RM Italy KL500 (with MRF455 transistors)

Discussion in 'General Technical Questions and Answers' started by N9RLI, Feb 27, 2009.

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  1. N9RLI

    N9RLI Ham Member QRZ Page

    Before I ask anything. If you're going to tell me that I have a CB amp, or that I should throw it away, or some other "witty" comment like that, please feel free to press your browser's back button. I'm looking for real answers from people who know what they're talking about.

    I bought this amp when I was 16 a few years ago, and now want to use it for the ham bands. I understand that the input SWR might be terrible, and the output needs filtering. I've even seen some mention that it's a class C and it needs to be rebiased to AB...anyone know of any mods that could be done to this amp to make it usable?
  2. PA5COR

    PA5COR Ham Member QRZ Page

    It is a very poorly class AB regulated CB amp

    It will work on 10/12 maybe 15 but then you need to make the poor bias circuit a lot better, and add switchable bandfilters.

    For a wider use a change in input and output circuitry is needed, and more bandfilters added.

    Choose on tthe green left side ombouw 27MC eindtrappen.
    It is in Dutch with lots of pictures to show what work is needed to make a decent amp out of it from 80 to 10.
    Follow the page to the end.
    On top click the link Besturing lowpassfilters to see how to control the several band pass filters.

    It can be done, but it is no weekend project...
    Last edited: Feb 27, 2009
  3. W9GB

    W9GB Ham Member QRZ Page

    Ramsin -

    Well, you are just over in Niles. Belong to North Shore Club?
    A number of Motorola engineers in your area -- good for Elmers and your educational project.

    You are not going to find a magic recipe for a weekend project. IF it existed you would have heard about it !!

    For the Motorola MRF455 RF transistor, START with Helge Granberg K7ES/OH2ZE (SK) and his Application Note for design and usage of this transistor.
    Helge was a Finnish-American amateur and was one of the earliest pioneers in SSB technology in 1950s (hence referred to as the "Art Collins" of Finland). An accomplished inventor, Helge was well-known for the "Granberg Transistor," on which he held one of his many patents. He passed away in 1996, he was only 63 at time of his death.

    Motorola Application Note AN762 (MRF455)
    Two MRF455 transistors give you a 100 watt amplifier design [Period]

    Your KL-500 unit uses 4 transistors. This is more complicated design and increases
    the requirements (larger torroids, etc.) of the Low Pass Filter to handle this power and adds additonal transformers.


    Datasheet for Motorola MC1723G (used as regulator on Helge's AN762 board design)

    Helge did not cover required output filtering in the Motorola Application Note.
    For the output filtering, there are many approaches -- depending on your desired usage.
    SINGLE or MULTI-BAND operation??
    Multi-band HF is more usable for your time and $$ investment --
    BUT adds more components and complexity (manual or automatic switching).

    G3TSO is an accomplished home builder and shows you how this can be accomplished

    G3TSO used the MRF454 transistor for a 140 watt HF amplifier.

    Communication Concepts (CCI) sells kits of parts and PC boards
    based on the Motorola Application Notes.

    IN ADDITION, the FoxDelta Group had an amplifier kit for a similar project a few years ago.
    Read about it here:

    Design / Usage Challenges in adopting the RM Italy (as is):

    1. The heat sink is inadequate (too small - minimal thermal design) for normal amateur radio SSB operations (duty cycle).
    2. Case design can not handle all of the components inside,
    unless you do outboard add-on -- which is a KLUGE setup.
    3. No LP filter -- so you need to add one.
    Here is a nice LP Filter board build by G3TSO, an award winning builder in UK

    4. As has been said many times,
    Strip your unit for parts;
    Get a larger enclosure with proper heatsink size -- as shown by G3TSO;
    Get PC board and missing parts from CCI;
    Build the LP Filter board;
    Learn by building -- it will cost you some $$ -- but the final product is far superior than where you started.

    Last edited: Feb 28, 2009
  4. N9RLI

    N9RLI Ham Member QRZ Page

    Thanks you for the decent reply. I wasn't expecting that to tell you the truth...

    Here's the Translated version for anyone that searches this thread.

    One question though, because even the translated version is a little hard, is the whole page dedicated to modding the KL500? Should I do everything on that page?

    Also, his has SD1446 transistors, mine has MRF455, do you think that matters?
    Last edited: Feb 28, 2009
  5. N9RLI

    N9RLI Ham Member QRZ Page

    Greg, I saw your post just now, I think you posted it when I was typing my previous post.

    Thanks for the info!...I've looked at Communication Concepts many times and the G3TSO site for his filtering, but lost the link for both and have been looking and looking for them...thanks for posting those. I don't mind building stuff at all, I'm actually a pretty decent solderer although that might not mean much if you don't exactly know what you're doing. Just a question, are the CCI app notes based on K7ES' designs?

    I'm going to take a further look into different amp designs, and see what I find to be suitable for me. In retrospect, it does seem as if though the hassle to benefit ratio is too big with the KL500. I was thinking of doing exactly what you said, use it for parts. Thanks again guys for the info.
  6. W9GB

    W9GB Ham Member QRZ Page

    Yes. Helge authored most of the Motorola Application Notes (he worked for Motorola) and CCI really exists in part to continue the support for his work, now that he is no longer with us. :(

    Helge also served as a technical advisor to ARRL.
    The ARRL Handbooks in early 1980s had a shortened version of AN-762.
    Page 6-41 in my 1982 ARRL Handbook.
    Figure 65 (page 6-43) of that same Handbook shows
    a nice 7-pole Chebyshev low-pass filter design with notes by Ed Wetherhold, W3NQN.
    Yes, look at the MFJ/Ameritron ALS-600 and ALS-500 ... that is what the RM KL-500 should be -- but they cannot build the same for less than Ameritron. While the ALS-500 is a mobile unit (using 12 V RF transistors), the ALS-600 uses 48 Volt MOSFET devices. The ALS-600 does not have an automatic antenna tuner -- which is another feature that adds to COST !

    Once you go beyond 2 bipolar transistors -- most good RF designers use 24 or 48 Volt RF transistors -- higher voltage -- lower current (This reduces the size of required PC board traces, wires -- by lowering the current requirements!!).

    You might want to look at the FoxDelta PC board (you could build 2 amps)

    Last edited: Feb 28, 2009
  7. N9RLI

    N9RLI Ham Member QRZ Page

    Haha, you're just throwing all the sites I couldn't find...

    I've looked at many many many amp designs that use 2 MRF455 transistors, but never 4, I always wanted to build 2 of them, or make a PCB and just double everything but don't know how to combine the power output. Is there such a way to do this?
  8. PA5COR

    PA5COR Ham Member QRZ Page

    The same site i mentioned in my first post also shows an amp running on 12 volts doing 1200 watts PEP, though with 4 x 2 other transistors.

    That amp was tested by the Dutch authorities and declared fit for use as amp.

    It does need a good 13.,8 - 15 volt 140 amp power supply.

    Completely homebrew.

    I work him regularly, we had many chats about thaat amp.
    Personally i have the heathkit SB 1000, completely rebuild and new tube.

    I build many amps from the Motorola "bible" including the 2 x MRF 454, and for 2 meters the 2 x MRF 247 etc.

    Lots of fun building stuff yourself.
  9. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    Using it for parts may be the best option but the amp is probably useable. If you look at the design specs for Class AB it biases the transistors so that they conduct a few degrees beyond 180. In a push pull Class B amplifier one transistor cuts off at 180 deg and the other turns on for 180 degrees. Distortion occurs at the point where they cross over, Class AB minimizes the distortion.

    If you keep the drive level a little lower, that is don't push the amp to it's rated limit the third order distortion products will drop and the second order products should be better than 35 db down. This is good enough for amateur use but just barely. I would use a low pass filter on the output that rolls off slightly higher than the band in use just to be on the safe side.

    If you have access to a spectrum analyzer you may want to try it and see before you tear it apart. As long as you don't over drive the input it should be acceptable. As for the input SWR, that can be fixed with a simple homebrew L network.
  10. W8JI

    W8JI Ham Member QRZ Page

    Generally you run into problems paralleling bipolar transistors directly.

    The best way to do it is to run push-pull with two, and then build two small PA's that you combine in a combiner.

    Stability is more of an issue when you combine PA stages, it often requires 20 dB of isolation on the input ports and what you can get on the output.

    Using magic T combiners and splitters, the only practical kinds for HF, the common port impedance sets the isolation along with the resistor across the CT transformer. Since you can control input impedance with a pad (because you have drive you can waste and the rig is a somewhat known source) that's where you should concentrate isolating the amps.

    Try looking at this:


    The ALS-500M uses transistors and op amps in the bias. The goal is a contant VOLTAGE source. There was a long thread somewhere where someone suggested a light bulb as a bias source limiter, but that's actually the opposite of what is needed. You need a very low dynamic impedance source, not one with high resistance that increases with current.

    This is why Helge used voltage regulators, and why I used an op amp driving a large pass transistor in the ALS500 amp. The idea is to build a constant VOLTAGE source with very low dynamic impedance. The last thing you want is a constant current source.

    This is because the transistors try to put significant current back into the bias source, and you want bias voltage to remain steady as the RF transistors try to force current backwards in the source and raise the source voltage.

    28 or higher voltage devices are much cleaner and easier to work with. You might consider such if you want IM as good as the radio, or IMD performance that is better than the radio.

    BTW, I know you want someone to mention this. That's a CB amp. ;-)

    73 Tom
    Last edited: Feb 28, 2009
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