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SB220 Is Reducing Drive Safe?

Discussion in 'Amateur Radio Amplifiers' started by N8PWM, Aug 11, 2018.

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  1. N8PWM

    N8PWM XML Subscriber QRZ Page

    I have an SB220 amplifier that runs nicely at 1KW with 100W input. There are times when I don't need 1KW and would like to reduce the drive and take the amplifier down to say 500W.

    I have heard that these amplifiers do not operate properly with reduced drive. Is this the case or can I safely reduce the drive to say 50W and still use the amplifier running about 500W out?

  2. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Tune it up like you’re running 100w drive and turn the drive down on the transceiver. No need to go to maximum suds (squeeze every last bit out of it), but you should notice the load and tune controls are set in the areas they normally are.

    What you don’t want to do is tune it at 50 watts and turn the drive up. It would be under loaded at that point.
    Last edited: Aug 11, 2018
    K4AGO likes this.
  3. W1QJ

    W1QJ Ham Member QRZ Page

    It depends on what mode of operation. Steady carrier modes like digital of CW you should load the amp for maximum output that the amp will be driven with when operating. If you want 500 watts output load the amp for max output at 500 watts. For SSB where there are higher peaks then load full and turn drive down do not retune.
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  4. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Of course, Lou's correct but I didn't want to blather on for 2 pages... :p

    For a more complete answer that includes data about your specific setup, what transceiver are you using and what modes do you want to run at 1/2 power?
    There are hidden variables that depend on the specific gear used.

    Last edited: Aug 11, 2018
    K4AGO likes this.
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Remember these are 'Linear Amplifiers', yes you can safely reduce the drive power once you've tuned and loaded the amp for maximum expected power output.

    When you operate in SSB mode this happens all the time. You tune and load the amp for maximum expected power (e.g. 1kw) but your actual voice energy drives much lower power most of the time and the amp is perfectly happy being driven with 10 watts, 1w or 1mw when your voice energy is less than peak. If the gain of the amp (e.g. 10dB) changes with drive level across its power range then the amp isn't linear and would distort SSB transmissions.
    K4AGO likes this.
  6. W1QJ

    W1QJ Ham Member QRZ Page

    I suppose the OP is completely confused now.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    "Unless you're running AM, tune up at the drive and output power you actually want to use."

    Switching to "CW" on the mode switch will reduce gain and output power.:p
  8. N8PWM

    N8PWM XML Subscriber QRZ Page

    Thank you all for your replies. I reduced the drive to about 50 watts, tuned the amp at that level and ended up with 500 out of the amp. Signal reports were great. Looks like it works great. This is exactly what I was after.
    K4AGO likes this.
  9. N8PWM

    N8PWM XML Subscriber QRZ Page

    Also I only use the amp for ssb.
  10. AI3V

    AI3V Ham Member QRZ Page

    A much better way to tune ANY amplifier is to pay attention to ALL it's meters and tune up for best efficiency.

    Pay particular attention to the anode circuit.

    The idea of retuning at different power levels is something to pay attention to,but there is no need to go overboard.

    Consider the average Sammy hammy talking away on SSB,

    His amp is all most NEVER at full power.

    Think about it.

    K4AGO and N5RFX like this.

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