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2x4 Davemade

Discussion in 'CB and Export Equipment and Accessories' started by Dean Street, Sep 26, 2019.

  1. Dean Street

    Dean Street New Member

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    Is it better to stay with 2 2290 driving 4 2879 or 2 2879 driving 4 2879.

    Feedback


     
    Shadetree Mechanic likes this.

  2. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Welcome! Two driving four is a lot of drive. Personally I would prefer a straight 4 or 6. What radio will you be driving it with?
     
    binrat and mrgumby like this.
  3. Dean Street

    Dean Street New Member

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    Connex 4300-300 in which the amp was bypassed
     
  4. Dean Street

    Dean Street New Member

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    I've used this radio with a straight 4 pill high drive and it did awesome.

    2 2290s driving 4 2879 would be much safer for my work truck with a 160 am Alternator instead of 2 SC2879s driving 4 SC2879s
     
  5. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Is that Connex a dual final radio or a single final radio (since the internal amp has been bypassed)? If it's a dual final radio, you'll cook the 2 x 4 amp, as 40W+ of drive is way too much for that amp. If the Connex is a single final radio only, that won't be as hard on the amp. Will you be running it AM only or SSB too?

    The 2 - 2290's will be better for the amp than 2 - 2879's in the driver section of that 2 x 4. Putting 2879's in the driver section will kill the final section pretty quickly.


    2 x 4 amps are not a good design, as it's very easy to drive the final section of that amp into saturation because of all the output from the driver section. It's better to run it as a straight 4, It'll last longer, won't run as hot, and will sound cleaner on the air. (y)


    ~Cheers~
     
    loosecannon, NZ8N, 543_Dallas and 2 others like this.
  6. nomadradio

    nomadradio Analog Retentive

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    One 2290 is more than enough drive power for four 2879s.

    Tricky part is finding a radio small enough to match a single 2290. If you put enough resistors in the input circuit to drop the drive level to the transistors you can use any radio you like. I'm still waiting to see a 300-Watt amplifier with a 400-Watt dummy load on the input side.

    A two-by-four is good for just one thing. Burning rubber.

    Just one problem. The tires are expensive.

    73
     
  7. Unit 75

    Unit 75 Sweet Home Alabama

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    Are you using that amp for AM only or for SSB as well, seeing it's class C biased?
     
    NZ8N likes this.
  8. loosecannon

    loosecannon Sr. Member

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    Dean Street,
    im afraid all you're going to find here is advice you don't want to hear.

    Things such as, a 2 driving 4 pill linear amplifier is a stupid idea in the first place and would either blast the 4 pills right into saturation or be swamped so much that it negates the need for the 2 pill driver section.

    or things like, bypassing the amp in a connex 4300-300 in order to run a 4 pill is not going to net you much more on the receiving radio, and will almost definitely be less than a 1 S unit gain.

    or maybe, switching out 2290's for 2879's is not a simple process, and requires knowledge, skills, and test equipment in order to make it work.

    please correct me if im wrong, but i get the impression that you are looking to make your watt meter move as far to the right as possible, all notions of good sense be damned.

    If i am correct, you can be re-educated on this forum, but only if you want to see the truth.
    many come here for similar advice and leave in a week claiming we are just a bunch of jerks who think they know it all, and don't know how things work in the CB world.
    LC
     
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  9. nomadradio

    nomadradio Analog Retentive

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    That amplifier was built by someone who understands his market. The only question that market asks is "How Many Watts?". So it's built to show as many Watts as possible, for the lowest-possible build cost. But the incentive is simple. The box that shows the biggest wattmeter reading sells best.

    If every customer ONLY asked the car dealer "How fast will it go?" and no other questions, the incentive would be to build and sell you a car that goes as fast as it can for the money.

    How many miles before it blew up would not be considered important.

    But there are many laws on the books that set standards for making a car that's legal to sell, concerning safety, fuel consumption and how much pollution it produces.

    As a result, your daily driver is designed with those incentives in mind. Makes it cost more than it would if going fast were the only design incentive.

    But a linear that's not legal to sell in the first place has no such limits placed on its design. Nobody asks "how many times can I key it before it blows up?". Or, "How many of my neighbors' TVs will it shut down?".

    Well, not until the third or fourth time it blows up. By then it's too late. Already designed and built for max wattage, and minimum expense to build it.

    You can't blame the amplifier's builder for listening to the buyers, and responding to what they "think" they want. That's usually called rational marketing.

    But a contraband market produces perverse incentives. And perverse products that match them.

    Like Glen Frey wrote: "It's the politics of contraband, it's the smuggler's blues".

    73
     
    BBB, Slowmover, wavrider and 3 others like this.
  10. 9C1Driver

    9C1Driver Sr. Member

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    Dean Street, here is something to consider. You want to be very careful with the Toshiba 2879 finals you have, by this I mean don't overdrive them and burn them up. This is what a set of 4 will cost you if and when you smoke the ones you have now.
    https://www.rfparts.com/2sc2879a-mq.html
     
    AudioShockwav likes this.
  11. loosecannon

    loosecannon Sr. Member

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    one of these days i'm going to calm the heck down and say cool things like Nomad does. ;)
    LC
     
  12. 9C1Driver

    9C1Driver Sr. Member

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    I actually thought you were spot on in everything you said loosecannon.
     
    AudioShockwav likes this.
  13. 543_Dallas

    543_Dallas Sr. Member

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    Anyone remember the 2x2? I forget who used to build them. It might have been stroker amps.

    If you're stuck on the 2x4 idea leave the 2290s in there. They will drive the snot out of the 4 2979s and there is no benefit to replacing them. Builders started using 2879 drivers when the Toshiba 2290 supply started running low.

    Since you've used that radio with a straight 4 and liked it I would bypass the driver and have a straight 4. With that amp all you have to do is disconnect the power wire to the driver, remove the resistor between the driver and final stage and move the coax from the variable to the input of the 4 pill.
     
  14. BBB

    BBB Well-Known Member

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    As noted, the amp's driver PA section can be well padded via the variable pot if equipped to help set things up right. This amp configuration is also about headroom not just max watts.

    Run only a single final radio with a low dead key on AM combined with a external 100+ Amp 15 VDC power supply, monitor your modulation, amp output (1-4 ratio AM), VSWR under 2, all with an accurate meter and a good low-pass output filter... then have some fun. If built well and properly cooled, it will last and sound just fine, trust me ;)

    [​IMG]
     
    Shadetree Mechanic likes this.
  15. 2NC995

    2NC995 WDX995 / DAN

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    I’d run the 4300 as is, and repurpose the amplifier for something else.

    Otherwise you’re just trading 4 transistors for 4 transistors. Not that a 1446 and a 2879 are the same, but 4 1446s can certainly talk!
     
    Shadetree Mechanic likes this.

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