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heathkit sb-220 sensitivity knob? tune?

Discussion in 'Amplifiers' started by 552fl, Jul 1, 2018.

  1. 552fl

    552fl New Member

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    what does the sensitivity knob do??do you put on rel,power and adj power that's going into it or through it when off..seller told me to tune with 100 watts and use whatever drive amount deadkeys around 250-300watts,,probably25-35 watts,,then you see around 1000 watts pep.. does this sound right??


     

  2. nomadradio

    nomadradio Analog Retentive

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    That knob is like the "Cal" knob on an old-style SWR meter. The "Rel Pwr" function of the right-hand meter would have cost more money to compensate for each frequency band. The cheap-and-dirty circuit they use to show relative output power becomes more sensitive band by band as the frequency goes up. Prevents you from using a single setting of that pot for all bands. If you did, the meter would barely register on 80 meters if you set it below the peg-out point on 10 meters. If they had used a frequency-compensated wattmeter circuit like you find in the Drake L4 amplifier models the added parts would have boosted the sale price. Would also have made this knob unnecessary.

    In theory, you could get it tuned up with only the built-in meter. I would never recommend running an amplifier this large without an external wattmeter. But in 1970, somehow this made sense to the folks who designed it.

    I recommend tuning for max modulated peak power. Never use a dead carrier to tune up.

    I would also suggest putting a SWR meter and coax jumper between the radio and the SB220. When you have the Tune and Load each peaked for max modulated power, check the SWR on this meter.

    When an amplifier is in line, the radio is no longer feeding the antenna. It's feeding into the amplifier's input circuit. A well-made amplifier will "fool" the radio into thinking that it's driving an antenna with a low SWR. The amplifier's input circuit may or may not be optimized for 11 meters. If not, the meter will show you a high input-side SWR when it's keyed up. This will affect the radio and the amplifier's output power, both.

    If you have not modified the amplifier for AM, it needs to stay on the "CW-Tune", or "low" side to prevent heat damage to the tubes and sockets.

    You should see around 700 Watt peaks with 100 Watts peak drive.

    The SB-220 was designed for sideband. No carrier, and no continuous heat load on the tubes from that carrier. To keep the SSB signal clean, they push the tubes to roughly half their rating with no drive. But only in "SSB" mode. Running on high side in AM mode will damage it unless it gets modified to do this safely. The tube heat from a 300 Watt carrier in SSB mode will push the tubes to their limit before you even say hello.

    It can be modified to safely run AM in SSB mode. Good chance that yours has not, if it previously belonged to a ham.

    73
     
  3. 552fl

    552fl New Member

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    tune up on the average rms side on the bird?or have it in peak,, pep modulating audio in to it..turning peak and tune back and forth dipping plate last,
     
  4. nomadradio

    nomadradio Analog Retentive

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    If you want to be spot-on, I would set the Load control reading peak modulated power, and switch back to average to peak the Tune knob on the carrier alone.

    The "dip" method on the plate-current meter is okay, but I still recommend peaking the Plate Tune for max power.

    73
     
    #4 nomadradio, Jul 1, 2018
    Last edited: Jul 1, 2018
  5. 552fl

    552fl New Member

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    ok sounds good,so do you think tuneing with 100 watts then turning it down to what ever keys the 250 or 300 dead key and swinging 1000pep does that sound right?
     
  6. nomadradio

    nomadradio Analog Retentive

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    Um, 100 Watts into this amplifier set for "SSB", the high side should get you 1100 to 1200 Watts. More than enough if the tubes are throwing the normal 400 Watts of heat with no drive at all.

    100 Watts into the "CW/Tune" side will get you closer to 700 Watts, plus or minus.

    Set the carrier for the desired 200 Watt ( or so) carrier, then tune the Load control for max modulated peaks.

    With the meter on "average", you will now probably see that the carrier level has changed. Peak the Plate Tune.

    This is where one of two things happen. Either the knob was ALREADY set to that peak, in which case you are done.

    -- Or --

    You will see that the knob had to be moved a bit to reach that peak. If it had to move very much, you may now find that the peak modulated reading with the meter on "Peak" now shows a peak with the Load knob set slightly up or down from where it was. Set it to peak modulated power.

    This is where "lather, rinse, repeat" comes to an end if the Plate Tune is now already at that peak carrier setting. If it has to be moved very much, lather, rinse, repeat until you find each knob ALREADY at the peak-power position. When you find this, you're done. Set the carrier for around 200 Watts, assuming you can get at least 700 Watts peak with audio.

    And if you have it modified for higher bias voltage, this will reduce the tube heat on SSB side, and make it safe to use in SSB mode for AM.

    The way it's built, that's not a safe thing to do. Gotta stick to Low (CW/Tune) if you want to use it for AM, so long as it is a stock unit.

    73
     
    Shadetree Mechanic likes this.
  7. 552fl

    552fl New Member

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    not happy with the heathkit sb-220..everyone says I sound better and louder on my texas star 500 with a littte mod-v than the heathkit .. bird showing 1200 pep coming out of the sb-220 and 750 on the litte texas star.don't make sense to me,,,just give a $1000 dollars for this sb-220???
     
  8. Dmans

    Dmans Well-Known Member

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    552fl,
    Are you using the SB-220 on AM? or SSB?

    73's
    David
     
  9. 552fl

    552fl New Member

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    channel 28 am
     
  10. Dmans

    Dmans Well-Known Member

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    552fl,
    Unless you are certain that this amplifier has been modified for safe AM use, I would stick strictly to Nomad's suggestions for AM use. (Quoted below)

    If you are seeing 1200 watts on AM chances are this amplifier will be short lived.
    The "CW" mode switch on the front of the amp will reduce plate voltage to a safe level for a continuous carrier (such as AM mode) provided input drive is limited to 100 watts (per Nomads suggestion) and output is limited to 700 watts plus/minus (per Nomads suggestion).

    I'm not sure of this amplifier's peculiarities but when using AM mode I have generally "overcoupled" an amplifier to allow for carrier power that modulation adds. Typically this is done by further advancing the "Load" control to reduce your un-modulated carrier by 10-20%. I know this is a standard practice with sweep tube amps but I have never owned a 3-500Z box. (Experts, Please chime in) Also, keep an eye on the SWR between your radio and Amp.

    Honestly, a receiving station will not notice the difference in whether your watt meter says 1200 watts or 700 watts but your received audio will be clearer.

    I'd hate to see you blow this thing up.

    73's
    David
     
  11. Dmans

    Dmans Well-Known Member

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    552fl,
    Read the thread linked below. Pay attention to the last 2 sentences in step 13. Captain Kilowatt is strictly a Ham operator (I'm pretty sure) so these instructions should apply to your 3-500Z amp.

    DON'T FORGET-CW MODE ON THE FRONT PANEL IS YOUR AM MODE FRIEND!

    https://www.worldwidedx.com/threads/how-to-tune-a-tube-amp.26225/

    Good Luck!

    73's
    David
     
    Captain Kilowatt likes this.
  12. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    The last sentence in the next step #14 is even more important. :ROFLMAO:
     
    Dmans likes this.
  13. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    What is the dead key power on the 220? Sounds like it might be too high, do you have forward swing? Also the 220 will need some work to do 11m.
     
  14. 552fl

    552fl New Member

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    Alright thanks for all the replies don't know what im going to do now maybe relist it on ebay or craigslist or facebook page make someone a low power boatanchor since the 4-pill is doing better than the sb-220,..all the old timers told me that glass was better than the transitors…. and 3-500z tubes was where the power was ??lots more than pill amps. lots of you tubes videos showing a lot of watts coming out especially bbi videos. using the sb220..I think I should of went with the 6-pill amp like I was going to get..the heathkit is such a beautiful amplifer and this one is in very good shape..
     
  15. Dmans

    Dmans Well-Known Member

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    552fl,
    They are great amps. But they do have to be set up correctly to run AM and survive.



    If your heart is in maxing out the watt meter, I would suggest selling it off now as running it wide open will leave you with a heavy metal piece of gear that you will not get your money back on. 3-500Z tubes are tough and have been around a long time, for a reason. But everything has it's limitations.

    If BBI modded amps are indeed doing 2k in AM, he is not showing the mods it takes to make the amp perform that way. Remember, he is in the amp repair and parts business. If you blow it up, he is more than happy to charge you to rebuild it.

    Best of luck!

    73's
    David
     

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