Sb-200 weird thing happening

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

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  1. AA5CT

    AA5CT Ham Member QRZ Page

    re: "If you really run into a brick wall I still wouldn't scrap the amp, I'd either get deep into diagnostics trying to figure out what breaks up the oscillations "

    Of course, NO ONE is going to treat this as a subject of study for forward and reverse gain, under non-powered and powered (but, be careful) conditions. Recall back in school during the study of two-terminal active devices, and the criteria for stability? Reverse gain (S12) must be at least some 10 dB less THAN the forward (S21) gain, but MORE 'reverse' gain means higher stability especially under CHANGING load conditions. Today's two-port VNAs can make these measurements.

    AND, on top of that, for more than just the narrow ham bands, a complete 'sweep' or characterization, if this was to be done properly (which is why we see so many 'hack' cures on the web) from 2 MHz up past 50 MHz (SINCE we don't know at what frequency the 'oscillation' takes place).

    Speaking of which - that would be HELPFUL for analysis and diagnosis - WHAT is the frequency of oscillation anyway?

    Some investigatory work with a GDO can also reveal where *strange* resonances are occurring, LIKE in the PA Plate choke maybe?
    Last edited: May 18, 2019
    K7TRF likes this.
  2. N8FVJ

    N8FVJ Ham Member QRZ Page

    If VHF oscillation, the hair pin type suppressors work better than resistor wound type.
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I almost wrote exactly that in my previous post. We tend to assume VHF parasitic oscillations but that might not actually be the case. A spectrum analyzer sniffing the amp's output could answer that question but something like a broadband receiver might detect the oscillation or perhaps a fast oscilloscope and working it back from the time domain if the oscillation is well defined to a single fundamental frequency.

    But yes, I keep thinking of the conditions of oscillation in terms of loop gain and phase shift. Something is apparently causing troubles on those two fronts and it should be possible to track it down though it might be something insidious like somewhat long lead lengths in the grid circuitry or grid bypass caps or RF chokes that have aged poorly or something else non obvious.

    One interesting test might be to see what happens in terms of amp stability with the rig RF disconnected but the amp's input terminated in a 50 ohm non-inductive load vs the input left floating when the amp is keyed. That doesn't point to a specific failure but it's more about trying to run down the conditions under which these oscillations occur. It could lead to possible kludge fixes like a suitably powered resistive T pad on the amp's input but personally I'd try to run down the root cause before introducing band aid fixes.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

  5. HAMHOCK75

    HAMHOCK75 QRZ Member

    Based on the new information here, I am inclined to believe this problem is still related to the parasitic suppressors due to the small change in plate current when it occurs. Once I had an amplifier break into oscillation on 10M due to a neutralization capacitor failure. The plate current pegged the meter. I could see the oscillation was in band with a 100 MHz scope. This small a change in plate current seems to indicate the tubes are running out of steam where the oscillation is occurring.
    K9AXN likes this.
  6. N8FVJ

    N8FVJ Ham Member QRZ Page

    Get the hairpin suppressors. Well know to perform better at VHF oscillation.
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    That's been debated heavily and it isn't a generally accepted conclusion that hairpin suppressors perform better.

    Here's Tom W8JI's take on suppressors in general and hairpin suppressors in particular:
    (emphasis added)

    With more detailed information on VHF stability and parasitic suppressors here:

    I'd personally go with conventional parasitic suppressors wound around non inductive resistors.
    WQ4G and K9AXN like this.
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    Just remove the 11 meter mod and all will be good.

    Or just use it on 11 meters only. Where it is legal. (Into a dummy load for test.)

    11 meters should be open soon. 10 meters also. :)
  9. HAMHOCK75

    HAMHOCK75 QRZ Member

    KA9JLM's comment reminds me that some time ago I was rebuilding another amplifier and found a series resonant circuit to ground at its input. It was not in the schematic but turned out it was factory. It's purpose was to ground any attempt to use the amplifier on 11M. If fact, I think it was required by the FCC

    I didn't mention it earlier because I have never seen that done at the output and that FCC requirement did not affect the SB200 until became the SB201. I suppose it is possible that a ham owner deliberately did that to discourage its use on 11M ahead of the FCC action.

    I checked the notch frequency with a signal generator and scope. It notched 27 MHz very well.
    Last edited: May 18, 2019
  10. KN4CTD

    KN4CTD Ham Member QRZ Page

    It never occurred to me before, but that could’ve been the reason that was there?? I know for a fact it’s never been on 11 meters least the 10 meter inductor shows no sign of anyone ever having tack soldered the tap in different places. If they did, they did one heck of a job! Personally, for me, I don’t operate 11 meters......heck, I generally don’t operate anywhere. If not for the obsession with how things work, the tinker factor, and meeting good people, I wouldn’t even be interested in ham radio at all. I don’t care if it ever works on the higher bands, but the next guy might. I can’t resell this thing one day to some unsuspecting guy and sleep at night. I’ll either get to the bottom of the issue or I’ll do something else.....part it out or, put a couple gi-7b tubes in it or something as a project to keep. I just hate to do that being as someone has spent quite a bit of time and money on these “upgrades.” I’ll dig into it a bit more next week. It’s got a pair of full output Cetrons, full output Taylor’s, and every Harbach replacement part/upgrade that I’m aware of.......both sets of tubes output 700 watts on the lower bands with less than 100 watts drive. It would be nice to have it work as I know it should. It would make someone a nice first amp but like I said, I won’t do like a lot of people on eBay do and pass it off to someone knowing full well it has an issue. Heck, I would probably do the grounded grid mod if I had some instruction on how to change the bias circuitry and metering. That might make it more stable. I am going to sit down and go through all the grounds and remove the bandswitch and plate/antenna caps and clean everything up really good......besides that, I’m probably at the limits of my capabilities short of ordering all brand new components from mouser and just rebuild every circuit in the amp.....I have no desire to do that. Even if I had a scope or spectrum analyzer (I would like to have these things one day) I still wouldn’t know how to use them! Thanks again for everyone’s suggestions and help!

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