Need help SB-200 BIAS high readings

Discussion in 'Amateur Radio Amplifiers' started by KJ4AQU, Mar 23, 2012.

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  1. KJ4AQU

    KJ4AQU Ham Member

    Hi all, I have a problem that has recently cropped up. When I key my 200 with my 746pro set at minimum RF power output my Grid current is sitting around 300ma which I feel is way to high! I can hear the relay clicking but my theory is that the -135v BIAS voltage is never being removed. And how would I check my theory? Does that make sense or am I barking up the wrong tree. Please any help apprciated. :confused:
     
  2. KJ4AQU

    KJ4AQU Ham Member

    I checked the BIAS voltage and at no Key it is -135v with ket down it is -6v. So any ideas folks?
     
  3. AF6LJ

    AF6LJ Ham Member



    if you take the keying line and just ground it.....
    What plate current and what grid current do you see on the amplifier??
    This would be with no input signal at all.

    There is a possibility you may have a shorted tube.
    But we won't know until we have both grid and plate current readings.
     
  4. KJ4AQU

    KJ4AQU Ham Member

    Dumb me! I forgot I added an inline fuse holder for the HV. Well it blew! Replaced fuse and now it's working again. I assumed that just because the meter said I had HV that the tubes had HV. Wrong assumption. Next time i'll look there first if i have high grid current. BTW Previous fuse was 750ma new is 1 Amp. Hopefully not over fusing.
     
  5. WX7G

    WX7G Ham Member

    An inline fuse holder (250 VAC type I assume) for plate voltage? That is not a good thing to do.
     
  6. AG6K

    AG6K Ham Member

     Indeed. Ignoring a fuse's V-rating is not good engineering practice. Also, no fuse is needed with a thoriated-tungsten filament-type tube because it is naturally I-limiting.
     
  7. AF6LJ

    AF6LJ Ham Member

    I hope that is not an AC line fuse.
    They tend to explode when used well beyond their voltage rating.
     
  8. WB2WIK

    WB2WIK Premium Subscriber

    Unless you use a specialty high voltage fuse (which is normally very l-o-n-g) adding a fuse to an HV line is not a good idea.

    Even if you do use a proper fuse, it's usually still not a good idea because if the B+ fails and you drive the tube you can fry the grid and then you'll be in the market for a new tube (or tubes).

    Usually people just use a "glitch resistor" for this, to limit arc current without actually opening up.
     
  9. KJ4AQU

    KJ4AQU Ham Member

    So what do I need to add a "glitch resistor"? What value and wattage. And I am guessing it has to non-inductive? I think I now understand why adding a fuse is not to smart. I did have that fuse blow up but it stayed inside the holder. I replaced the standard 12Gauge wires that were leading into it(fuse holder) with HV wires. But still probably a bad idea. Please advise. Pic is of fuse holder. Sorry a little blurry. I guess I'll pull it then.
     

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    Last edited: Mar 30, 2012
  10. WB2WIK

    WB2WIK Premium Subscriber

    Scroll down to "glitch protection" here: http://www.somis.org/D-amplifiers2.html

    Maybe also look at this: http://www.flickr.com/photos/79358868@N00/3258869687/

    Some stuff on improving the SB-200, including adding a glitch resistor: http://blog.kotarak.net/2008/03/sb-200-part-1.html

    It doesn't have to be non-inductive, since it's not in the RF path. This goes in the B+ line between the power supply and the plate RF choke in the amp, so it's not exposed to RF. It does need to be well isolated and installed as far away from other components and the chassis as possible. I usually just mount a power resistor on a couple of ceramic pillar standoffs screwed to the chassis in an open space not near other stuff.
     
  11. KJ4AQU

    KJ4AQU Ham Member

    Ok I made up a Glitch resistor. Put 2 Plastic tie wrap holders down on bottom of amp. One you could see was holding my fuse holder in place. Surrounded by Electrical tape. Solder in line a 10 ohm 10 Watt wire wound resistor with 2 layers of heat shrink on both ends and put it inside a plastic "shelf" if you want to call it that. See picture. Sorry blurry getiin old ya know! Opinions please! Anything else I should insulate?
     

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  12. KJ4AQU

    KJ4AQU Ham Member

    Does this resistor get hot? I have not turned on the amp since doing this and I am thinking hmmm plastic tray and plastice tie wraps. Possible melting? I guess I'll find out later. Been a long week and am tired.
     
  13. WB2WIK

    WB2WIK Premium Subscriber

    It will generate some heat. 10 Ohms will generate close to 4W (of dissipation) when you're key-down. Less average power than that on low duty cycle modes like SSB.

    I usually use about a 25W resistor with an air core that let air flow through it, keeps it cool. These things come with "clips" that push into the ends of the resistor and provide a mounting hole for them so they can be screwed down to insulated (ceramic) standoffs.

    Photo below.
     

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  14. KJ4AQU

    KJ4AQU Ham Member

    I have seen those before. Now to get one and some ceramic standoffs. For the standoffs what height do you recommend? Don't want to be too tall and touch housing.
     
  15. WB2WIK

    WB2WIK Premium Subscriber

    I have a lot of 1" standoffs, so I usually use those. Make sure the mounting screw (to the chassis) and the screw holding the clamp for the resistor are "separated" by either ceramic or at least an air gap of 1/4" or so inside the standoff to prevent fireworks.
     
  16. KJ4AQU

    KJ4AQU Ham Member

    May I ask where do you get the Resistor from? I mean what supplier?
     
  17. WB2WIK

    WB2WIK Premium Subscriber

    I get most of my stuff from Apex Electronics in Sun Valley, CA. I just walk in and buy them over the counter.

    But Digi-Key and Mouser should have this stuff.
     
  18. KJ4AQU

    KJ4AQU Ham Member

    Ok I will search for one. Guess I'll have to do some running around to try and locate.
     
  19. WB2WIK

    WB2WIK Premium Subscriber

  20. W9GB

    W9GB Ham Member

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