Heathkit SB-220 with 120 watts Drive

Discussion in 'Amateur Radio Amplifiers' started by N8FVJ, Feb 2, 2020.

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  1. N8FVJ

    N8FVJ Ham Member QRZ Page

    Using a tube radio without variable power output, will a SB-220 be ok with 120 watts peak input and using the CW position to reduce power output?
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd think such operation will create excessive grid current. But if heavy loading can keep the peak Ig down to 250mA or so, probably fine.
     
  3. W1QJ

    W1QJ Ham Member QRZ Page

    what radio would that be?
     
  4. N8FVJ

    N8FVJ Ham Member QRZ Page

    My favorite- National NCX-5. Has very clear receive audio with a 2.8kHz IF filter and about no noise with an outboard DNR speaker. HF bands sounds like 2 meter FM. Great for my poor ears. Dual RF receive amp is very sensitive on 10 meters.

    The radio kind of embarrasses my TS-950SD as it sounds much better. Drift settles down to about nothing after 15 minute warm up. Digital display will get you within 1kHz so you can find any frequency.
     
    Last edited: Feb 3, 2020
  5. W1QJ

    W1QJ Ham Member QRZ Page

    If you wanted to back the final output power down about 30 watts so that the output is around 90 watts why couldn't you just load the final for a bit less than 300ma? Can't you dip the final at around 250ma? The final should still be linear at that point.
     
  6. N8FVJ

    N8FVJ Ham Member QRZ Page

    Detuning the RF amp also detunes the receiver. Yes I could do that, prefer the receiver tuned to maximum sensitivity. Best way to detune is using the drive control. I do this detuning a little as on lower bands the radio swings to 140 watts at times.
     
  7. W1QJ

    W1QJ Ham Member QRZ Page

    The load adjustment should not effect the receiver, but anyhow I did not see any mention of the drive control until the CW mode. I had an NCX-3 back in the day. I know the exciter tune will effect the RX signal but the load shouldn't. That NCX-3 was my very first SSB rig.
     
  8. N8FVJ

    N8FVJ Ham Member QRZ Page

    The load control does effect receive, but not much like other controls. To detune to 100 watts I have to turn the control to almost 180 degrees opposite of the peak output. Kind of poor to operate that way? It will not detune to say 60 watts to not overdrive some amps.
     
  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    You might consider adding a feedback resistor (aka swamping resistor) between the amp’s tuned input circuits and the filament signal coupling capacitor. Something like a 10 ohm to 20 ohm, 10 watt resistor. It won’t have a big impact on normal drive power but will allow you to drive a bit more power without excessive grid current. It also tends to improve the amp’s input matching and input SWR bandwidth.
     
  10. W1QJ

    W1QJ Ham Member QRZ Page

    You won't over drive an SB-220 with 100 watts or probably just on the edge with 120w You should not have to go down to 60 watts
     

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