SB200 Issues & Questions

Discussion in 'Amateur Radio Amplifiers' started by WB9EGG, Nov 30, 2021.

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  1. WB9EGG

    WB9EGG Ham Member QRZ Page

    Yes, I have been doing that - my internal Yaesu tuner doesn't like it very much - I may experiment by reinserting the MAT-TUNE mAT-30 tuner in between the Yaesu and the SB200 - that may work better...WB9EGG
     
  2. WB9EGG

    WB9EGG Ham Member QRZ Page

    I am looking at the SB200 manual on page 33 - are you referring to the .001 ufd 3 KV disc capacitor? One end is connected to the the RF choke that is on the load capacitor...the other end is connected to those two resistors which connect to the anodes of the two tubes...
     
  3. W1QJ

    W1QJ Ham Member QRZ Page

    The Sb200 as originally designed should give you damn near a flat input swr even with fussy SS radios on 80-20meters. The slugs may need a bit of adjusting but it should be that good. If the swr can’t be adjusted down then chances are some mica caps gone out of spec. On 15 and 10 some playing around with values may needed or simple adjustments but that is on a case by case basis. There’s no reason to ever have to use an external tuner unless to finagle the WARC. A properly tuned input is a MAJOR ingredient to smooth operation. Not unlikely to get 700-800 watts output with a properly tuned amp.
     
  4. WB9EGG

    WB9EGG Ham Member QRZ Page

    Hi W1QJ, Thanks so much! Yes, like I mentioned before, I used to own one of these SB200s in the 1970's and I recall getting 800++ watts out quite easily! So you mean those small mica caps that are across the inductor coils in the input circuit? Our other friend on here suspected it might be more the HV Capacitor - that connects to the Load capacitor by the RF choke (and then goes to the resistors connecting to the anodes of the power tubes) - the reason being is when I turn my Load knob - not much happens - on all bands I have to have it all the way counterclockwise - it doesn't really change the power or interact with the Tuning knob that much...
     
  5. WA2CWA

    WA2CWA Ham Member QRZ Page

    With my SB-200 on 80 meters, the loading capacitor is maximum CCW no matter where on the band I operate. Been that way for as long as I can remember.
     
  6. WB9EGG

    WB9EGG Ham Member QRZ Page

    Does the Loading capacitor make any difference in tuning on other bands - or is it maximum CCW on all bands?? Ron
     
  7. WA2CWA

    WA2CWA Ham Member QRZ Page

    From memory: I think on 40 meters it's fairly high in CCW position. 20 to 10 it's fairly mid way. Let me also point out that I never do dip and load. I just look at the outboard forward power meter (not the one in the SB-200 which really sucks for accuracy) and just tune for maximum power output generally in the AM mode (generally about 25 watts drive) of the transceiver. Sometimes check and/or touch up power output quickly in the CW or FM mode.

    I always found the meter switch to be intermittent so generally just keep it in the HV position. This SB-200 was built roughly in 1969 and still has all the original parts, tubes, and probably even dust.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Note: Assuming the tubes are good and the plate tank works, having the LOAD control fully CCW is always an indication of insufficient drive power. Pretty much always.

    If you can drive the amp harder (more power arriving at the cathodes), the LOAD control will need to be advanced and won't be fully CCW.

    Sounds like your particular amp has an input tuning problem that does not allow sufficient grid current (drive power). If that is fixed, I'll bet you find the LOAD control works okay.
     
  9. W1QJ

    W1QJ Ham Member QRZ Page

    The cap at the bottom of the plate choke to ground isn’t your problem. That’s just a bypass cap. If the amplifier doesn’t make sufficient output power the load cap will always want to be at or near zero. On 80 and 40 you need to be up around 400 watts output before the load cap begins to lift off zero. It remains pretty low on 80 and 40 anyhow and the less power output the more it stays close to or at zero. It’s remotely possible the 80 meter padding cap is bad you can easily check it in circuit. The later model load caps, the ones that look like the tune cap generally have less effect on the loading than the older style, but doesn’t hinder output in the long run.
     
  10. WB9EGG

    WB9EGG Ham Member QRZ Page

    Thanks so much! Well, yes, I never have any grid current....I tried retuning the input inductance coils - but nothing happened - there was no change at all when I turned the lugs on any of the bands - I found that unusual. I have a Yaesu FT-DX10 that has a maximum output of 100 watts - which should be enough to drive the SB200 - I will, again, put a real watt meter between the Yaesu and the SB 200 to see how much power I actually am getting into the amp...
     

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