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In Reply to: RE: I don't ...RE: I Think You're Good to Go... posted by Tre' on September 10, 2014 at 07:03:28
"The grid current is not being sourced by the bias supply but instead is being sinked through it?"
Not necessarily. Here's another explanation, and I apologize if it's too simplistic. I'm just trying to lay out the whole concept from A to Z...
To set this up, the secondary of the IT swings voltage at its upper terminal referenced to ground. In order to do that, its lower terminal must be held as closely to AC ground as possible. We can't actually ground it, because that would short the negative bias needed by the grid. So, a bypass capacitor is connected from the bottom terminal to ground.
Here's what happens when this arrangement is driven in A2...
When the audio signal at the upper terminal of the transformer swings more positive than the grid bias, electrons are drawn from the grid. They flow down through the transformer secondary winding toward AC ground. The electrons are pulsed at an audio frequency, because in this design, the grid only draws current on positive-going peaks. The current pulses flow out of the secondary at the bottom terminal and through the bypass capacitor to ground. That's the complete path for grid current if the bias supply is isolated with a series rectifying diode, as most are. 10 ma of grid current creates exactly 10mA of current through the capacitor.
The problem with this arrangement is that the pulsed electrons entering the capacitor cause the average DC potential on the capacitor to become more negative. In other words, when grid current is drawn, the capacitor immediately becomes more negative than the bias supply that charged it up in the first place. If there is no DC path to ground at this point in the circuit, the capacitor will continue to charge as long as grid current is drawn. It won't stop until the capacitor and the grid become so negative that grid current no longer flows.
What's needed is a way to prevent the DC voltage on the bypass cap from increasing. That can be done in several ways, including a Zener of exactly the right voltage (as Paul noted), a bias supply that drains off the excess voltage by sinking it to ground, or even a resistor. If the purpose of the amplifier's A2 capability is only to handle musical peaks without clipping, and if those peaks push it into A2 only occasionally, the resistor will work just fine. The capacitor will charge up slightly when a peak draws grid current, but the resistor can be selected to discharge it in time for the next peak. Of course, this requires that the bias supply provide more current, because it will see the resistor as an additional load.
We could look at this from the perspective that all these techniques actually fall under the role of the bias supply. However, I think it's important to realize that DC stabilization of the capacitor when grid current is drawn is really a separate function. If a DIYer has a bias supply capable of providing 1 mA of negative current (current that charges the cap to the desired negative potential), it can be used in A2 as well as A1. We only need to make provision to drain the excess average voltage off the capacitor when grid current is drawn. Adding an additional circuit to do that might be a better alternative than scrapping the entire bias supply.
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Buy Chinese. Bury freedom.
Follow Ups:
"I apologize if it's too simplistic."Simplistic works for me!
So if we had a shunt regulated bias supply, with the shunt component being the last element of the supply and fully capable of shunting that kind of current, then we're good?
And (conceptually) it would only be the shunt component that would need to be rated for the higher current, not the supply itself?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/10/14
"And (conceptually) it would only be the shunt component that would need to be rated for the higher current, not the supply itself?"
Exactly right. I'd say you're down with this one! :)
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Buy Chinese. Bury freedom.
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