Fender Champion 600 Preamp Bias Part 5

Now I'm finally going to get down to talking about biasing. Instead of giving a wordy definition of bias I'm going to try to focus on some details of the circuit that will give you an intuitive sense of what bias is and why we need it. Once you have a grasp of those the textbook definitions should begin to make more sense.


Part 4 of the preamp bias posts showed where to connect your meter leads to measure the B+ voltage, plate voltage and grid voltage in the first 12AX7 gain stage in a Champion 600. Those measurements will all come into play later on. Right now we're going connect the meter in another way to read the cathode voltage.






Zooming in on the schematic, we see this label (which I've highlighted in blue):






It says +1.7 VDC / TP6. This means we should see a reading of positive 1.7 volts of DC at Test Point Six. The line extending from the label box indicates where the meter would be connected to take this reading.

All of the test point measurements (and in fact the majority of the measurements you'll take inside any amp) are "referenced to ground". What that means practically is that the point you connect the black lead of you meter should have no components between it and ground.

This is the symbol for ground on a schematic:






So looking back at the schematic above you'll see that the way our meter is connected is measuring the cathode voltage "referenced to ground".

Now back to our cathode resistor. Since a capacitor blocks DC current and it's a DC measurement we're concerned with we'll ignore the cap for the moment:






That leaves us with 1.7 volts measure across a 1.5K resistor. That means we have enough information to calculate the current through the resistor using Ohm's law.




1.7 volts divided by 1,500 ohms is .0011 amps - or 1.1 ma.

It's no mistake that this 1.7 volts at the grid and 1.1 ma of current fall right at point C on the graph of plate curves we've been looking at:






The circuit designer chose point C to be the bias point. Given the load line they had to choose the 1.5K resistor to make that happen. The next post will explain why.

Fender Champion 600 Preamp Bias Part 4

In the last post we left off with three points whose combination of current and voltage fell along the load line:






This whole load line business gets a bit abstract and that can make it hard to think about it in terms of the actual circuit. To try to bring it back to earth a bit (or at least back to the schematic) here's a picture illustrating where that plate voltage appears in the circuit*:






If you were to connect your meter's red and black leads to the points indicated in the schematic the voltage you'd be reading on the meter would be the voltage that appears on the X axis of the plate curves graph we've been working with. We'll see later that this voltages changes a great deal depending on the bias point and the nature of the signal that is connected to the input. The fact that this voltage changes according to the input is exactly what makes the tube function as an amplifier.

Move the red lead to the other side of the plate resistor and you'd be reading the B+ voltage. This voltage should remain pretty much constant with signal applied or without:






Now move both leads and you'll be reading the grid voltage:






The grid voltage is indicated on the plate curves as well. Here is the graph with the load line again. Have a look at the top end of the curves - you'll see a voltage label on each of them:



By following the grid voltage curve to it's intersection with the load line, you can see what grid voltage (-0.5 volts) that corresponds to the plate voltage and current we've chosen at point A:






Since the grid voltage curves are pretty widely space you have to guesstimate when the points on the load line fall in between curves. I've done that here for points B and C.






Using these points and the three values associated with each of them we can calculate the value of the cathode resistor in order to set the bias. We can also use them to calculate the gain of the stage. We'll do that in the next post.




* This is the plate voltage relative to ground. There are times when you need to measure the plate voltage relative to the cathode but this measurement will work fine for what we're concerned with here.

Fender Champion 600 Preamp Bias Part 3

In Part 2a we plotted the maximum current for the first 12AX7 gain stage in a Champion 600. That point was connected to the maximum voltage point to find what's called the load line for the stage.


This is the place to come to terms with a essential concept. Every point on the graph of plate curves represents a possible combination of plate voltage and plate current for a 12AX7. Let's choose a few for the sake of illustration. Let's pick Point A at 1.1 milliamps of plate current and 140 volts at the plate. Here's that point graphed onto the 12AX7 plate curves:



Now we'll pick another one we'll call Point B at 0.4 millamps of plate current and 230 volts at the plate. Here's that point graphed onto the 12AX7 plate curves:


Now we'll a third, Point C, at 1.1 milliamps of plate current and 230 volts at the plate. Here's that point graphed onto the 12AX7 plate curves:

Here's the fundamental concept. The load line we've drawn crosses all the possible combinations of voltage and current that can occur in our circuit given the 100K plate load we've chosen - so any dot which plots a combination of current and voltage must lie on this line for it to be a possible occurance in the circuit. We've plotted three different points but only Point C lies on the load line. When the circuit is drawing 1.1 mA of plate current the plate voltage must be the 230 volts indicated by point C NOT the 140 volts indicated by Point A. And with 230 Volts on the plate the circuit must draw 1.1 mA as indicated by Point C, NOT the .4 mA indicated by Point B.

The load line indicates that Point A and Point B do not exist for the circuit given our chosen 100K plate load. Points A and B can still exist as long as we adjust either the current or the voltage so that they lie on the load line:

This all may seen somewhat abstract, but it's essential to understanding the biasing of the preamp stage. It should all become clearer as we begin to discuss bias in the next few posts.