Fender Champion 600 Input Voicing Mod "Kit"

Instead of selling "kits" for the Champion 600 mods on this site, I'm putting together a series of posts with direct links to sources for the parts involved.

Below is a chart of capacitor values and -3dB points for the Champion 600 low input revoice mod.

This mod will work in a great number of amps with high and low inputs including the 5E1, 5F1 Champs and a host of other Fender Amps. You can find the the measured frequency response for a .012 uF cap in Part 2 of the mod post.


Clicking on any of the capacitor values in the chart should take you directly to a suitable part for the mod on the Mouser Electronics website. You can order directly from there.

If you have any questions about the parts links here, feel free to drop me an email.

Fender Champion 600 Tone Stack Bypass/ Fat Switch Mod

This pic shows the shows a few of the mods I've made to this Champion 600. The two small red switches on the bottom of the chassis are for my Presence Plus and Input Voicing modifications. This particular post concerns the tone stack mod associated with the two bigger switches to the upper right.



This mod takes the coupling cap mod from the Tone Stack Test and hardwires it to a switch so the amp can use the stock tonestack or bypass the tonestack with a single coupling cap to eliminate the mid cut in the stock circuit. The right had switch below handles that job. The left switch acts as a three position "fat" switch.

First....

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

Here's the finished mod from above:


The combination of resistors on the "fat" switch are selected to combine for three settings:
15K (stock), 30K (Frondelli Mod fat boost value), and 47K (for a little extra boost). The boosts effect the mids most dramatically, but they provide extra gain across the whole spectrum too.

All three resistors connect to the same pin on the right hand switch - the one just to the left of the center pin with the black wire.





The switches I used here each have one more set of contacts than are needed to make the mod. I frequently do this when I'm experimenting so that if I decide to add something to the switch later on I don't need to disassemble the circuit and solder in a new one.

The resistors in the fat boost circuit replace R19 on the pc board. Here's how the wiring runs:



Here's the coupling cap, prepared for installation:


And here it is soldered in place. Since only one end is really fixed and the other will be supporting a wire I put a bead of silicone underneath to make sure it stays in place.


Here's how the wires are run to the switch. Point "A" on the switch runs to point "A" on the board. Same for "B" of course.

Here's the frequency response for the four settings measured at the amp output with a 4 ohm resistive load:




Click on the graph for a high res version. From the bottom to the top the curves are for Stock, Fondelli Mod Fat Boost, Extra Fat Boost and Tone Stack Bypass. The tone stack bypass curve is up about 16dB up at the mid cut frequency!

A final note...if you try this one yourself, be careful to place the switches low enough so they clear the cabinet when you put the chassis back in. It a tight fit.

Fender Champion 600 Tone Stack Test

The stock Champion 600 circuit is roughly based on the Vibro Champ AA764 circuit (without the vibrato circuit of course). Fender has incorporated a tone stack from the AA764 but eliminated the bass and treble knobs by replacing the pots with fixed resistors. The result is a tone with a strong mid cut and a significant gain loss between the first and second stages.

This was my quick test to see how the amp sounded with the tone stack removed from the circuit and a coupling capacitor substituted for it. I've used a fairly large value here - .1 uF. The 5E1 and 5F1 Champ circuits both used a .02 uF coupling cap instead of a tone stack. This is a good place to start if you're trying to get your Champion 600 closer to one of those earlier circuits.



The coupling cap is tagged in from the top of C1 to the bottom of R21. The end of R19 needs to be lifted to disconnect the tone stack. The result is a more even frequency response and a good deal more drive to the second stage. I've done my 12DW7 mod to this amp, so I needed a bit of extra volume. With the tone stacked bypassed in a stock amp the breakup of the extra gain of the dual 12AX7 stages will make the amp lot dirtier. I'm looking for more clean headroom but a lot of people might go for the added drive.

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

Like most of the mods I've done on this Champ 600, I'm going to make this one switchable. You can leave the mod like this if you choose but if you do it's good practice to tidy up the end of R19 by either removing it completely or insulating the lifted end with shrink tubing.

Fender Champion 600 Output Transformer

Another difference between the new Fender Champion 600 and the older Fender Champ 5E1 and 5F1 circuits is the higher supply voltage (also called the B+ voltage) and an accompanying higher impedance primary on the output transformer in the Champion 600. I've never measured an original myself but replacements like the TF103-48 have a 5K ohms primary impedance. My measurement for the Champion 600 transformer at 1kHz is about 11K ohms.

I was a bit surprised by this so I called Mercury Magnetics to ask them about the impedance of the output transformer in their C600 Champion Upgrade Kit. They said their measurement was around 9K for the original and that they raised it a bit to 10K for the upgrade.

I don't have the details for their measurement, but here's the details of mine:



The tone generator (center) is set to input 1 volt @ 10Khz into the secondary of the output transformer (shown on the left side meter). The right hand meter shows the voltage output on the primary with 1 volt on the secondary. So here's how the primary load is calculated.

A transformer impedance ratio is equal to the square of it's voltage ratio.
Here the voltage primary to voltage secondary is 59.4:1
59.4 squared is 3,528
So for a 1 ohm load, the load reflected at the primary would be 3,528 ohms.
But the Champion 600 doesn't have a 1 ohm speaker for the load, it's 4 ohms.
That will raise the the impedance reflected at the primary 4 times.
So 3,528 times 4 is 14,112 or about 14k ohms.

But didn't I say I measured about 11K?

Yes - but at 1kHz. The above measurement was for 10Khz.
Transformers (especially cheap ones) are imperfect devices and their response varies with frequency.

Below is the same procedure, but at 1k.





With 1 volt on the secondary at 1 kHz, the reading on the primary is 52.94 volts (rounded to 53).

Here the voltage primary to voltage secondary is 53:1
59.4 squared is 2,809
So for a 1 ohm load, the load reflected at the primary would be 2,809 ohms.
So 2,809 times 4 is 11,236 or about 11k ohms.


Here's the reading at 100Hz:



With 1 volt on the secondary at 100 Hz, the reading on the primary is 44 volts.

Here the voltage primary to voltage secondary is 53:1
44 squared is 1,936
So for a 1 ohm load, the load reflected at the primary would be 1,936 ohms.
So 1,936 times 4 is 7,744 or about 8k ohms.


From this you can see that if you load the 4 ohm secondary of a TF103-48 with a 8 ohm speaker you raise the primary load to 10k - same as the Mercury Magnetics. Of course there's a whole lot more to a transformer than just the loading, but if you need to replace the OPT in a Champion 600 and are short on dough an 8 ohm speaker on the 4 ohm tap should get you back in business.



UPDATE: For comparison, I've posted measurements for an AB764 Vibro Champ output transformer here.

Fender Champ - comparing the 5F1 and 5E1 circuits

In modding their Champion 600's, some people are trying to get closer to earlier versions of the Fender Champ circuit, the 5E1 and 5F1. This post is to clarify the differences between those two early circuits. There are two significant ones.

The first is the missing cathode bypass capacitor in the 5F1. Eliminating this cap reduces the first stage gain. Reducing the value instead of eliminating it completely will give a bit of a treble boost (see my earlier post "Champion 600 cathode bypass mod" for a chart showing how different capacitor values will effect the response).


Secondly, in the 5F1 champ the choke has been eliminated and a screen supply node consisting of a capacitor and resistor has been added. This saved Fender some cash, as a choke is a good bit more expensive than a cap and a resistor. In a Class A amp a choke doesn't contribute to sag they way it does in a Class AB amp, but the change still has an effect. The screen supply resistor in the 5F1 drops the voltage for the screen below that of the plate. Since the screen voltage actually has more effect on plate current than plate voltage, this changes the character of the output section. If you really want to hear if the choke makes a difference between the two circuits you need to put the choke in series with the screen supply resistor, not eliminate the resistor completely.