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In Reply to: Here is what's a bit puzzling about C minus supplies. posted by Michael Samra on July 10, 2016 at 13:24:06:
"If you put a scope on the negative bias supply rail while music is playing,you can see the audio there at the source riding on the filter caps. [snip] ...wouldn't it make sense to build the lowest impedance C minus rail that you can"I really want to answer in the affirmative, because this seems so intuitive. However, I think the question indicates a possible misconception about the cause and effect of the observation.
To analyze the cause, it's necessary to first consider the relationship between the C- filter cap and the audio signal at the power tube grid. Regardless of how that signal arrives at the grid, it's decoupled from the bias circuit through either a high value resistor (150K, 220K, etc.) or a high value of inductive reactance (choke or transformer). Both methods serve the same purpose - to isolate the signal on the grid from the impedance of the bias supply.
In the case of resistive decoupling, the effect can be reasonably well quantified. For example, let's say the bias supply filter includes a 47uF capacitor to ground, and the cap is connected to a 220K series resistor that delivers C- to the grid. If this RC is analyzed by applying a 100V P-P signal at 20 Hz (worst case frequency) to the resistor, only 56mV P-P appears across the cap. That's a reduction of more than 62dB, and that number doesn't include the additional attenuation created by any of the other bias supply components.
As a result of this analysis, I think it's very unlikely that grid signal comprises the majority of what you see on the scope. If it is, the simplest remedy would be additional C- capacitance. However, there's a subtler path for audio to the top of the C- filter, and it's more likely to be the cause of the effect. Specifically, when most power stages are modulated by signal, they in turn modulate the B+ supply. This creates voltage and current fluctuations that reflect back through the supply and into the power transformer. The transformer, in turn, modulates the C- supply voltage. This represents a source of audio that acts on the C- supply at a much lower impedance than the power tube grid, and for which additional filtering across the supply will likely be less effective.
So, the short answer to your question is that a C- supply exhibiting a lower impedance to ground will indeed reduce audio signal appearing on the C- filter cap. However, reducing the impedance between the cap and the power transformer probably won't. The latter is what (marginally) occurs when the supply is changed from half- to full-wave rectification.
Edits: 07/10/16 07/10/16
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- Bias Supply Z - Triode_Kingdom 15:27:15 07/10/16 (0)
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