help solving an HF radio amps issues

Gary Shaddick

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Sep 11, 2023, 11:19:53 AM9/11/23

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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

Peter W.

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Sep 11, 2023, 3:05:12 PM9/11/23

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> 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

legg

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Sep 12, 2023, 7:56:14 AM9/12/23

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On Mon, 11 Sep 2023 11:19:48 -0400, Gary Shaddick <no...@nowhere.net>
wrote:

Feeder lines, connectors or lightning arrestors need replacing.

RL

Tim R

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Sep 13, 2023, 7:24:20 AM9/13/23

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

legg

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Sep 13, 2023, 10:57:00 AM9/13/23

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On Wed, 13 Sep 2023 04:24:17 -0700 (PDT), Tim R
<timoth...@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

Allodoxaphobia

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Sep 14, 2023, 7:48:05 AM9/14/23

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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.

legg

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Sep 15, 2023, 11:59:57 AM9/15/23

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On 14 Sep 2023 11:48:00 GMT, Allodoxaphobia <trepi...@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)

Tim R

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Sep 16, 2023, 8:55:02 AM9/16/23

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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).