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Asymmetric modulation in AM

S

Studio1

Had a colleague recently recount to me a situation where a station he looks after has a 5kW AM TX.

The modulation became asymmetric - ie minus 95%, plus 70%.

He checked the modulator out and it was perfect.

Replacing the final (bottle) cured the problem.

Can anyone shed some light on what would cause this?

My valve theory is limited and doesn't extend to weird situations of this nature!
 
Those positive peaks are the first to go when a tube gets soft. In the old plate-modulated rigs, having good modulator tubes was more important than having good finals. You could stick a weak mod tube in the final position and get a couple months more out of it if you were lucky.
 
Studio1 said:
Had a colleague recently recount to me a situation where a station he looks after has a 5kW AM TX.

The modulation became asymmetric - ie minus 95%, plus 70%.

He checked the modulator out and it was perfect.

Replacing the final (bottle) cured the problem.

Can anyone shed some light on what would cause this?

My valve theory is limited and doesn't extend to weird situations of this nature!
Click to expand...

Had similar issue with a CCA xmtr back in the 70s (but then who didnt?)......Since it has a tube, I woudl assume the transmitter is high level modulated (using plate modulation or some other form in the final PA stage).....as the tube gets soft (old), the modulation as well as power output will fall off.....usually the bias/screen voltage to the tube can be played with to compensate for it (the bias will affect the modulation.....it sets the idle point of the tube when there is zero modulation...much like a bias voltage on an op amp's positive input for single supply use..its sets the output DC level used for the Zero AC level in single ended usage).

The mod section will be putting out +/- audio swings just fine...but if the tube is not biased correctly or its getting soft, the zero mod point is changed and you end up with screwy modulation like your friend did (My CCA 5KW was doing 99% neg, 85% max pos.....changing the tubes didnt help...we finally tore it apart one weekend with the 250w aux on the air and redid the bias stage completely....sure enough, she came back up to 100% pos...trying to asym the xmtr was somewhat impossible though....JUST to do 100% mod meant pushing the audio stages, a pair of 4-1000s, past their max rated plate rating!!! Forget 125%)
 
In an old 1kw RCA transmitter that was at a station that I was at - it used 4-400C (pair) for the modulator and the same for the finals. Common to put the used modulators over tot he finals and the new tubes in the modulators. If you had a good schedule of separation between the 2 sets you got improved tube life and kept the sound great.
 
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