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Author Topic: What happens when overmodulation occurs?  (Read 1788 times)
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K8DI
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« on: October 15, 2019, 05:57:47 PM »

Trying to learn a little before I break things....

Not caring at all about the rf output...assume a dummy load and a Faraday cage.

Considering plate modulation with a transformer, class C, one tube (or parallel tubes).

What happens when you modulate 150% negative, in terms of the tube, and the transformer? Anecdotally, many writers here and elsewhere talk of blowing up transformers.  Why? How?

If I want to play with various negative peak limiters, which means heavily overmodulating a transmitter in testing, will something break? What, and why?

Obviously, negative (or very low) plate voltage will stop the tube conduction. Will it conduct backwards if sufficiently negative?  What happens in the modulation transformer? Why is it bad? Again, not talking about the spectral issues or how it sounds, talking about blowing stuff up....

Ed

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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
KK4YY
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Your best isn't as good as you can be.


« Reply #1 on: October 15, 2019, 07:00:12 PM »

Here's my explanation (I'm sure others can do better at this that I can, but here goes):

When the plate voltage of the class C RF amp is driven negative (below zero) it will no longer draw current and thus provides no load to the modulator. With no load present at the output of the modulation transformer, no current can flow and the voltage will rise very sharply. This very high voltage can cause the insulation of the transformer to break down - ruining the transformer.

The 3 diode limiter seeks to provide a proper (matched) load under those conditions which will draw current and prevent the voltage from soaring. Alternately, a spark-gap will limit the voltage rise to the point at which an arc occurs and provide a current path. Other protection methods have been developed, but these two are probably the most common.

I use the 3 diode method, myself.


Don
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All your worries won't add a day to your life, or make the ones you have any happier.
kb2vxa
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I modulate, therefore AM


« Reply #2 on: October 24, 2019, 09:59:33 AM »

You can take your cue from Leo Meyerson W0GFQ and family when they designed and produced one of the best AM transmitters I ever operated, the Globe Champion 300A. The beauty of this beast was the built in compressor circuit that when fine tuned kept the modulation at about 95% (I like headroom) with sufficient density to have considerable talk power. A great addition to any transmitter is a home-brew heart of a Gates Sta Level (schematic available on the Internet) compressor.

My friend who bought the Globe at a hamfester cleaned it up and before seriously operating with it discovered it needed some new tubes. No problem back in the 60s and 70s when tubes were easy to come by at reasonable prices, but the 809 modulators had almost become unobtanium. After much searching he found a source and had them imported from the UK.

Whatever you do don't forget the heart of every ham shack, a MUST is a modulation monitor. An O scope with a common 2.5mH RF choke across the V input to bypass 60Hz hum, the gnd terminal earthed, and a hank of wire on the hot terminal works fine. One is essential for adjusting and monitoring modulation, over modulation causes distortion resulting in splatter, hams operating next to you won't appreciate it.

As a footnote, a while back the "wide band sideband" thing started gaining ground, SSB Gangstas? That's a big no no and the FCC let them know it. Lesson learned: You don't have to transmit telephonium audio as Timtron calls it to have a nice sounding signal within legal limits.
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73 de Warren KB2VXA
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