Sb-200 weird thing happening

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, May 15, 2019.

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  1. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    If the amp stops its bad behavior on 10m with NOS CC resistors in the suppressor job, you will have done an interesting experiment in the long running "SB-200 suppressor saga."
    KN4CTD likes this.
  2. WA7PRC

    WA7PRC Ham Member QRZ Page

    A parasitic suppressor functions to reduce gain above an amplifier's highest operating frequency. Using more XL than needed causes the suppressor to dissipate power below the amplifier's highest frequency.

    Heathkit's SB-series amplifiers used a 3-1/2 turn inductor (14ga solid tinned copper wire). I measured mine as 80 nH. The resistor's function is to reduce the inductor Q, to broaden the effect. Its value isn't terribly critical -- anything from about 27Ω to 100Ω is OK. What IS critical is, the resistor(s) need to have low inductance. CC works, as will other types specified as suitable for RF.
    WQ4G likes this.
  3. KN4CTD

    KN4CTD Ham Member QRZ Page

    I have zero desire to modify this amplifier in any fashion that strays from the original design. My desire is to have this thing operate correctly (as designed) and make power out on the higher bands (where it was also originally designed to work) into anything other than a 50 ohm dummy load, without oscillating. Besides a couple of extinct, out of value parasitic suppressor resistors, a missing plate choke bypass cap, and some everyday Harbach mods, I can’t find anything else that’s been altered or is out of spec. W8JI suggested on a forum that those 200pf caps where always a bad design and that the design would’ve been much better either using .01 caps or hard grounding of the grids. He designs amps, I don’t. I swapped them out, it works. Now I have an amplifier that tunes and loads above 40 meters into an actual antenna. I’ll swap some 200pf micas back in after I rewind the suppressors on those resistors that cost $3 each. If that “fixes” the problem, great, I’ll leave it alone. If not, I’m going with what works.
    K9AXN and KD2ACO like this.
  4. W1QJ

    W1QJ Ham Member QRZ Page

    The Internet is flooded with some really crazy and unnecessary "mods". Why some hams see it fit to do EVERYTHING they see on their computer screens that other do to their amps really baffles me. If they see it they do it. Many of the hams that publish some of the rubbish seen have no idea what or why they are doing it. It sort of seems like a "money see. monkey do" thing. There are some mods that are acceptable and a few are actually necessary to keep up with the change of exciters over the years. I would seriously take anything you do into deep consideration before you do it. If you are unsure, come here and ask.
    K9AXN, WA7PRC and N6UH like this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    C'mon, Lou, some are really useful.

    I especially like a "flare fire" option, where if the amp gets too hot it lights the fuse on a flare which then shoots out the top of the amp to get your attention.

    The guy who wrote that one up forgot to say "only use this outdoors," so first time mine worked it set the ceiling on fire.:(
  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    SOME modifications/upgrades are worthwhile. The trick is figuring out which changes are of value...

    I recall changing the ignition points in my then new-to-me 1968 Plymouth Barracuda 340-S -- in the rain. That was the only time I did that; I soon changed over to reluctor-driven electronic ignition. Yes, it was "pointless". :p
  7. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Read this --- will help you to choose for yourself if appropriate.

    That was a substantially stable and well designed amplifier in its original state. Stability was tested with nothing attached to the output, then nothing to the input, then nothing on either. That's a lot to expect from any GG amp. If it worked that well then, what the hey makes people think that it is a design issue????? It is called a design issue by people that simply do not understand the complexities of what it was meant to contribute; sorta like pulling the engine out of a BMW and replacing it with a model A 4 banger cuz I don't understand it or didn't agree with the engineer who designed it. It's essentially a Collins design.

    The genesis of that design is the Collins 30L1, 17,000 sold to mostly military. There are no engineers today that would be carrying coffee for the Stanford of Hallicrafters, Bruene and Senti of Collins and many others of that era. I have yet to hear a person who criticizes the design of the that grid circuit, to also understands it.

    People like to blame the sins of the 811A and the foolish choice of using unmatched plate and grid 811As on the 572B. Read the above web page then decide for yourself what makes sense to you.
    You'll feel better about your choices. If you find anything difficult, let me know. Be happy to explain not cuz Dad says so.

    Regards Jim
    KN4CTD likes this.
  8. KN4CTD

    KN4CTD Ham Member QRZ Page

    Thanks for posting the link. I had actually read that before, believe it or not. If I 100% truly understood the true technicalities of the argument between grounded grids/floated on caps, I probably would be able to make a decision based upon “what I think” and not the opinions of others......and I probably wouldn’t be on here asking others for advice on how to troubleshoot my problem with the limited resources I posess either. I appreciate the help and opinions of everyone who has given me tips on how to troubleshoot this sb-200 on the higher bands. Using my limited testing resources, I simply cannot find any other components out of spec and to the best of my knowledge, it appears to have been built by someone who was at the very least competent laying stuff out and knew how to use a soldering iron. Remember, this amp acts fine on the higher bands into a 50 ohm dummy load. I’ve fixed several problems that I’ve found so far. The plate bypass cap was completely gone and the parasitic suppressor resistors were out of range. I didn’t have NOS resistors on hand but I will soon. I’d be surprised if they make any difference, but I hope I’m wrong! I’ve checked every component on the bottom of the tubes for being in spec and for proper layout according to the schematic. I’ve even redrilled the holes for the grid to ground solder lugs and moved them as close as they could possibly be to the socket so the capacitor lead lengths are as short as they could ever possibly be. Those 200of mica caps also test good according to my fancy Chinese LCR meter. The original builder didn’t ground the input wafer end of the input coax.....of course the schematic doesn’t show this, but I added a solder lug there and grounded the shield on that end also for good measure. This thing just absolutely doesn’t work correctly on 15 and 10 meters into my Atr-30 tuner and 80 meter dipole. I even made up a quick 10 meter dipole cut for 28.5. It doesn’t work into that either. Neither did it work correctly into the same dipole cut for 15 meters (before I shortened it, obviously)! Did I mention, I have two pairs of pair of Cetrons and another pair of Taylor’s? Amp acts the same with either pair. I have no desire to deviate from the original design, but I do not know what else to check in this thing that might cause it to exhibit this behavior. I’m not advocating redesigning the wheel, but it sure is hard to ignore the night and day difference in this particular amplifier, into my particular antenna setup after I changed the value of those caps. I may never fully understand why. If you can think of anything else that I haven’t already mentioned having checked in this amp....that may cause it to exhibit these symptoms I’ve described, I would surely like to hear because I’m tired of fooling with this thing.
  9. K9AXN

    K9AXN Premium Subscriber QRZ Page

    The easiest way to get to the source is to use a frequency counter to determine the frequency of oscillation. If the frequency is a result of neutralization, it will be very near the tuned frequency and will respond to the position of the input and output tuning dials. If it's VHF, the the parasitic suppressors are very likely --- as the earlier posts suggest.
    As one suggested, it wreaks of parasitic because the plate meter does not peg.

    The input coax should not be grounded at the wafer. Grounding two ends of a coax invites a ground loop and in the input of an amplifier, ugly.

    The plate choke bypass should remain exactly as it was; do not change the value. The type of capacitor used by design will be a high quality class 2 capacitor. do not arbitrarily use a Z5U or other class 3 high dielectric capacitors. They suffer Hysteresis effects and the capacity varies wildly with voltage.

    You could check the plate choke to be 50uH but never have seen one fail. The choke is another enigma for folks. Lot of folks believe the 50uH choke is poor design. however it's a very well thought out design. It was first introduced by Gene Senti (Collins engineer). Won't go into detail here but one common belief is the low value causes it to overly participate with tuned circuit. This is not true.

    Choke properties are as follows. The impedance must be greater than the output impedance of the tube on the lowest frequency used. When this is true NO AC current flows, only pulse varying DC: The current in the choke never reverses. The current flowing in the choke that occurs during the conduction cycle of the tube is stored and during the off cycle it is returned, a major contribution to the flywheel effect.

    This may help with the reason for retaining the value of the plate choke bypass. There is a plethora of theories regarding the reasons for the terror of the series resonant frequency of a choke.

    The SRF of a plate choke has nothing to do with lumped properties: It is a distributed property. The way to determine the first and most dangerous SRF is unwind the choke and measure the length of the wire. Calculate the frequency where it's a half wavelength. Now what do you have if you short one end to ground and drive the other end ---- Voila, A very effective short to ground. It is not some unmeasurable loop to loop capacity at all. If you wish to find the various very miner resonant and anti resonant spots, use your dipper and scope. They will appear as tiny little tilting waveforms.

    What I'm essentially saying is, don't mindlessly change the original design of the grid or plate circuit based on arguments that cannot be substantiated or explained. There are design enhancements for keying and hot keying that I would recommend as others have.

    Some of those vintage designs were created by world class engineers that are now history.

    Put it aside for a moment to catch your breath then give it a go later. The folks on this list have helped to resurrect some impossibly shabby radios.

    Good luck ET Regards Jim K9AXN
    KN4CTD likes this.
  10. KN4CTD

    KN4CTD Ham Member QRZ Page

    I don’t have a grid dip meter or a scope but I’d like to have better testing equipment on hand and learn how to use it one day also. I’m not even sure how to use a frequency counter to determine if the oscillation is due or neutralization or vhf parasitics. What kind of decent testing equipment is commonly available on the used market at reasonable prices that I might look into purchasing and learn how to use? I’ve seen grid dip meters and I think Ameritron makes a set of coils for my antenna analyzer that is used for the same purpose. I understand the very basic workings of a grounded grid amplifier but when it comes to figuring how every value of every individual component resonates, oscillates, series tunes, blah, blah blah etc, I’m lost. I was hoping to find bad component, replace bad component, have good amp! Even find error in build, correct error in build per schematic, have good amp. I’m a good parts changer though. I guess I could get on Mouser and make an order of every single component from the build manual......with my luck, it still wouldn’t work right. I’ll remove that plate choke and check the inductance, but it appears factory and undamaged just by looking at it.

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