SB-220 C4 Overheating

Discussion in 'Amateur Radio Amplifiers' started by NE6I, Oct 18, 2014.

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  1. AC0OB

    AC0OB Subscriber QRZ Page

    Or a 1 ohm Thick Film resistor with the present caps.

    if you feel it necessary to replace a choke with a choke, I would recommend the following choke:



    AC0OB - A Place where Thermionic Emitters Rule!
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    Nice work.

    Sometimes you can repair a coil, But what made it open ?

    Old age does not count. lol

    I think I would replaces all of the associated parts before you need a new power transformer.

    Or get a new can of smoke to add, just in case you need to add some back.

    Have fun.
  3. NE6I

    NE6I Ham Member QRZ Page

    Why the suggestion to ground the grid directly or use a resistor? Why is that better than the original design? P presume the choke is there to prevent some sort of RF or A/C off the grid.

    And again, huge thanks to all of you for helping me find the problem!
  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    The reasons it is better to directly ground the grids;

    1.The amplifier is more stable, adding extra inductance to the grid leads in the form of those extra parts lowers the overall stability of the amplfier.

    2.Grounding the grids directly lowers the drive requirements.

    Doing this has no negative impact on performance except for a couple of DB increased IMD which is already low enough.

    And by all means don't forget to put a 1 amp 250V fuse in series with the center tap of the filament and bias transformer filament winding. (the green and yellow wire)
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    In order to keep XL down, this is how I did that:
    The path from the socket terminal to chassis is
    as short as you're going to get. I used soft
    copper strip, cut with shears. It goes up & over
    the up-turned tab on the socket terminal and
    then down. Hardware is #4-40.
  6. AF6LJ

    AF6LJ Ham Member QRZ Page

    And that should work really well.
  7. AC0OB

    AC0OB Subscriber QRZ Page

    A good upgrade for sure!



    AC0OB - A Place where Thermionic Emitters Rule!
  8. W1QJ

    W1QJ Ham Member QRZ Page

    I have original 1MH chokes if anybody needs them.
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    One Megahenry? You'll need a forklift to move it. :p:rolleyes:
  10. KM1H

    KM1H Ham Member QRZ Page

    There are 2 schools of thought about those grids.

    If you ground them directly the gain and power go up with the same level of drive. To me that is asking for trouble in that amp. The choke saved your tube by acting as a fuse, if it was grounded and arced internally you likely would have several parts to replace, including the tube. Instability is most often due to the parasitic resistors going way up in value from decades of RF, voltage and heat. Stay FAR away from nichrome voodoo.

    Your choice but there are thousands of that amp still running as designed.

    Other 3-500 amps with direct grounded grids regularly have a tube arc and often the tube, bandswitch, zener, plate choke and maybe a few other parts go with it.

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