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The SB-200 on 6: A MONUMENTAL Project!

Discussion in 'Amateur Radio Amplifiers' started by WA6MHZ, Oct 2, 2012.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    400W or 250W output from a pair of 572Bs is pretty low in both cases.

    My old SB-200 (when it was "new") produced 550-600W output on all bands with <100W drive and fairly low grid current. If pushed harder it would do ~650W but the grid current would get higher than I'd like it to be.

    I think possibly all your tubes are a bit soft.
  2. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    That was just in the TUNE position of the Kenwood TS-830S. In the SSB mode, the power output went well in excess of 500W on the Cetrons, but not quite to 500 on the RF Parts. So they are definitely soft, but not completely DEAD like the 6M amp leads you to believe! Something ELSE is fishy. maybe I can work on it again tomorrow after the QCWA meeting. No football games I HAVE to watch in the afternoon.
  3. AF6LJ

    AF6LJ Ham Member QRZ Page

    Keep us posted.
    I have computer work tomorrow so I won't be around until later....
    The suspense will drive me up a wall.
  4. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    It is VERY DANGEROUS to work on LIVE!!!! TWENTY FIVE HUNDRED VOLTS running around in there and under there, and if I just make ONE LITTLE MISTAKE, it will be my LAST!!!

    Imagine them coming in Monday Morning and finding me laying on the floor, deader than a doornail, ZAPPED by that voltage. It is GOOD to be SCARED of that kind of voltage. Being the SLIGHTEST bit careless or overconfident is FOOLHARDY! We aren't messing with BLACK WIDOWS here. This is what ELECTRIC CHAIRS are made of!! And I haven't done anything bad enough to be BARBECUED!!!!

    SO I am very reluctant to try any testing of circuits with the monster turned on. It is SAFEST inside the cabinet, with it's lethal voltages FAR AWAY from messing hands! I need to do all my testing with the HV OFF and WELL shorted to ground! Even messing with the thing on just AC is dangerous. Losta people are belly up from contacting just 110VAC!

    So I need to proceed very calmly, cautiously and with a firm, well studied plan before I light this DEMON UP!!!
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    Do what I do; hook up the test gear and then power up the amplifier under test.
    Besides the grid connections are in the back of the amplifier.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Or you could do what I did once, have a metal D-104 connected to a grounded rig in one hand while probing around inside an amp having B+ turned on with the other hand.

    After that, can I have all your stuff?:D

    I actually did do that once, working on my first homebrew amp at the ripe old age of about sixteen. I was actually on the air transmitting with the case off the amp, attempting to feel how hot my screen dropping resistor was getting on a tetrode amp. Meant to bring my hand an inch or two away from it to feel the heat...oops.

    When I woke up a few minutes later (on the floor) the D-104 was still in my hand and its cable had pulled the rig about a foot closer, almost falling off the edge of the bench.

    I remembered not to do that again.
  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    I don't think I would try that a second time Steve, you might have used up all but one of your lives. :)
    Besides I would miss you.
  8. KB4MNG

    KB4MNG Ham Member QRZ Page

    $300 already and you have not gotten anywhere? How much do you estimate you will have in this amp at completion? How much is a 6 meter amp that works?

  9. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    Looks like I already need a new set of 572Bs, so there's another $100 down the rathole. But this is the FUN and ADVENTURE of building! What fun would it be to just go out and BUY one? Nothing learned or experienced there!
  10. N2EY

    N2EY Premium Subscriber QRZ Page

    So the RF Parts tubes aren't completely bad.

    Why do people describe worn-out tubes as "soft"? That has a different meaning to me. ("Soft" tubes have a not-so-good vacuum in them, as opposed to those with a "hard' vacuum. In the days of audions, a "soft" tube was often prized as a detector.)

    How were they confirmed?

    When you fired up the unconverted SB-200, and put it in tx mode with no drive, there was some idling plate current, right? 160 mA or so?

    What was the idling plate current in the converted SB-200 in tx mode and no drive?

    That's the REAL verification of the DC path. The tuned circuits and such have no effect on the no-drive/tx mode plate current. If you don't have similar idling plate current, the RF will never be right.

    As AF6LJ says, you have to get the DC stuff right FIRST. She's also right about the grid bypassing.

    It's NOT the tubes. If that amp was anywhere near right, you'd get SOME RF out of it. But you report getting NOTHING.

    If the meter circuit isn't right, fix it! You need to get that stuff working properly FIRST.

    Once all that is functional, the next step is the RF tuned circuits. They need to be DESIGNED, using either paper & pencil, charts, or one of the online pi-net calculators. And you have to include tube & stray capacitances. They matter on HF but on 6 meters they REALLY matter!

    It's not hard at all. Even I learned how to do it, decades ago.

    Yes, sometimes a couple of turns and a dipper and you get lucky. This isn't one of those times. I think your input circuit is WAY off.

    That's because you WON'T take a step-by-step approach to getting it to work. You want to just slap in some parts and have it work, without checking all the details. That slap-it-together approach might work if you had a perfectly working SB-200 and a lot of luck, but that's not the case here.

    I wrote a step-by-step procedure some posts back - did you read it?

    You were right not to convert the SB-230. A working one with a good bottle is too valuable to hack up.

    73 es GL de Jim, N2EY
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