Heathkit SB220 Problems

Discussion in 'Amateur Radio Amplifiers' started by K9NRA, Jun 11, 2015.

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  1. WA7PRC

    WA7PRC Ham Member QRZ Page

    After hard-grounding the grids in my SB-220, I found it to be unconditionally stable. No amount of misadjustment caused it to take off. Others have reported the same.

    Eimac shows the grids hard-grounded in their 3-500Z datasheet here. In Class AB2 w/ 3KV EP, IMD3 is specified as -40 dB. Worst-case is shown as -33 dB (w/ EP = 2500V).

    Apparently, not enough (if any) to cause concern.

    vy 73 es gl,
    Bryan WA7PRC
    N2EY likes this.
  2. G0HZU

    G0HZU QRZ Member

    I'm a bit suprised by the claim above... a typical (ballpark 90nH?) suppressor coil will be self resonant well up into the UHF region so the claim above clearly can't be correct. i.e. it's easy to disprove it by just measuring the SRF of a typical coil.

    In theory, a simple RL suppressor just needs to be a perfect resistor in parallel with a perfect inductor. But obviously neither perfect component exists and so you will get some stray capacitance in realisable components (along with other imperfections such as stray inductance in the resistor and some loss resistance in the inductor).
    But the dominant feature is that it is a simple inductor in parallel with a resistor.
    Just make a typical 90nH suppressor coil and measure it for self resonance. The SRF is going to be up in the UHF region and nowhere near the typical frequency of instability.
  3. KM1H

    KM1H Ham Member QRZ Page

    About 2-3dB worse which at 2500V puts it into the same category of most SS rigs, GIGO on a quiet band.

    Ive build dozens for others and repaired many in recent decades, not a trace of instability as designed and age or construction flaws got them to me.

    That series of amps is marginally neutralized on 10M which is where it takes off when tuning. An out of phase third winding over the filament choke and a wire looking at the tubes fixes that by an additional 20dB isolation.

  4. G0HZU

    G0HZU QRZ Member

    To try and predict/measure the SRF of the coil part of a classic RL based suppressor I tried winding a replica of a typical suppressor coil here and I used about 14cm of wire in total to get about 90nH. The SRF can be crudely predicted to be somewhere near 1GHz when the coil is in free space simply because of its wire length.
    If it is tested in a jig near ground this will fall quite a bit due to stray capacitance.

    I tried measuring the free space SRF with a remote exciter/receiver (I used my the ports of my VNA and some RF probes) and it was close to 1GHz as predicted. I then made a two port test jig and measured the coil the regular way on a VNA and the SRF had fallen to about 600MHz. But this is still way above the frequency of instability of a typical amplifier. Adding the resistor isn't going to be enough to make the suppressor self resonant at (say) 120MHz.
  5. KM1H

    KM1H Ham Member QRZ Page

    3-4 turns of #14 space wound about .75" in diameter sure resonates a lot lower than you claim. Sounds like a possible equipment/operator error plus testing out of circuit is meaningless since there are a lot of capacity strays involved.

    Its simple enough to use a GDO for this.

  6. G0HZU

    G0HZU QRZ Member

    Note that I described a 90nH coil above and your 3.5 turn coil of diameter 0.75" and a length of maybe 1" is going to have an inductance in the ballpark of 140nH. However, I'd still expect this larger coil to have a SRF up around 400MHz if measured close to a ground plane and with a VNA.

    I'd also guess that its total wire length would be about 7" so when fed against ground it would be resonant at around 400MHz based on a very crude approximation.

    There are other (much more detailed) calculations/calculators available to predict its SRF and I would hope they would all agree with about 400MHz. Note that this is just the SRF of the coil. However, I expect that when a typical resistor is included the behaviour of the network will be dominated by the various stray inductances within the suppressor and so any self/stray capacitance will only have a minor influence on the impedance of the completed resonator at 125MHz anyway.

    Note also that I'm trying to describe a typical RL suppressor and not some custom and more complex design involving extra components.
  7. G0HZU

    G0HZU QRZ Member

    I had a quick go at measuring the SRF of a 3.5 turn coil with diameter approx 0.75" using 1.25mm diameter wire.
    The first plot below shows the free space resonance is up around 680MHz.
    The second plot shows resonance when imaged against a ground plane and this is now down at about 480MHz. This is now like a passive 1/4 wave helical resonator.
    The third plot shows the effect of putting an aluminium corner/wall right next to the coil. The resonance only drops slightly. The last plot shows the free space SRF of a 10 turn coil using 1mm diameter wire. The SRF is 280MHz.

    The way the test jig works is that the round H field probe (RF source from the VNA) will induce energy into the test coil and the E field probe thenpicks up energy from the resultant E field around the coil at resonance and this gets fed to the receive port of the VNA. Without the test coil the E field probe has high rejection of the H field produced by the round H probe. So the E probe 'needs' the test coil to produce an E field at resonance in order for it to detect anything significant.

    On the 10 turn coil you can see that the coil behaves as a transmission line (or a bit like a helical dipole?) because it has resonances at approx 1/2 wave, full wave, and 3/2 wave.

  8. K7BIT

    K7BIT Ham Member QRZ Page

    I used 10 ohm 35W
  9. N2EY

    N2EY XML Subscriber QRZ Page

    That's exactly how properly-designed parasitic suppressors work. The goal is for the inductance, and loss, to be negligible at the operating frequency, yet so great at the parasitic frequency that oscillations cannot start.

    Nichrome wire for parasitic suppressors is pure snake oil because, as you say, it is lossy at both VHF and HF. It will "work", because it is lossy at VHF.....the price you pay is the loss at HF.

    73 de Jim, N2EY

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