobile43, Jan 1, 2009
    Last edited: Jan 2, 2009
  9. mackmobile43

    mackmobile43 Jock Supporter

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    I ran another wattage output plot after changing the wraps, I changed the input wraps to 2 in to 4 out and could not get the input swr down when leaving the output wraps at 4 out so I put put the input back to 3 wraps and the output down to 2 wraps and found a nice compromise the the input and output match at 1.1:1.

    Here are the wattage again produce in 1 watt increment increase from 1 watt to 14 watts from the RCI-2950DX.

    Radio......Amp AB.......AMP C
    1.....22....................10
    2.....47....................38
    3.....68....................59
    4.....99....................79
    5.....122..................99
    6.....165..................120
    7.....188..................165
    8.....199..................170
    9.....218..................182
    10...235..................200
    11...245..................220
    12...260..................237
    13...280..................242
    14...280..................242
     
  10. NorthStar

    NorthStar Administrator

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    Damn good numbers
     
  11. that doesnt exactly support folks saying 2879's will do more watts in class C because of the more efficent class c design .

    of course i could be missing something . i was called a moron in another forum the other day . lol , maybe they are right ? :(
     
  12. mackmobile43

    mackmobile43 Jock Supporter

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    The purpose of this thread is not to dispute the efficiency of the 2sc2879 but merely to show that a bias circuit will create as much or more wattage with less drive input, granted the power consumption may be more for a given drive level but what you sacrifice there is more than compensated for in audio quality in every mode of tx.
     
  13. mackmobile43

    mackmobile43 Jock Supporter

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    Oh, did I mention this board is switchable? when using a separate switch to power the board it will switch the input choke back to Class-c operation when not powered.............(y)
     
  14. dudmuck

    dudmuck Active Member

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    might be better to use dBm

    Code:
        in      AB out            C out
        dBm     dBm (gain)        dBm (gain)
     1w=30......43.4(13.4)........40.0(10.0)
     2w=33......46.7(13.7)........45.8(12.8)
     3w=34.7....48.3(13.6)........47.7(13.0)
     4w=36......50.0(14.0)........48.9(12.9)
     5w=37......50.9(13.9)........50.0(13.0)
     6w=37.8....52.2(14.4)........50.8(13.0)
     7w=38.5....52.7(14.2)........52.2(13.7)
     8w=39......53.0(14.0)........52.3(13.3)
     9w=39.5....53.4(13.9)........52.6(13.1)
    10w=40......53.7(13.7)........53.0(13.0)
    11w=40.4....53.9(13.5)........53.4(13.0)
    12w=40.8....54.2(13.4)........53.7(12.9)
    13w=41.1....54.5(13.4)........53.8(12.7)
    14w=41.5....54.5(13.0)........53.8(12.3)
    
    
     
  15. mackmobile43

    mackmobile43 Jock Supporter

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    I was absent the day the professor showed this
     

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