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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 QRZ Page

    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 QRZ Page

    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 Premium Subscriber QRZ Page

    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 QRZ Page

    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. KH6AQ

    KH6AQ Ham Member QRZ Page

    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 QRZ Page

     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 Premium Subscriber QRZ Page

    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 QRZ Page

    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 QRZ Page

    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.

    Attached Files:

    Last edited: Mar 30, 2012
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Scroll down to "glitch protection" here:

    Maybe also look at this:

    Some stuff on improving the SB-200, including adding a glitch resistor:

    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.
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