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SB-200 oscillating with an antenna tuner

Discussion in 'Amateur Radio Amplifiers' started by KS9Q, Mar 29, 2017.

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  1. KS9Q

    KS9Q Ham Member QRZ Page

    Anyone want to take a crack at this one?

    SB-200, total rebuild, all connections proper and all solder is solid. Mechanically sound. Original Cetron tubes. Makes 650-700W into a dummy load, tunes well. Here's the thing - all is well and good until I try to connect it to an antenna via a tuner (Palstar AT2k although same issue with Ameritron ATR-20). So... using the tuner on 80 and 40, all is right with the world. However, on 20m and up, if I have the tuner in circuit, the amp starts to oscillate immediately when the unit is keyed (no drive), regardless of antenna selection. If I select bypass to drive the antenna without the tuning network OR if I select the dummy load, the oscillation doesn't occur. It only happens with the tuning network in circuit between amp and antenna.

    Parasitic suppressors are original style except using 100 Ohm resistors (all I had on hand). The connections to tube bases are as short and as tight as I can make them. No excess lead length. Using RG-400 coax from output rail to the antenna jack. Relay contacts are whistle-clean. The oscillation shows up both on my monitor scope and on the reflected power meter. I do not have another set of tubes to try. If I connect a matched antenna to the amp directly or if I connect a dummy load, it works fine. Driving unit with an Icom IC-7410.

    Thoughts are welcome.


    Jim K. - KS9Q
  2. AC8AC

    AC8AC Ham Member QRZ Page

    that's bazar not even sure what to think . what makes it stop ? unkeying or do have to turn it off for it to stop ?
  3. KL7AJ

    KL7AJ XML Subscriber QRZ Page

  4. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Is the tuning network set for min VSWR (or close) at the time you engage the amp, or are the settings way off at that time?
  5. KS9Q

    KS9Q Ham Member QRZ Page

    Unkeying makes it stop instantly. As soon as cutoff is applied to the tubes, they go right to standby.
  6. KS9Q

    KS9Q Ham Member QRZ Page

    If you are referring to the antenna tuner's tuning network, it is set for minimum VSWR using only the exciter as a source prior to enabling the amp. If you are referring to the input network to the amp, those settings, while not perfect, are reasonably close. I have not however gone through and minimized every coil in the center of the band, which I suppose would be worthwhile. The VSWR to the exciter has been within acceptable limits, so I haven't messed with the input network as of yet.
  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    This shouldn't happen
    You tune the tuner first than tune the amplifier into the tuner.
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    How have you adjusted the amp's Load control prior to seeing these oscillations? Have you adjusted for max power or loaded beyond max power to where power drops off a bit?

    Similarly given the rebuild, what's your idle mode plate current when the amp is keyed into the dummy load without any drive power applied?

    Also what did you retain and or change in the overall amp circuit topology with the rebuild? Did you strap the grids to ground and change bias accordingly? Any changes to the input coupling of the amp?
  9. KS9Q

    KS9Q Ham Member QRZ Page

    And that is exactly how I do it. The last thing I want to do is put the amplifier output into a badly mismatched load. That I think would be asking for it. So I make sure that the tuner is set properly at the hundred watt level, then I tune the amplifier up into the dummy load. Then, with the amp unkeyed I switch in the tuner. As soon as I hit that mic button and the amp goes in to transmit mode, it starts to oscillate.
    Last edited: Mar 29, 2017
  10. KS9Q

    KS9Q Ham Member QRZ Page

    Before I engage the tuner I have already tuned the amplifier into the dummy load. It will have been tuned at full drive for proper grade current and proper plate current and loaded for best output. Curiously, when I mesh the plates on the loading control, the oscillation either stops or drops to almost nothing.

    No substantial changes to the circuit. I did not directly ground the grids because that would require redesigning the bias circuit. The grids each go through a 220 pF capacitor to ground. I have heard that it is wise to increase that value upwards of around 1000 pF. As of yet I have not attempted that.

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