Peaked Grid current at 0 Drive? SB-221

Discussion in 'Amateur Radio Amplifiers' started by W1LLO, Mar 25, 2019.

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  1. W1LLO

    W1LLO Premium Subscriber QRZ Page

    Hi there folks!

    Long time reader, first time poster. I have a SB-221 Heathkit amplifier. I got it at a local hamfest and the man who sold it to me told me it worked fine (which it does for the most part). Let me preface this by saying everything on the amp seems to work fine. The tubes have a nice healthy yellow glow. It puts out as much power as it's supposed to (800 CW to 1100 SSB into the dummy load measured by an external meter), the plate current reads fine and dips at maximum output, and it tunes up just great. The problem I'm having is with the grid current. Whenever the amp is keyed (even at 0 drive!) the grid current on the meter peaks full. The only time it doesn't is in SSB mode and it will still peak if I speak higher than a whisper.

    What I'm wondering is if this is a serious problem, or a bad meter? Of course it could be operator error as well. This is my first amplifier after all.

    If anyone has any insight, i'd appreciate it. Figured I'd try this before I cracked into it.

    Thank you!


    Other information
    Radio IC-718
    Amp 120v
    bypass switch and soft start mods
    wet paint can dummy load
    tuned with straight key in CW mode
    Last edited: Mar 25, 2019
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    You may just be over driving it, If a whisper maxes out the grid current.

    Good Luck.
    Last edited: Mar 25, 2019
    KD2ACO likes this.
  3. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Nahhh, something is wrong. There are a couple things that can do this. I’m at my shop later to look over a few things and come up with something to try so we can get more clues.

    I'm trying to figure out what you're seeing. If you pick a band (say, start out on 80m with the dummy load), where are your Tune and Load controls sitting when the amp is tuned up?
    Last edited: Mar 25, 2019
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Check the grid metering resistor which should be between the - return of the HV power supply and chassis ground.

    It's impossible to overdrive an SB-221 with an IC-718; something's wrong with the metering, and probably not the meter itself.
  5. AA5CT

    AA5CT Ham Member QRZ Page

    Wow ... first thought was, something is oscillating, but it might also be the metering is messed up, per WIK.

    Not clear when the Grid Meter was showing 'peaking' in the OP especially when he says it does it with "0" drive: (even at 0 drive!).
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    I never really thought about that. :confused:

    Measuring the SWR on the input of the amp would measure very high, If it is not a metering problem.
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    But the person that sold it said it worked fine. :rolleyes:
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If the meter shunt blew open or changed to a much higher resistance, instead of the meter being 0-350mA it will be 0-1 mA (I think -- have to really look at the meter specs) but in any case will be a hell of a lot more sensitive, so the slightest whisper would drive it up against the peg, if not blow the meter out entirely.

    Grid current on that amp would normally run in the 200mA - 250mA range somewhere when properly tuned up and into a good load. If the shunt drifted very high or blew open, be really careful.
  9. W1LLO

    W1LLO Premium Subscriber QRZ Page

    Thank you for your response! Well here’s the thing. I have no idea if these old knobs are even on straight at this point. Here’s some photos that I took of the controls and of the meters while keying. The one of the computer screen just shows the 0 transceiver output.

    Attached Files:

  10. W1LLO

    W1LLO Premium Subscriber QRZ Page

    Haha. Yeah. That was my reaction too. “Works fine!” He says.

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