2M VHF amp received audio distorted

Discussion in 'Amateur Radio Amplifiers' started by N4TWX, Feb 21, 2017.

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  1. N4TWX

    N4TWX Ham Member QRZ Page

    I was trying to describe the receive audio to my friend that owns the amp and to me it sounds as if he is talking through a fan was the best way I could describe the distortion. He tried a couple of different microphones and no change, but without the amp he sounds normal. I don't have many particulars on the amp but it will produce legal limit and is homebrew.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Is this on FM or SSB?

    If it's SSB (I'll assume that for now), how far away is he from your location and how strong is his signal?
  3. N4TWX

    N4TWX Ham Member QRZ Page

    90 miles and it is ssb. With a preamp on he is 10 over 9 with the amp on but if he decreases the drive the problem still exists.
  4. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Is this a new problem?
    Has the amplifier performed normally prior to this?

    There are a few things that could be wrong but to be certain, there needs to be more details.

    Do you have a schematic of the amplifier?
    How is it setup?
    What is the bias on the stages?

    Hope this helps
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Good questions.

    The part about the signal sounding like he's talking through a fan gives me a couple of "guesses" about what's wrong:

    1. The amp is squegging due to some instability or the tank circuit unable to fill in the RF cycle (like it's biased for Class C and then used on SSB);
    2. It might be 120 Hz hum from the power supply, modulating the signal due to inadequate power supply filtering/regulation.

    Those are just guesses, but I wouldn't be surprised if it's one or the other.
  6. WA4SIX

    WA4SIX Ham Member QRZ Page

    Also, as a guess, too small of a power supply could drag the voltage down to an unacceptable level.

  7. N4TWX

    N4TWX Ham Member QRZ Page

    O.K. thanks for the replies. He found something to do with the center tap on a grid bias transformer. He took it to a friend with more expertise than he and I have and they made a change, I think it was that the center tap was cut off and not used and they used it. Not sure, but I did listen to it afterwards from my location and it is much much better. Thanks again.
  8. W1QJ

    W1QJ Ham Member QRZ Page

    What tube(s) does this amp run?
  9. N4TWX

    N4TWX Ham Member QRZ Page

    Russian GS35B
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    How does he sound with your preamp off ?

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