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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