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I can't identify this resistor!!!!!!!!!

Discussion in 'Amateur Radio Amplifiers' started by KM6CND, Aug 26, 2017.

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  1. KM6CND

    KM6CND Ham Member QRZ Page

    To all who helped me ID this resistor, thanks a bunch! :cool:

    I went to my surplus electronics store, all the resistors were on display were not, non inductive, so the clerk goes to an old box and pulled out 2, old carbon 62 ohm, 2 watt resistors for 50 cents each. I installed the resistor and my amp is running just fine.

    I have less than one hour of talk time on this tube. If it needs any "care and feeding", please let me know.
    WA7PRC and N2EY like this.
  2. N2EY

    N2EY Ham Member QRZ Page

    The power supply filter capacitors, for one.....

    Well, you just got a very expensive lesson....

    Here are some data sheets for the 3-500Z. Note that the grid dissipation rating is only 20 watts. More important, note that for full output, a single 3-500Z needs only 30 watts of drive in grounded grid.

    Anybody who tells you that your amp needs more than that does not know what they are talking about.

    73 es GL de Jim, N2EY

    Attached Files:

    N2SR and WA7PRC like this.
  3. KM6CND

    KM6CND Ham Member QRZ Page

    Duly noted. Thanks for the info.

    N2EY likes this.
  4. N2EY

    N2EY Ham Member QRZ Page

    No prob. Just trying to help you avoid the really bad advice that sometimes is given online.

    You were given an amp with no manual and no tuning instructions, so a mistake was a real possibility.

    You'd be surprised how many hams I have seen, with much more "experience" than you, with amplifiers that come complete with manuals and instructions, proceed to blow them up by not doing what the manufacturer recommends.

    That amplifier does not have a tuned input circuit. This means the input impedance is likely to not be 50 ohms. Not a problem with some rigs; an issue with others.

    73 es GL de Jim, N2EY
    N2SR and WA7PRC like this.
  5. K6BSU

    K6BSU Ham Member QRZ Page

    Just a few months back, I posted an identical problem with the parasitic suppressors for my home brew amp.

    Pretty sure I knew the answer, but got some opinions just to make sure.

    The parasitic resistors were burning up, indicating VHF oscillations.

    Finally, I ordered some 100 Ohm 2 Watt A-B carbon comp resistors from an E-Bay seller. cost a lot, but they worked just fine.

    Attached Files:

  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    There're more reasons. Tom W8JI s'plains other reasons it's a good idea to use a tuned circuit on the input of a grounded grid amplifier:
    That seller would be bigsmythe74 ( He offers quite a selection of NOS Allen-Bradley CC resistors.
    The resistors in your photo are brn-red-brn = 120Ω
    Last edited: Aug 27, 2017
    N2EY likes this.
  7. K8ERV

    K8ERV Ham Member QRZ Page

    Luvit when a plan comes together

    TOM K8ERV Montrose Colo
  8. W5THT

    W5THT Ham Member QRZ Page

    If the parasitic suppressor blew up then you have a parasitic oscillation in the amp in the VHF frequency range.
    This will kill the tubes. At HF the small amount of inductance across the resistor should not drop enough RF voltage to burn the resistor.
    Get the manual on the amp and look at the pictures. Someone may have moved or left loose a ground connection or component that allowed the VHF oscillation to start. The suppressor resistor is to reduce the gain of the stage on VHF frequencies, If someone set up the overall gain of the amp, then it is time for restoring the original circuit. Check that you do not have a VHF oscillation in the exciter. Good Luck.
    Pat W5THT
    K1ZJH likes this.
  9. KM6CND

    KM6CND Ham Member QRZ Page

    Pat, thanks for the reply. I have about given up on this amp. I did replace the resistor and fired up the filament and it was in the 5 volt operating range. I have a new tube in it and thought all was well... no so.

    The amp (when it worked) had very little operating hours, maybe 4-5 hrs monthly with short TX's of generally less than a minute. Then I discovered FT8 and the amp was on for 30 seconds and off for 30 seconds, with no more than 250 watts as that's all the antenna was rated for. I had logged about 100 digital contacts before the failure. I was running an FT8 session and I looked at the amp at just the right time, to see the left side meter SLOWLY crawl up the scale. I was not sure if I saw what I saw, so it cycled again and that's when the amp would key up and ZERO watts came out. This was on 40. Not knowing any better, I moved to 20 and after a couple cycles, 20 went to zero. I have not had it back on since. Oh and BTW, I heard a sizzle sound just before the failure on 40.

    Many have questioned the schematic and I can't be sure it's the final version of the drawing. Without question, the SK that built this amp knew exactly what he was doing.

    Any ideas - suggestions are welcome.

  10. K8ERV

    K8ERV Ham Member QRZ Page

    Luvit when a plan comes together
    TOM K8ERV Montrose Colo

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