plate transformer selection

Discussion in 'Amateur Radio Amplifiers' started by KB1SEL, Jan 18, 2011.

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  1. KB1SEL

    KB1SEL Ham Member QRZ Page

    If a transformer is rated for 1500 watts 1.5kva will it peak much more then 1500 with a tube type amplifier running 70 percent efficeint.

    Please correct me if im wrong but if on am you should key no more then 262.5 watts.With a tube type amp running 70 percent efficeincy.So i would have 4x the peak power.

    What happens if you dont have the proper carrier to peak ratio.
     
  2. KB1SEL

    KB1SEL Ham Member QRZ Page

    I forgot to add 1500 watts ccs
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Depends on the rest of the circuit. Choke input or capacitor input? Also depends on duty cycle. A 1500W amplifier at 70% efficiency (quite high) would consume 2143W DC. But for SSB or CW service, this shouldn't be a problem since duty cycle is low. For 100% d/c service, it could be a problem.

    Where'd the 262.5W come from? Legal output power limit in the U.S. for AM is 375W with 100% modulation. With a 70% efficient amplifier (which would absolutely NEVER happen when running a linear amplifier on AM -- it would always be lower than this), that would be theoretically 536W DC input power for the carrier. However a linear amplifier properly tuned for 1500W PEP output would never achieve anything close to 70% efficiency on the carrier. You'd probably be lucky to hit 35%, in which case the DC input power for the carrier would be 1071W. Of course if you reduce % modulation, you can increase the carrier power correspondingly, but there's no advantage there that I can think of.

    What are you trying to do?
     
  4. KM3F

    KM3F Ham Member QRZ Page

    The answer is not so simple without taking into account other factors in the overall operation.
    Starting with the plate transformer at 1500 watt rating; there is tranformer efficiency to take into account., It usually is in the 80% range depending on the quality of construction.
    So the total AC power draw will be more than 1500 watts out of the power line.
    For AM operation, the tube device used needs overall plate dissapation capability to cover long time keyups associated with AM operations. Usually called head room.
    The carrier to peak power ratio you ask about is dependent on the amplifier having enough linear dynamic input/output ability such that during the percentage of modulation to peaks 100% or less does not result in current/power putput limiting at any point called flat topping or any other distortions that cause other artifacts to be outputted with the RF signal.
    Reveiw AM modulation operation and amplifiers in general to see all this info.
    As well, over driven amplifiers on SSB can also cause flat topping or limiting that generates splatter and makes for unfriendly relationships.
    The 1500 watt output limit by law does not care how much power the input has to expend to reach that limit, so efficiency and losses are included. Generaly for AM power limit the rule for 100% modulation is a 375 watt carrier times 4 " at 100%" modulation" = 1500 watts, no matter how you arrive at it.
    70% eff. with the usual amplifiers would be very high.
    Only such new designs as class E etc reach higher efficiencies.
    This is a good time to make example of the way radios are rated for AM operation. The usual radio is rated at 100 watts SSB but 25 watts for AM.
    It's the same thing as above. The radio's 100 watt capability on SSB is a 25 watts carrier on AM again due to the 25 watt carrier times 4 = the final power ability of 100 watts with some room for making sure the signal is linear and clean
    I may have not fully understood your question but tried to give the underlying basics for the overall operation.
    For full understanding of this, an expansion of detail is in order.
    Good luck
     
  5. KB1SEL

    KB1SEL Ham Member QRZ Page

    Thanks for the response,I used the transformer rating just for an example.

    I came up with the 262.5 by taking the 1500 watts and dividing it by 70 percent.Then i divided that number by 4 to get the am carrier level.

    So how do you figure what a power supply is capable of providing for output if you know the transformers kva rating and the amplifer efficiency.On am mode of operation.

    What difference would i see between a choke/capacitor input besides maybe a little bit better regulation if the choke was made properly.

    Im not building anything im just curious
     
  6. KB1SEL

    KB1SEL Ham Member QRZ Page

    Thanks again for the responses. What will a tube type amp run at at best maybe 65% on am.

    Is there a easy formula that i can use to figure out the proper size transformer for the legal limit.For which ever tube i should decide on some day.

    I should have added the 1.5 kva rating is for the secondary side.
     
  7. KM3F

    KM3F Ham Member QRZ Page

    Just considering the transformer, you would need to know the mfger rating for ICAS or CCS, then design the output circuit for the desiered voltage output at the current needed, keeping within the transformer rating.
    The amount of core iron is the first tip-off.
    Normally a capacitor output is used for amateur amplifiers.
    An example in one of my amplifiers is the transformer is rated nearly 1 amp at 2200 volts. Here the unloaded voltage is 2400 DC out of the supply and drops to 2200 under nearly 500 ma of load current.
    The amount of output filter capcitance determines to a certain degree how fast the supply voltage drops with SSB voice load other wise a much lower filter value will do enough smoothing as a filter only.
    Other parameters that enter are recitifier drop, bleed current, transformer winding DC losses etc.
    You need to study the mechanics of each part of it rather than begin with any sort of formula other than possibly output = input plus all losses, then figure operational dynamics that can be used within the limits.
    good luck.
     
  8. KB1SEL

    KB1SEL Ham Member QRZ Page

    Ok i understand

    what is the highest efficiency a tube type amp can run at.
     
  9. KB1SEL

    KB1SEL Ham Member QRZ Page

    Is it true eca transformers had problems and thats why they sold out.
     
  10. AF6LJ

    AF6LJ Ham Member QRZ Page

    If it's a linear amplifier, maybe 55% to 60% maybe a little better on a good day.
     
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