Monday, March 2, 2009

Fender Champion 600 Preamp Bias Part 1

In order to understand what's behind the new the bias point for the 12DW7 mod, we'll first have a look at how the initial 12AX7 gain stage in the stock Champion 600 is biased.

There are three critical factors in the effecting the 12AX7 stage - the power supply voltage, the value of the plate resistor and the value of the cathode resistor. They're indicated on the schematic below. Keep the first two immediately in mind. The cathode resistor will come in later.

Fender Champion 600 preamp schematic B+ Plate Resistor and Cathode resistor indentified

Like all the images here, you can click on the schematic image for more detail.

Now we'll reference the B+ and the plate resistance to "plate curves" for a 12AX7 to determine the "load line" and from there determine the bias point.

Don't let this scare you off. It sounds much more complicated than it actually is. It's really quite straightforward once you grasp the basic concepts.

The graph of plate curves is part of the collection of tube data generated by the tube manufacturer. You can find them posted in a host of places on the web. I frequently use Duncan Amps TDSL.

Here's an image showing the plate curves for a 12AX7:

Fender Champion 600 12AX7 Plate Curves

The X Axis shows the voltage at the tube's plate, the Y axis shows current flowing through the tube and the numbers labeling each of the curves indicate different bias voltages.

The first thing we need to do is determine the maximum possible plate voltage.

That one's easy. Since the plate voltage will never exceed the available supply voltage we can simply use the B+ voltage for the preamp section. We can read that off the schematic - 340 Volts. So we place a dot on the plate voltage axis of the graph:

Fender Champion 600 12AX7 Plate Curves with Maximum Plate Voltage indicated

Now we need to find a place to a dot on the plate current axis. This dot should indicate the greatest amount of current that will flow through the tube.

We can't just read this number off the schematic - we'll have to calculate it.

We'll do that in the next post.


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