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.
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.
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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