"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. KA5LQJ

    KA5LQJ Ham Member QRZ Page


    I think I have this right. I'm wanting to build a FULL, LEGAL, OUTPUT AMP
    for 160, 75, 40, and 10 meters.
    1kW of PEP ssb = 2kW of AM or FM. ???

    I'm getting throughly confused as to what tubes I would need to run to
    accomplish this. I have a Heathkit SB-220 with two, 3Z-500s that have not
    been run in probably 20 years. I was told to scratch that and use 811's,
    813's, or 572's (5762's?). Then that changed to just ONE, 3-500z!

    Now, I do know that class AB1 draws about 35% of current through the
    cycle, class B draws about 50% and class C draws 80%, but has to have one
    heck of a modulation transformer.

    I guess my solution is to get stinkin', filthy, rich, and buy a 5kW Gates AM
    Broadcast band station and convert it over, LOL!

    There is boundto be another, better solution. 75 meter AM caught my eye
    on Sunday afternoon. There were some area hams that got on and everyone
    claimed to be mobile. But just the mike cord hung out the window to their
    car, LOL! It was called The Caravan Club. Those gentlemen would talk for
    hours about circuits, contacts they had made, tell clean jokes and reminiss
    about the good old days of spark radio. It was a pleasure to listen to them
    and if you found one, each one was MORE than glad to take the time and
    patience to teach you the Morse Code, Rules & Regulations, and circuits.

    Occassionally, you'd hear them of 40 meters and 10 meter AM. If someone
    blew a tube or capacitor, then the others would search their junk boxes and
    send that ham the part by mail, no charge. THIS is what I'd like to do, rag-
    chew about circuits and find out a little something about the other hams.

    IS there a gentleman who can help me find a circuit to BUILD such a transmitter?

    Thanks in advance,

  2. K9STH

    K9STH Ham Member QRZ Page

    Amateur radio operators within the jurisdiction of the FCC are limited to a maximum of 1500 watts PEAK output on most bands. Exceptions are 50 watts e.r.p. on 60-meters and 200 watts on 30-meters. Of course, Novice Class and Technician Class operators are limited to 200 watts below 30 MHz in those band segments allocated.

    AM, unfortunately, is not like the "goode olde dayes" when the maximum carrier power was limited to 1000 watts INPUT. Since when an AM (actually a double-sideband carrier present signal - SSB is an AM mode) carrier is 100 percent modulated the peak output power is 4 times the carrier power. Therefore, for AM operation, when plate modulated, the maximum carrier power is 375 watts output.

    For FM, the carrier power is the peak power and you can run up to 1500 watts output on FM. Maximum efficiency for FM is class C. Also, you can run class C for AM IF you use some sort of modulator, preferably plate modulation although screen and cathode modulation will work but not modulate the signal 100 percent.

    You can use a class AB-1, AB-2, or B linear for both FM and AM. However, the efficiency is not going to be as good as with class C. Also, remember with AM you still have to limit the carrier to 375 watts if the exciter is plate modulated. If using another form of modulation in the exciter (i.e. screen, cathode, controlled carrier) you still have to make sure that the peak output power does not exceed 1500 watts.

    When using SSB or AM, you cannot really estimate the peak output power from input power because of the complex nature of voice modulation. The only way to estimate AM is to assume 100 percent modulation (which it definitely may not be as high as 100 percent) and then you still have to look at output power and not input. The average efficiency of an HF transmitter is usually around 60 to 65 percent. However, depending on the band, the efficiency may be a little higher (especially on 160, 80/75, and 40) and a little lower (especially on 15 and 10). For VHF and UHF the average efficiency is usually even lower than on HF. Not always, but usually.

    Glen, K9STH
  3. W1BR

    W1BR Premium Subscriber QRZ Page

    As Glen noted, you can use a linear to amplify an AM exciter, but you will have to keep
    the RF output at 375 watts carrier if you are running 100% modulation. That would
    probably strain the limit of a small dual tube 3-500Z amp, like the SB-220.

    If you go Class C, you will have to use high level modulation on the plate of the final,
    that equates to a hefty audio modulation stage and quite a bit of vintage iron.
    Class C is the only class that can be high level modulated--the power output has
    to follow a square law response where a doubling of the instaneous plate voltage
    produces a 4X peak power output. Only Class C will that.

    There are advantages and drawbacks to both approaches. A vintage DX-100
    or B&W 5100 isn't too shabby for a beginning AM station.

    A lot this has been rehashed over on the AMFone.net forum; and there are
    endless threads regarding the use/limitations of linear amps vs. high level
    plate modulation.


  4. K9STH

    K9STH Ham Member QRZ Page

    Several thing about HF FM:

    FM/PM operation is allowed on any frequency where phone operation is allowed except for the 60-meter band which is USB only. The only stipulation is that the modulation index of an angle modulated signal (angle modulation means frequency modulation or phase modulation) below 29.0 MHz cannot exceed 1. Modulation index is defined as the absolute value of the deviation divided by the highest modulation frequency. Most FM operation on the 10-meter band uses +/- 5 kHz deviation. The absolute value of +/- 5 kHz is 5. Virtually all modern transmitters limit the maximum modulating frequency to 3000 Hz (3 kHz). Dividing 5 by 3 gives a modulation index of 1.6667. To get a modulation index of 1 the maximum deviation cannot exceed +/- 3 kHz. Now most of the HF transceivers that are now available on the amateur radio market which do have FM operation have a "wide / narrow" provision for FM. The "narrow" position limits the deviation to +/- 2.5 kHz which results in a modulation index of 0.8333 which, of course, is less than 1 and therefore is completely legal for operation below 29.0 MHz.

    Most amateur HF FM takes place on the 10-meter band and in the frequency segment reserved for repeater operation which is 29.5 MHz to 29.7 MHz. However, FM simplex operation can, and does, take place below 29.5 MHz with 29.300 MHz being a popular frequency since that frequency is used in other areas of the world (i.e. Japan) like 29.600 MHz is used in North and South America. One thing that seems to confuse many amateur radio operators is that although FM/PM operation is allowed by those holding General Class, Advanced Class, and Amateur Extra Class operators anywhere that phone operation is allowed (with the 60-meter exception), Novice Class and Technician Class operators cannot use FM/PM in the 28.3 MHz to 28.5 MHz segment. Those persons holding either class of license are restricted to CW and SSB ONLY in that segment even though higher class licensees may use FM/PM without any regulation violations.

    Although FM/PM operation is allowed on other HF bands, this is generally not done although perfectly legal. During the period from just after World War II until the mid to late 1950s, "NBFM", which was FM with a deviation of +/- 3 kHz was fairly popular on 75-meters and on 40-meters (after phone operation was allowed in the early 1950s). The modulation index of 1 or less is a restriction which was included in the regulations after that period of time. During that period, a number of commercially available amateur radio transmitters had NBFM capability. Also, even though FM/PM can be demodulated using a "normal" AM receiver (using "slope detection"), a number of receivers actually had NBFM adapters available as an option (my Collins 75A-3 receiver does have the optional NBFM adapter installed).

    Glen, K9STH
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The SB-220 with good tubes can run about 200W carrier power on AM (800W PEP with 100% modulation); more than that will overheat the tubes and the power transformer. But, that's a LOT of power, and is more than half the legal limit. With a very good sounding AM exciter, nobody can tell the difference between "high level" (plate) modulation and a well modulated transmitter followed by a good linear amplifier.

    A Johnson Valiant can run about 150W of plate-modulated AM, which again is nearly half the legal power limit for AM today, all by itself! Globe Kings and such can run more than that. They're big and heavy and quite expensive today.

    The advantage of a good linear amplifier is you can use it on all modes. Again, with a great sounding exciter and keeping the amp will within its linear operating range, it's impossible to tell the difference with a receiver, a scope, or a spectrum analyzer. Full modulation is full modulation.

    I like AM and although the big old boat anchors are cool, they are very heavy and take up a lot of space. I'd rather use a good-sounding exciter and a linear amplifier. To achieve "legal limit" AM power with a linear amp, though, requires a pretty big and well cooled linear amp. 1500W PEP on AM with a carrier is much more stressful on the amp than 1500W PEP on SSB without a carrier.

    If I were to build a legal-limit amp for HF today (and I've built several over the years) with the intention of using it on all modes including AM, I'd use an 8877/3CX1500A7 probably, with a very good 3000V 1A CCS power supply, and a lot of air through the tube. A pair of 3-500Zs might do it, but it would take great cooling and a similar power supply (3000V at 1A CCS). The 8877 can do it better.

    I'd never use 811As or 572Bs or even 813s for this.
  6. KM1H

    KM1H Ham Member QRZ Page

    Ignoring the legal stuff for a moment most ham amps dont have a strong enough power supply to run AM at what the tubes are capable of. The SB-220 family is a prime example and is barely able to handle typical long winded (old buzzard) transmissions much beyond 200-250W carrier.

    Sometimes I use a LK-500ZC with the big heavy duty remote factory transformer and that handles 1500W PEP+ with ease. A TS-950SD in the wide AM position is used as the driver and it gives excellent audio quality.
    But most of the time I use a variety of 30's to 50's commercial and HB transmitters followed by big Class C amps with modulators. Also remember that the modulator stage also needs its own power supplies from low level to 1500-2000V or more if you want a strapping signal. My avatar is not my modulator but those are the tubes I use in one modulator, PP 304TL's.

  7. KA5LQJ

    KA5LQJ Ham Member QRZ Page

    GOD BLESS each one of you Gentlemen!:) It's so nice to run into real hams that don't mind sharing their knowledge with others. I'd forgotten
    a lot of what you all mentioned.

    What I need to do now is research and find a schematic, photo lay-out, and parts list for a 8877 amp and build one myself. I used to LOVE to do
    those kind of construction and repair projects. Now, I just need a shop to do them in :D

    Well, it's my nap time.


  8. K9FON

    K9FON Guest

    Im always amazed at how these new hams are soooo obsessed with power. Smacks of CBer to me!!
  9. W4AFB

    W4AFB Guest

    Want to get all that power going and probably going to push it through a G5RV.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    This is from the ARRL Handbook and is a good article although does not include 160m. 160m could be added; I'd probably copy the Ameritron AL-1500 for the tank design, it seems to work.

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