LINEAR AMPLIFIER Model SL250DX AM SSB

Discussion in 'Amateur Radio Amplifiers' started by YO2LTC, Apr 9, 2021.

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  1. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    For CB use, it might be good to check what your country allows (if you care about stuff like that). Even if there are no laws against it, these type of amps in CB use generate strong harmonics that cause interference on higher frequencies, e.g. police, fire, emergency, etc.
    Harmonics could be reduced if an additional low pass filter was added.

    The data sheets show an amp with bascially no bias, so the circuit is made for a class C operation like FM.

    The amp pictures show diode biasing which is a pretty bad method.

    Here is a good reference for making a good bias circuit for it and explains what's wrong with the first two methods (if you want linear operation): http://www.ifwtech.co.uk/g3sek/tr-bias/tr-bias1.htm

    You can see from the data sheets, that the transistors are made for up to 80W out each. So the pair of them should make max 160W -- anything over is distortion and splatter I would expect.

    73.
     
    YO2LTC likes this.
  2. YO2LTC

    YO2LTC Ham Member QRZ Page

    Thank you for your reply...so should I go for selling it? :)
     
  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    You could be right. I was basing the Class C from the inductor that comes from the center tap of the input transformer. Looks to go to ground & that would be Class C. Of course, it could have tracking, as I didn't notice the diode.
    Good eye.

    Ed
     
    YO2LTC and PA5COR like this.
  4. N7EKU

    N7EKU Ham Member QRZ Page

    Depends on what you want to do,

    The difficult part of the amp is already built and the quality looks fine. You could head over to communicationconcepts.com and find their amp that uses those transistors and then copy the bias circuit over to that amp. It would simplify the interior wiring a bit without having the level selector and SSB/AM switch. Most operators would also remove the carrier operated relay and use a PPT circuit instead. And most would probably remove the receive preamp since most rigs already have their own and they are probably a lot better than that one. Then the amp would just need a switchable lowpass filter setup. Kits of these are available I'm sure. It would be a project for sure, so it depends on what you're up for.

    Or you could sell it, and use the money to help fund a new Hardrock50, or miniHFPA2 amp that are designed just for the FT-817 type rigs.

    73.
     
    YO2LTC likes this.
  5. YO2LTC

    YO2LTC Ham Member QRZ Page

    Usefull info! Thank you.
    I found that amp, http://www.communication-concepts.com/an762-140/
    I'll have a look and figure out the mods I should be doing to mine or...buy the EB104 KIT (aiming higher :) ) with the filters.

    73
     
  6. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    Looking closer at your current amp, the link I sent about biasing hints that the single diode biasing method may not be that bad if it has a high enough standing current. You would need to measure the amp and see what that is. It could very well be that the amp is a copy of the eb63. It kind of looks like it is actually, because the diode current looks to be set by that 5W 30ohm resistor which is about what the eb63 design uses.

    The main changes then would be LPF's, if you want to stick with the COR or not, and find out what the power level switch does (different input attenuators? changing the bias?); one way to check the latter would be to monitor the standing current while manually triggering the relay and then switching among the different power settings to see if the standing current changes. Of course you could also just draw up a schematic of what you have now by tracing the circuits out.

    While using the same transistors, the an762 design seen on the CC site is also very good one; it uses a superior biasing method, and also a superior output transformer system than the eb63. So one could switch over to that design too (order all but the transistors for about $55+ship).

    The eb104 would be interesting too!

    73.
     
    YO2LTC likes this.
  7. YO2LTC

    YO2LTC Ham Member QRZ Page

    I'll take it step-by-step. I just ordered all the LPFs from com-concepts. Uppon arival I'll do some tests and measurements using the amp as is + filters, to see how it runs. Depending on results I'll see further actions.
    I might buy the AN762-140 or even 180 depending on what I'll find.

    73 and Thank you
     
  8. YO2LTC

    YO2LTC Ham Member QRZ Page

    So, I'm returning with some findings.
    It appears that SL250DX amp layout is 95% similar to the EB63 (https://www.communication-concepts.com/eb63a-140w-hf-amplifier/) I randomly crosscheck the amp's layout against EB63 schematics and they matched with some exceptions regarding some components values (eg. 33ohms vs. 30ohms rezistor). In order to do some live tests with it, I found and bought a external LPF (https://www.radioworld.co.uk/comet_cf30mr ), it should do, I guess.
    As soon as I'm done with the live tests I'll post the results.

    73

    Here are some more pictures and the EB63 schematics.
     

    Attached Files:

    WA4SIX likes this.
  9. YO2LTC

    YO2LTC Ham Member QRZ Page

    Done some testing! It works! External Comet LPF connected...
    On 20m band SSB, I got 162W output, on a 10W input. Hopefully I'll run some testing on other bands in the days to come...Easter Bunny is interfering... :))))
     

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