A New Amplifier Project

It seems difficult to get a twelve volt RF amplifier to work well on sideband at over 1.5kw.
The Texas Star DX1600 and 1600x work well up to around the 1-1.2kw PEP range but are getting pushed in their discomfort zone at much more. Doesn’t seem to much else out there at the moment and Texas Star were not interested in reproducing the 16 transistor DX3200 they once produced as they could not stand up to life in the real world and the return rate was close to 100%. The amplifiers use the Toshiba 2SC2879 device and as far as I know it is still the king of bipolar 12v devices.
Newer mosfet devices are emerging now with power levels approaching 1.2kw PEP per device but the cost is still quite high. The heatsink technology required to remove the heat generated by these small devices will also need to be further improved. While looking around we came upon a device that has been out for some time but has 3 times the power output of the 2879 but runs at 4 times the voltage..50v.
There seems to be a correlation between the IMD products and the voltage that a device runs on when using it in AB class to amplify at HF frequencies. Higher the voltage the better the IMD. The cleaner the amp…all other things being equal…
So if 8 2879’s can produce 1kw OK then 8 of these hi voltage mosfet parts should produce 3kw for the same footprint. Having 8 devices the thermal dissipation can be spread over a larger area of heat sink, but the associated components will be more and special combiners will be needed as well. When it is all said and done a decent tube amp will produce the same for probably less cost and work but I prefer to use solid state and DC. Personnel thing.
First thing was to find the power supplies. 57A 50v supplies are available on the used market. I bought 2 of them for US$135 plus shipping. They look like missiles. 2 feet long. I think they have come out of telephone exchanges. 2950w max output. Although one PSU should run the amp OK in SSB we will use 2 of them so we always have enough headroom…I am not building this ..it is being farmed out and the maker is plodding along slowly getting the parts assembled. he knows what he is doing and has the experience to carry out the job. At the rate things are going it will take a year but fortunately I am in no rush so “steady steady; catchee monkee”..(old chinese proverb!)

The PSU's

The PSU’s

2 ea 4 transistor boards and the combiner

2 ea 4 transistor boards and the combiner

The transistors are the SD2933 devices. 8 of them. Looking for a relaxed 1.5-2kw PEP output.
The 2ea 4 transistor modules

The 2ea 4 transistor modules


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7 Responses to A New Amplifier Project

  1. 19mex001 says:

    Nice build there dave,
    so if i understand correct you use a excisting board/PCB just with different transistors and some other parts to make it work.
    I wonder what the cost would be of this system then, would be nice a transistor amp like this.
    I have a 8 x 2sc2879 amp in my car and building another one for in 14 division remote, but this draws allot of Amps at 12v and indeed ive read about the IMD that it could produce.

    I got a tube amp of 2.5 Kw in the garage but its way to big to put over there and then i have the problem of remote tuning, have to make all kind of stepper motors on the knobs, so transistor is the solution.
    Ill keep following this thread

    • dd182003 says:

      Yes Vince, That is pretty much it….The texas Star DX1600 /X is a nice sounding amp when run reasonably but no protection and anything over 1kw is pushing things into the land of no return if something goes wrong. The 2933’s are 3 times as powerful as the 2879’s in my current SS amps. The 8 transistor 2879 amp needs in excess of 100A @13.8v to run easy and that brings all sorts of losses on the way to the amp. The new 50v amp will not need as much amperage for the same power which I hope will keep the heat and IMD down and the efficiency up. No tune and no filters, other than a low pass…can’t see it putting out much below 10-12m as that’s where I intend to run it….It will cover all the HF spectrum if required but the low drive suits my 30w 10-12m radios. More photos coming soon I am told.
      Also looking at another 5kw tube amp for 10-12m. Actually may work out cheaper than the SS amp if we can source the required bits cheaply enough and they are out there! Mike working on his one and will write something up on that build too.

  2. dd182003 says:

    Vince, The board is a special board designed to run these transistors. While based on the motorola designs of many years ago it has been further upgraded to increase output and efficiencies especially on the higher bands. Helge did not care about gain after 20m so most of the original designs perform dismally around 30mhz.. Be interesting to see how it performs in real life.

  3. dd182003 says:

    Parts for the 5KW tube amp using 2ea GS35B would be in the vicinity of US$1400 with out the transformer…

  4. 19mex001 says:

    Hey dave,
    Thanks for the info.
    This new amp you are designing i am interested in i mean the transistor amplifier as i said, maybe something nice for use for my remote station 😉 🙂
    Any idea if the amp works like expected to sell another one ?
    The price you mentioned is without the building costs of course for the tube amp ?, i have googled a but on the tube your using and there are some nice amplifier builds with those and nice output power is available.

    ill keep following this


  5. Dave, i found this, to be honest another vince found it that i know 🙂 http://www.youtube.com/watch?v=8ziYqjMQGEQ
    check it out, i think its much more interesting to use this transistor as 1 allready gives also 1.2kw and seems unbreakable.

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