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  • help solving an HF radio amps issues

    From Gary Shaddick@21:1/5 to All on Mon Sep 11 11:19:48 2023
    A fellow ham operator is having a problem: every time he transmits with
    an amplifier in place, he either blows the amplifier fuses and/or
    destroys the tubes if using the tube unit he has. He has been keeping
    drive to the amp as recommended and even has tried low drive first, but
    once he tries full output on either amp, they go out. He's tried
    different antennas but to no avail. He can run the radio by itself at
    200 W into any of his antennas with no issues whatsoever (the amp is 500
    W). His SWR's have been normal, even at the time the amps stop working.
    Notice I said amps because the same behavior happens on both of the two
    amps he owns.

    I've suggested possible RFI issues somehow causing the issue, but this
    is just a guess. I'm just as perplexed as he is.

    Any thoughts you have would be welcome and I could run suggestions by him.

    Thanks in advance,
    Gary

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  • From Peter W.@21:1/5 to All on Mon Sep 11 12:05:08 2023
    I've suggested possible RFI issues somehow causing the issue, but this
    is just a guess. I'm just as perplexed as he is.

    I am going to offer a solution based on an audio model as it seems to cover both your conditions. But, first, let me reframe the conditions and results to make sure I am understanding it correctly.

    Condition A: When he modulates rF into an rF amplifier at a low level, all is well.

    Condition B: When he modulates rF into an rF amplifier at full output, he either blows the fuse (solid-state amp), or fries the output tubes (tube amp):

    If that is a correct understanding here is a potential diagnosis - and suggestion:

    Under Condition A, the secondary amplifier is not modulating anywhere near 500 watts. And so is not under any stress. Just as I have a 250wpc stereo amp at home being fed from a pre-amp with roughly 2 volt maximum output, but running, typically, at about
    0.02 watts. Peak-to-average in this case is 20 dB. The power-amp may run the very rare 200-watt peak, but most of the time it is running at far less.

    Under Condition B, the system is modulating at the "FULL" 500 watts. Were my power-amp to modulate at its maximum outlet, even briefly (more than a few seconds), it would be pulling very nearly 30 amps at the wallplate - i.e.: Blowing fuses. My 75wpc
    tube amp would handle it somewhat better, but were it to be expected to do it for over those few seconds, it would red-plate the tubes in short order. Given its class, what it draws at full output and quiescent is not substantially different - lots of
    waste in tube designs, all those filaments, so it would red-plate before blowing a fuse - they exist to protect the transformers.

    So, I think your friend has a fundamental misunderstanding (if my contentions and interpretations are correct) on how amplifiers work. They have a rating - which is what they can deliver cleanly for some period of time at a certain input level. But they
    are not (usually) designed to operate at full peak modulation, all the time.

    Peter Wieck
    Melrose Park, PA

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  • From legg@21:1/5 to All on Tue Sep 12 07:55:55 2023
    On Mon, 11 Sep 2023 11:19:48 -0400, Gary Shaddick <none@nowhere.net>
    wrote:

    A fellow ham operator is having a problem: every time he transmits with
    an amplifier in place, he either blows the amplifier fuses and/or
    destroys the tubes if using the tube unit he has. He has been keeping
    drive to the amp as recommended and even has tried low drive first, but
    once he tries full output on either amp, they go out. He's tried
    different antennas but to no avail. He can run the radio by itself at
    200 W into any of his antennas with no issues whatsoever (the amp is 500
    W). His SWR's have been normal, even at the time the amps stop working. >Notice I said amps because the same behavior happens on both of the two
    amps he owns.

    I've suggested possible RFI issues somehow causing the issue, but this
    is just a guess. I'm just as perplexed as he is.

    Any thoughts you have would be welcome and I could run suggestions by him.

    Thanks in advance,
    Gary

    Feeder lines, connectors or lightning arrestors need replacing.

    RL

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  • From Tim R@21:1/5 to All on Wed Sep 13 04:24:17 2023
    He's run it into a dummy load, right?

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  • From legg@21:1/5 to timothy42bach@gmail.com on Wed Sep 13 10:56:41 2023
    On Wed, 13 Sep 2023 04:24:17 -0700 (PDT), Tim R
    <timothy42bach@gmail.com> wrote:

    He's run it into a dummy load, right?

    Didn't say it was at the end of the feedline.

    He views his 'antenna' as what's plugged into the
    transmitter.

    RL

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  • From Allodoxaphobia@21:1/5 to legg on Thu Sep 14 11:48:00 2023
    On Wed, 13 Sep 2023 10:56:41 -0400, legg wrote:
    On Wed, 13 Sep 2023 04:24:17 -0700 (PDT), Tim R wrote:

    He's run it into a dummy load, right?

    Didn't say it was at the end of the feedline.

    He views his 'antenna' as what's plugged into the
    transmitter.

    He has a license?!?!?!?!?!!

    Jonesy
    --
    Marvin L Jones | Marvin | W3DHJ.net | linux
    38.238N 104.547W | @ jonz.net | Jonesy | FreeBSD
    * Killfiling google & XXXXbanter.com: jonz.net/ng.htm

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  • From legg@21:1/5 to All on Fri Sep 15 11:59:38 2023
    On 14 Sep 2023 11:48:00 GMT, Allodoxaphobia <trepidation@example.net>
    wrote:

    On Wed, 13 Sep 2023 10:56:41 -0400, legg wrote:
    On Wed, 13 Sep 2023 04:24:17 -0700 (PDT), Tim R wrote:

    He's run it into a dummy load, right?

    Didn't say it was at the end of the feedline.

    He views his 'antenna' as what's plugged into the
    transmitter.

    He has a license?!?!?!?!?!!

    Jonesy

    None of his issues involves a license, nor would one
    solve them.

    RL (VE3-UTE)

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  • From Tim R@21:1/5 to legg on Sat Sep 16 05:54:58 2023
    On Friday, September 15, 2023 at 11:59:57 AM UTC-4, legg wrote:

    He has a license?!?!?!?!?!!

    Jonesy
    None of his issues involves a license, nor would one
    solve them.

    RL (VE3-UTE)

    My guess, it's really a CB.

    Yes I know, the FCC rules say:

    Citizens Band Radio Service (CB)

    CB service allows two-way radio communications.
    CB service operates on 40 shared channels on a "take-turns" basis, meaning no CB channel is assigned to any specific individual or organization. Users must never talk with another station for more than 5 minutes continuously, and must wait at least one
    minute before starting another communication on the same channel.
    CB equipment used in the United States must be FCC-certified and labeled as such by the manufacturer.
    You may not raise the power output of your CB unit, attach any type of power amplifier or modify the unit internally. The maximum authorized power levels vary depending on whether the station is transmitting a single side band (up to twelve watts Peak
    Envelope Power or PEP) or an AM signal (up to four watts PEP).

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