Heathkit SB220 Problems

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

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  1. AF6LJ

    AF6LJ Ham Member QRZ Page

    From my reading in the old handbooks the principal of operation of this type of parasitic suppressor goes like this..

    The coil is self resonant at the frequency the amplifier naturally wants to oscillate at.
    The self resonant circuit provides a high impedance to the parasitic frequency...
    The parallel resistor is simply a place for that energy to be dissipated.

    That's how I understand it...

    There have been arguments here and elsewhere in the past over the tube or tubes being the cause of the parasetic osciallations. That really isn't iomportant from out standpoint since nobody designs amps with those tubes anymore and the tried and true methods work.
     
  2. KM1H

    KM1H Ham Member QRZ Page

    MOX resistors have some inductance thus the Ameritron versions which the "engineer" copied from me when I described them on Amps in the mid 90's and had been using since the mid 80's or so. Give credit where it is due and not to a copycat who claims everything as his own.....just ask him.
    Metal film doesnt have the surge capacity to be useful but the modern Ohmite OY series do very well without external caps.

    Nicrap or copper, their inductance is obviously wrong for suppressors in those photos.

    No extreme, just a coil, a MOX resistor and 2 small silver mica caps to cancel the inductive reactance. I developed them for 6M in the mid 80's when good carbons were getting scarce and some were pure import crap.

    Correct but it is high Q while parasitics can be over a wide range.


    The resistor IS NOT supposed to dissipate any power. Its sole purpose is to prevent an oscillation from even starting by presenting a very low Q broadband resonance. Sort of like using 100' of RG-58 on upper HF and higher and then claiming a new miracle broadband antenna. When the parasitic is close to the highest operating frequency then excessive inductance will include that frequency. Ive had no problem with 572B's on 6M and the 3-500Z is a breeze.

    The tube is the sole source of the parasitic and influenced by its surroundings. A parasitic doesnt start in a tank circuit as the nichrome meathead and his poorly RF educated groupies believes.

    Ive no idea what you mean by that as just about all tubes from the 39-49 pre/post WW2 era are still being used by some, The 811, 813, 814, 4-125A, 4-250A/4-400A. 4-1000A, 4CX250, and more are still in wide use. Older tubes such as the 250TH, 810 and dozens of other low mu types just didnt have the gain to develop a parasitic.

    Carl
     
  3. W1BR

    W1BR Premium Subscriber QRZ Page

    I think the trick is to have just enough inductance to be effective at the parasitic frequency that the amp is most likely to oscillate at. The resistor ruins the parasitic VHF Q, and suppresses tendencies toward self oscillation at some VHF frequency. The trade off is having enough inductance to be effective, along with the resistance, to spoil the Q at VHF while not introducing noticeable losses at the highest frequencies that the amplifier operates at.

    It would seem that nichrome wire really doesn't do much... providing the conventional suppressor was designed properly. Nichrome wire losses would be the same at VHF as HF.

    Pete
     
    N2EY likes this.
  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    Cool...
    That's good to know.

    And this makes sense since for oscillation to occur there must be some place where there is gain and in phase feedback.
    Shift the phase and oscillation cannot occur.
    811s scare the crap out of me, the internal construction has so much lead length, I sometimes wonder how those tubes manage to work on fifteen meters lat alone ten with four of them in parallel.

    572s aren't that much better a pair seems to be bullet proof up to ten in terms of stability, and lots of folk have made six meter amps from them.

    Good Post Thanks Carl.
     
  5. KM1H

    KM1H Ham Member QRZ Page

    No tricks involved as that is what competent designers have known and been doing for many decades.

    Nichrome does absolutely nothing useful in this context and having it as part of the desired RF path is moronic, Ive seen them run bright red on 10M. In addition it ads no measureable lowering of the circuit Q, at least to 70-75 mHz which is the limit of my HP and military Q Meters.

    Carl
     
  6. AC8AC

    AC8AC Ham Member QRZ Page

    only valuable use for nichrome in an amplifier is for replacing impossible to find meter shunts . i use them for metering my home brew projects using 1 and 5 ma panel meters . i remote shunt my meters so i never can have over .7 volts on the meter under any fault or normal operation but thats a topic for another day

    for suppressors a use 1/4"wide .028 brass strips up to 1 kw plate then i use 1/2" just bend in a horse shoe measure 4" resistor lead to lead . mark with a 250* paint crayon to check for over heating at the tip of the shoe 10 meters should be maybe 1% less efficient than 40 . this approach is not too scientific but works . on 6 meters it will over heat and youll have to run them about 2" resistor lead to lead

    but a 3-500 contrarily to what some will say they are quite stable and so is the sb220 especially if the grids are grounded well like yours is . there is no real reason to be super scientific . ive seen more sb200's take off than 220's even with open grid chokes due to flashover
     
    N2EY likes this.
  7. W1BR

    W1BR Premium Subscriber QRZ Page

    Carl, I think you replied ahead of me as I was typing my post... good double! I didn't see your post when I started typing.

    73

    Pete

    ps: how is that Daiwa RF processor working out? I have mine in line, but haven't done much SSB work to form an opinion.
     
  8. K9NRA

    K9NRA XML Subscriber QRZ Page

    Ok, I have new suppressor resistors in the unit, it powers up ok, but my grid current is approx. 1/2 of my plate current (Ig=300ma ish, Ip=650ma ish with 90w in ~1300 out. Before directly grounding the grids, old suppressors etc. it was 1/3 of the plate current. Is this a problem? The old suppressor was 2 100 ohm in parallel (2x5w), new ones he sent me are 3 100 ohm in parallel (3x5w)
     
  9. KM1H

    KM1H Ham Member QRZ Page

    Yep, that is what happens when you ground the grids. The gain goes up along with the output and so does the chance of instability.

    You can either reduce the drive, or add a 10 Ohm noninductive resistor in series with the RF at the sockets; this also adds negative feedback which will bring the IMD back to original.

    Carl
     
  10. K9NRA

    K9NRA XML Subscriber QRZ Page

    I wondered if that could have an effect. I don't have a way to check IMD , as a rule, how much worse doe it make the IMD by directly grounding the grids?
     

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