[AMRadio] Modulation transformer ID and repair

[Donald Chester]

>>While on the subject, in the circuit where there is no ground connection,
>the audio return path is through the power supply filter capacitor.  This 
>is
>effectively reducing the amount of capacitance since the coupling capacitor
>and the power supply filter capacitor are now in series with the audio 
>path.
>This is OK if the power supply filter has enough capacitance. It even has
>one advantage, the need for a very HV capacitor as a coupler is reduced to
>the DC voltage drop across the modulation reactor.
>
>In the circuit of Don and mine where the audio path is to ground, there is 
>a
>slight reduction in hum from the power supply as well.

Another factor often overlooked, especially when the HV filter capacitance 
is marginal and a common power supply is used, is second harmonic distortion 
caused by loading of the power supply by the modulator.  Over each audio  
cycle, the class B modulator will pull current as each modulator tube's 
plate current is maximum, but will pull zero current as the audio cycle 
passes through baseline and each modulator tube is drawing only static plate 
current. That means that over each audio cycle there are two times that the 
power supply is loaded, at both the positive and negative modulation peak.  
This pulsating load will introduce a second harmonic component at the power 
supply output.

If the audio is returned directly to ground, this 2nd harmonic component, 
along with most of the residiual power supply hum, will be filtered out by 
the modulation reactor. If the audio is returned back to the power supply, 
this 2nd harmonic component, plus any hum at the power supply output, will 
appear as modulation voltage to the final.

Don K4KYV

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