attn: Anthony, re: SEX amp circuit operation
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| Posted on October 4, 2009 at 19:29:32 | |
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Posts: 7296
Location: Seattle, WA Joined: April 23, 2001 |
The thread scrolled off the screen so I'm bumping it up to a new one. I said "...This creates a voltage below ground, which biases the grid of the output triode to a negative voltage." and you asked "Is this because the high negative voltage below Rg yields a positive voltage at the Cc-Rg-Rstop node which then gives you a negative bias at the second tube's grid because of Rstop?" The grid is connected to the "-bias" node through Rstop and Rg (diagram in the manual). Since the grid draws no current, and that resistor chain is isolated from the driver by Cc, the grid DC voltage is the same as "-bias". Now you have to look at the power supply at the same time; you will see that all the power supply current goes through Rbias. Most of it is the output triode current, about 27mA if I recall correctly. That current, through that resistor places "ground" about 18 volts more positive than "-bias". Thus the grid is about 18 volts below the cathode which is grounded. "...does Cpf stand for parafeed capacitor?" Yes, I tried to label the components with descriptive names. Generally "cathode bias" means the cathode is raised above the grid which is grounded, so my use of the term is not precise. Another meaning is that the cathode current generates the bias voltage (through a resistor usually; sometimes we use an LED for another example) - this is the way I used it. Sorry for the confusion! Anyhow, if the cathode current is too high, the resistor Rbias will drop more voltage, placing a more negative voltage on the grid and counteracting the excess current. That is the negative feedback action. Notice that Rbias is shunted with a large capacitance; that makes the negative feedback ineffective at audio frequencies; it only works at DC. Hope that helps!
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