The SB-200 on 6: A MONUMENTAL Project!

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

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  1. N2EY

    N2EY Premium Subscriber QRZ Page

    Don't do it that way. It is NOT a good idea. You are adding complexity where simplicity is needed.

    The need to shield and bypass a meter in the final compartment should be avoided. Either put both meters outside the final compartment or use a single meter.

    What about grid current? Grid current is much more important than HV.

    So you need a meter switch anyway.

    You could just measure grid and plate current, and let the external SWR bridge do relative power output and SWR. A Dow Key relay isn't essential if the pi-nets are properly designed.

    You don't need to measure HV with a front panel meter any more. In the bad old days when the legal limit was 1000 watts DC INPUT, FCC required all rigs rated more than 900 watts INPUT to have provision to measure both final plate current and final plate voltage. And the SB-200 could do 1000 watts DC INPUT on CW. That's why the HV meter.

    Or use GI-7Bs.


    It's still a conversion, though. Just getting more extensive.

    73 es GL de Jim, N2EY
  2. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    Now I am thinking of stacking the meters Vertically, and mounting an outboard display (bar graph) to measure Grid Current and Relative output. It would stick out about 1/2 inch from the front panel and can be mounted in front of the final compartent with no wires or holes into the compartment whatsover. The cables from the display unit will go into Power Supply compartment through a small hole. So I am looking for very thin Bargraph displays. Don't want to go LCD. It will look a little klugy, but that is the only way I can keep it out of the Final Compartment. I already have the 2 meters, a HV and a Plate Current meter I got when I first envisioned the project. Paid dearly for them too. Of course, it would be easiest just to go back to the Heathkit meter affair. Or, I could go to ALL bargraph displays which would be MUCH easier to put behind the Front panel in the Power supply compartment.

    Bargraph meters are hard to show actual values of current and voltage though. I have worked with LM3914s before and they are good for RELATIVE voltage/current, but not PRECISE. Bargraphs are great for tuning for maximum smoke. The more LEDs lit, the better. It would work for the HV, showing most of the LEDs lit shows there is PLENTY of HV there. If suddenly NONE were lit, that would show the HV is KAPUT! Since this is being run off of 120VAC instead of 220, the HV is going to be low anyway. The meter I have is a 3000V full scale meter, so 2400V will ready about 80% up the scale. I could make the Final Current 0-1A, Grid current is 0-100mA
    and relative output 0-1000W. I probably need isolated power supplies for each of the LM-3914s as some of those are reverse polarity.

    So I will work on that now before I cut up the panel. 4 separate bar graph meters. That would look alot prettier than the old Wiggle type meter movements.
  3. KF5FEI

    KF5FEI Ham Member QRZ Page

    You could use a 4x20 character LCD and have 4 bar graphs with labels -- just use sample your voltages with an A-D convertor on a microcontroller and display them. I'm doing this on my remote display on the 817 for the RF out and S-meter.
  4. N2EY

    N2EY Premium Subscriber QRZ Page

    Stacking the meters vertically is a good idea. The rest isn't.

    All you really need to measure are grid current, plate current and output. Since you are going to use an external SWR bridge, the output function is taken care of. As explained before, HV measurement is only provided because it was required in the old days when FCC went by input power (for amateurs).

    If you really want more metering, a new rotary switch or a couple of toggles can do the job. The plate meter can measure either plate current or HV at the flip of one switch while the grid meter can measure grid current or relative output at the flip of another.

    Bargraphs and such just make the thing more complicated than it needs to be. Power supplies for a meter? Sheesh! Plus if RF gets into them it's even more of a mess.

    Focus on the basics of getting the thing working.

    73 de Jim, N2EY
  5. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    Working on calculations...
    The SB-200 meter is 200uA at 1400 ohms. That comes out to .28V Full scale. OH OH! Might need a very linear OP AMP circuit to drive the Bar Graph display to atleast 10 times that. 2.8V full scale would work nicely for the LM3914. I am not familiar with how to program a LCD Bargraph display with a microcontroller. That is way beyond my technical abilities and I heartily salute those of U who can do that. I am still set back in the TTL and CMOS IC days.

    For HV, if I ran 3 ten segment displays, that would be 3000V full scale for 30 segments, or 100V per segment.

    For 1A plate current full scale, scaling is more difficult. 30 segments would make 1.2A easier, but the last third would never light. In reality, you probably would never see over 900mA in plate current, and if you did, the tubes would be ON FIRE!!! So think I will make it a 900mA full scale instead! Most of the time, Plate current is usually around 500mA at most.

    For Grid Current, the White area on the meter is 0-100mA, but the meter goes to 200 mA. Maybe I will make that 150mA full scale for the 3 bar graphs (30 segments) and the "white" area would be the first 20 segments.

    For the Relative power, since it is relative, 30 segments are all relative and the max can be set to 900W

    One thing you can do with LM3914s is when a certain segment is reached, U can make it flash the whole display. That is cool, so, for example, when I exceed 100mA grid current, the display flashes indicating I am driving it too hard. And when the Plate current exceeds 700mA, I am in danger of MELTING it down!

    Wiring up 120 segments will be a DAUNTING task, and use up lotsa ribbon cable. The Control board will have to hold 12 LM3914s plus all the support ICs, power supplies for isolation, op amps and such.


    You are right, Jim. This is WAY WAY complicated. But it SURE would be pretty!
  6. N2EY

    N2EY Premium Subscriber QRZ Page

    It is too complicated, Pat. Too many chances for a wiring error, programming mistake, RF in the electronics, etc.

    What you need to do is to look at the GOOD, WORKING ANALOG meters that you have on hand, and figure out how to use them in SIMPLE, EFFECTIVE metering circuits in the "SB-206".

    You are getting distracted by things that don't matter and forgetting about things that do matter - if the goal is to actually produce a working SB-206.(Sometimes I wonder about that).

    What meters and switches that are good and working do you have? Do you need help with the circuitry and calculations?

    73 de Jim, N2EY
  7. W1QJ

    W1QJ Ham Member QRZ Page

    I think the simple thing to do would be to install a good original meter. I have them in stock, then move on with the intended project of getting the amp on 6 meters.
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