What all is included in the DC load current a transformer sees? - diyAudio
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Old 12th April 2013, 01:47 AM   #1
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Default What all is included in the DC load current a transformer sees?

That's the question.

Not including heater currents.

My understanding- could be way off, I'm sure- anode DC currents and screen currents for all tubes, as you see on the datasheet graphs. This would seem to be the bias point values at quiescence. As the anode voltage approaches 0, the screen current increases. As the bias point shifts, either due to cathode bias, or overdrive conditions forcing it towards the center, the anode DC current changes.

Basically it seems to be the things the power supply is supplying- not any of the AC action.

?
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Old 12th April 2013, 02:17 AM   #2
gootee is offline gootee  United States
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Unless you also have another power supply, your power supply supplies everything except the input signal. And the input signal usually supplies almost no power.

In a power amplifier, the small input signal only controls the "current valve", whether it's a tube or a transistor. The control signal can be thought of as changing the resistance of the device's main channel, allowing a larger copy of the input signal to be created, as current, which flows through the main channel of the device. (And the current almost all comes from the power supply reservoir capacitors.)

So yeah, the "AC action" comes straight from the power supply. But it's always as CURRENT. We try to hold the supply's voltage constant, so that the same change in channel resistance called for by the input signal will always produce the same change in the output current. That's "linearity". But often we can come up with a circuit topology that will compensate for supply voltage changes, meaning that they have some PSR (power supply rejection).
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Old 12th April 2013, 11:43 PM   #3
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Thank you, that helps my understanding quite a bit.

How do I go about calculating the DC load of a super simple valve amp?

I have load line charts of al the valves, operating points and all that, and a pretty clear idea of maximum current values as indicated on the charts. How do I translate all that into a solid number I can use to choose capacitors etc?

Thank you!
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