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.






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:






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:






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.

Fender Champion 600 cathode bypass mod

This post is from the series of mods I've done to this Champion 600. I'm finally getting down to changing the circuit a bit. My main goal is to clean the amp up a bit, especially in the low end. It's probably going to require a speaker replacement eventually, but I'm starting with the cathode bypass caps.

With most 12AX7 circuits, smaller bypass cap values will reduce the low end response by a bit less than 6dB. The frequency at which the response is down by 3dB is, logically, the "3dB down point" or the "knee frequency". Changing cathode bypass caps can be a bit more subtle than changing coupling caps since the low end response is never reduced more than 6dB - only the knee frequency is changed.

There are two cathode bypass caps in the preamp section of the Champion 600. Either or both of them can be changed out to shape the frequency response of the amp.

Here they're indicated on the pc board layout - C3 is for the first stage and C10 is for the second:





A standard tuned 6 string guitar isn't going to produce anything below 80Hz and even the low frequencies it does produce aren't going to reproduce well though the Champion 600's speaker. Below is a chart with values calculated for the 3dB down point with a series of common cap values. Note the stock value of 22uF has a -3dB point of 9Hz. I'm going to start with a 1.5uF in the first stage and a .68uF in the second and work from there.

If electrolytic caps are used (and they probably will be for the larger values unless you've got deep pockets) it's very important to maintain the polarity of the cap or it will explode under use.




This amp came to me with a printed copy of the Frondelli Mod . The mod itself is fine, but a number of the changes detailed there do the opposite of what this amp owner is looking for. He's looking for a cleaner sound with more low end headroom. Even though Frondelli is limiting the low end somewhat, he is also increasing gain for a more saturated sound. If that's what you're interested in, though, you can tweak the bypass cap value to your taste there as well. The blue resistors in the diagram below indicate the ones changed for the 1st stage in the Frondelli mod (R8 and R2). The bypass cap associated with that stage (C3) is indicated in orange.


Jon Frondelli recommends a .68 uF cap here, but you're free to change it to taste. Below is a chart showing calculated -3dB values adjusted for the new plate and cathode resistor values in the Frondelli mod.



If you're interested in how these values are calculated, drop me an email - or visit Randall Aiken's site.

Ampeg V4 curiousities part 2

This is a look a some of the less common tubes used in the preamp section of the V4. There's also a view of the circuit board revealing one of the major differences between the V4 and the V4B.