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SB-220 HV and line voltage Concern

Discussion in 'Amateur Radio Amplifiers' started by N8PWM, Aug 19, 2016.

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  1. N2EY

    N2EY Premium Subscriber QRZ Page

    That's a good no-mod way to do it. Also permits you to take the amp offline for signal comparisons easily.

    73 es GL de Jim, N2EY
     
  2. N8PWM

    N8PWM XML Subscriber QRZ Page

    Hi Jim. All put together. I may lengthen the cables but it's ready to go. Taking a break from canning but will test it tonight.
     
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    Even with a "stiff" AC source, you can still have series loss. In the SB-220, it's the HV transformer... loss in the magnetic coupling between primary and secondary, and resistance losses in the wire itself. The variation in line voltage between no load and full load will tell you where the loss is.

    When it comes to replacement electrolytics, the best you can reasonably do is the CDE 381LR series (datasheet). You should NOT go crazy with the capacitance... 270μF is plenty (OEM was 200μF) and will not require a step-start circuit. Then, 270μF/450V will be pn 381LR271M450A032 (35mm dia x 35mm L). Digi-Key has them in stock for $5.98 each (#338-2283-ND).

    While you're replacing the electrolytic capacitors, the 30K/7W wirewound resistors should be replaced with something that won't cook the heat-sensitive electrolytic capacitors. Since most modern electrolytics are much more consistent in value, you can use 100K/3W MOX types. Vishay #PR03000201003JAC00 (datasheet) fits that requirement. Digi-Key has them on hand for $0.68 each (#PPC100KW-3JCT-ND).

    A common failure is the carbon composition HV metering resistors (R6-R8, 4M7/1W). They absorb moisture and then soar in value. They should be replaced with types that are rated for the VOLTAGE. Vishay pn VR68000004704JAC00 (datasheet) fits that requirement. Digi-Key has them on hand for $0.78 each (#VR68J4.7MCT-ND).

    In the event of a tube arc, the meters are at stake. It's a very good idea to add protection. A 3A (or greater) rectifier (anode to ground) from -B (terminal 'B' on the rectifier/metering PCB) does that. See http://www.w8ji.com/metering_amplifier.htm.

    vy 73,
    Bryan WA7PRC
     
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  4. N8PWM

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    Hi Bryan. Thank you for the information. I am planning on continuing with the reconditioning so this is good information.

    UH OH HOUSTON WE HAVE A PROBLEM

    I am now seeing a strange issue. When I load up on 40 meters. I can get full power out (yes still on CW/Tune) and all is well. I have been getting top notch signal reports. When I load up on 20 meters, it tunes fine and as I increases the drive and retune, I hit a point where all of a sudden I hit the key and there is a pretty blue arc that appears to be coming from the area of the tune variable cap. I can't really tell because I am very cautious and immediately unkey. This happened to me yesterday and today when tuning up on 20. It does not appear to be effecting the amp any as I am able to back off the drive and retune and then it will load up fine and work perfectly.

    Note that this does not occur until I am driving with about 75-80 watts.

    I am at a loss. I thought that it might be parasitic oscillation, but that would immediately kill the tubes. They do have a strange cap resistor wound with heavy wire like an external coil around them and they look nothing like what the manual shows. I can only assume that they are a parasitic oscillation suppressor.

    I cannot explain this arcing, but visual inspection shows no burn marks anywhere. Anyone have an idea? I am wondering if the cap plates might be dirty and when I increase the drive it starts to arc on the dust/dirt? Since it is not doing it on 40 meters, that the only thing I can think of but I will leave it to you amplifier experts!
     
  5. W1QJ

    W1QJ Ham Member QRZ Page

    If the plate tune capacitor is arcing and making a spark which can appear blue, it will ALSO present a hissing or spitting sound as well. If you do not hear any sound nd you see a blueish arc, it can be one of the tubes that is gassing out and going plasma. If that is the case then that tube is mildly gassy. The thing to do in that case is to operate the amp in the CW mode and run as much power as possible and get the plate of the tubes red for a while. Use the amp like that for several hours. Hopefully the tube will getter the gas that is in there. However, if you do hear a sound that happens at the same time, then it is probably the tune cap arcing which means you are too lightly loaded.
     
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  6. N8PWM

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    I am hearing the hissing and snapping. That being said please explain being too lighty loaded in relation to tuning the amp.
     
  7. N8PWM

    N8PWM XML Subscriber QRZ Page

    Ok I went back through the thread and it was suggested that once the amp is tuned, that I should advance the load control until the power reduces by about 50 watts. I assume that means heavier loading. I understand that but this is happening as I am increasing drive. How can I deal with this when tuning so that I am able to increase the drive without the arcing?
     
  8. W1QJ

    W1QJ Ham Member QRZ Page

    Ok, most likely you have one OR MORE plates on the tune cap that are "bubbled" already. Once the plates get bubbled it is much easier for them to arc. At this point you should have the plates replaced with new ones or good ones. Once you get those bd plates replaced you then hve to start fresh on properly loading the amp. I run an exchange program on tune caps. You can contact me off list for info on that. You also have to look at your parasitic suppressors and make sure you have the correct ones and they are working correctly. Once all this is done you start fresh with properly loding to avoid a re-occurrence. There are ways to avoid arcing at the tune cap. The major way is to have presets on the load and tune or pre-load the load cap before re-tuning after increasing drive.
     
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    I may have missed it... did you remove/disconnect the added variable capacitors?
     
  10. N8PWM

    N8PWM XML Subscriber QRZ Page

    Hi and thank you for the info. I will contact you off list about the tuning cap replacement.
     

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