"AB1", "B", or "C" class for 2KW AM/FM amp.......

Discussion in 'Amateur Radio Amplifiers' started by KA5LQJ, Sep 17, 2011.

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  1. W1GUH

    W1GUH Ham Member QRZ Page

    If you're planning AM linear service from an amp, be prepared for 1) very low efficiency in the amp and 2) a 'scope will be necessary for tune-up. Tune-up is like no other mode except for screen-modulated AM. Not simply a matter of dipping the plate or peaking the output power.

    Just a for instance...I ran a 4-1000A in linear AM service. When tuned for 375 watts output, I was dissapating all of the tube's rating (1 KW) and then some. (Nice red color!) Found that that tube is darned rugged - it took it! (For how long, who knonws?).
  2. WA7KKP

    WA7KKP Ham Member QRZ Page

    None of the combinations will give you a full legal limit (1500 watts output) amplifer. The 811's and single 3-500z are only good for a kw PEP INPUT, about half that out. The 572b is just an 811 with more plate dissipation, and a pair of these is good for 1200 watts input, half that out.

    A rule of thumb is that you'll need 1/3 of your plate input as plate dissipation. For example, the 4CX1000A will do 1680 watts output with 1000 watts plate dissipation. For a reasonable tube life (commercial broadcast) you'll find the plate dissipation equal to or higher than the rated power output.

    If you're wanting to amplify just FM or CW signals, go Class C -- higher efficiency means more RF per dollar you pay to the power company. If you want to do SSB, you'll have to go class AB or B, but the efficiency drops. AM linear amplification of a modulated signal -- expect to have three times the dissipation for the power output. That's why CBers burn up tube amps left and right!!!

    Grounded grid is most popular because the drive power needed is usually around 100 watts, same as most rigs. Neutralization is often not needed, but some tubes do need it.

    Grounded cathode give the most gain, at the expense of requiring neutralization of some sort. The legendary Johnson Desk Kilowatt used a pair of 4-400's both in Class C and Class AB2 for linear operation. Class C required about 30 watts (the Viking Ranger was nicely suited, and the 6L6 modulators would drive the 810 modulators in the final for best efficency at a KW AM); Class AB2 required maybe 10 watts PEP. An external resistive 3/6/10 db pad was sold so that any transmitter <100 watts would drive it without overdrive.

    Get yourself any of the W6SAI (Editors & Engineers, later SAMS) Radio Handbooks . . . as a design engineer for Eimac, he knew how to design a tube PA better than just about anyone else dead or alive. In those books, he shows basic designs for triode/tetrode PA's, tables for Pi and Pi-L networks, grounded grid design, etc.

    Then pick and choose your tubes according to (1) availability and (2) suitability. the 4CX1000A is a nice PA, but not commonly used in broadcasting, where cheap pulls are the rule. Not to mention the tube socket costs nearly as much as the tube itself. 4-400 tubes are quite common, along with the 4CX250 used in FM transmitters as the driver stage.

    Gary WA7KKP
  3. KM1H

    KM1H Ham Member QRZ Page

    The 3CX3000A7 is still a popular pull that is very easy to use and about impossible to hurt within reason. Aftermarket (not Eimac) sockets are available.
    You will be able to run as a GG AM linear at any power up to 1000W carrier with just a 2500-3000V PS. With a 4000W Pd its still loafing.

    Another very popular tube with hams is the YC-156.

  4. KA5LQJ

    KA5LQJ Ham Member QRZ Page

    Hi All,

    Yes, Eric, I won't disappoint you. I got my CB license in 1965. At that
    time, I had passed my Second Class RadioTelephone exam and started
    my own two way radio shop repairing COMMERCIAL two-way radios. I ONLY used the CB in my service truck to talk to MY "base".

    I closed the business in 1973 and went to work at the local Western
    Electric telephone manufacturing plant. I dropped my 2nd Class and my
    Citizens Band Class D license, but kept my class C for model R/C planes
    active. I became a licensed Amateur Radio Novice in April of 1981 and a
    Technician in October of that year.

    At that time, I was working 2 full-time jobs to keep my family and relatives heads above water. Both my Mother-in-law and Father-in-law had become seriously ill and lost their jobs.

    Since then, We've lost my dear Mother-in-law and Father-in-law and I've become both physically and mentally disabled.

    I'm not 'power-hungry', it's just the guys that I to on 3880 are running 500 watts or more and my puny 40 watts from the 706MkII-G doesn't cut it some mornings and they can't hear ME. There are some jerk sidebander's over on 3872 kcs that fun over a kW and deliberately interfere with our AM conversations. Are you satisfied now? :mad:


    Last edited: Sep 25, 2011
  5. KA5LQJ

    KA5LQJ Ham Member QRZ Page

    Ya Know....?

    I came here looking for help and information. Most of you were kind enough
    to explain and try to help. I appreciate that.

    But, seeing as I'm judged now as less than a human being, I'll quietly close
    the door behind me and visit no more. :(


  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I didn't see much of anything derogatory here.

    AM is a tough mode. Frankly, I'd pick up a couple of "boat anchors" and use them if I was more into AM. A Valiant can run about 125W carrier power of full high-level plate modulated signal, which is about 500W PEP on AM. Not much difference between that and "legal limit," frankly. And a mating "boat anchor" receiver like an NC-300 or something would be a real pleasure to use on AM compared with the little Icom. Try it! You might fall in love with old gear, and it doesn't cost nearly as much as building a legal limit amplifier.
  7. NZ9Y

    NZ9Y Ham Member QRZ Page


    How do you think the Ameritron AL-1500 would do for AM? They claim thier 8877 amp will do 1/2 hour continuous carrier at 1500 watts and 30 seconds at 2500. Could you even run that amp too hard with a modern SS rig?
  8. W9GB

    W9GB Ham Member QRZ Page

    Don -

    I really don't bother with 80 meters .. too many attitudes and bad manners .. I prefer 40 meters (old 7240 kHz AM gang), since early 1970s.

    I have found that on 160 and 80 meters ... mnay AM stations are using converted commercial AM transmitters.
    Plenty of these surplus over the past 20 years as some AM stations have gone dark and others converted to solid-state designs.

    HOWEVER, there are BAD conversions, as you can see from 3840 - 3890 kHz activities now on You Tube (Gates BC-1G and other BCB AM conversions)
    Since you had your Second Class License in 1960s/early 1970s -- you know these issues.



    Many 8877 amplifier designs in the books and literature over the past 30 years.

    Paul Hewitt, WD7S offers some modules for the DIY amplifier builder.

    Paul's photo gallery is also great inspiration of what can be done.


    Last edited: Sep 25, 2011
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The AL-1500 will support legal limit power on AM (375W carrier power). I hear guys using them quite often on AM and they're not blowing them up.

    Some have expressed concerns that its blower may not be big enough, and it a hot environment that might be true. In a typical household environment where the ambient is 30C or lower, I don't think this is ever a problem.
  10. KM1H

    KM1H Ham Member QRZ Page

    Since the blower can handle around 2500W in a CW/SSB contest and close to 1500W in a RTTY contest I wouldnt worry about 1500W PEP on AM. The 8877 certainly wont be straining.

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