Friday, January 2, 2009

Fender Champion 600 Input Voicing Mod

This is probably the simplest of the mods I did to this Champion 600. Since I put in the 12dw7 the "low" input wasn't really very useful (I never use the low inputs myself anyway). Just one capacitor can revoice it to make it more useful. With the cap in place the "low" input gets a big volume boost for everything but the bass frequencies making it sound more like another channel than a lower volume input. The effect is a more dramatic for the "low" input but to my ears it gives a very subtle boost to the "high" input as well. Here's a plot of the frequency response.

To test the mod and see if I liked it, I just curled the ends of the leads on this orange drop cap and hooked them underneath the leads of R5. The leads are spread a bit wide and then compressed when the cap is hooked in. The spring tension of the leads holds the cap in place for a listening test. There isn't really any significant voltage in this part of the circuit. So worst case, if the cap falls over and shorts a nearby lead, there isn't going to be any damage.

This mod will actually work with any amp that shares this input circuit: That includes a whole lot of Fender amps. The schematic above was excerpted from the Bassman 5F6A - but you'll see that it's identical to a host of others. The red resistor is the one being jumpered over with the .012 uF cap.*

I put the cap on a switch just for the sake of comparison. Here's a modified schematic for the Champion 600:

Now, before the actual modification....


So here it is - just a cap a switch and one wire!

Here's the underside showing the switch. The switch for this mod is to the left. The one to the right is for the "presence plus" mod.

This is probably the quickest and easiest mod I've done for this amp. It's real simple to drop a few different caps in and see if you like it. Higher values will give a bit more low end, smaller values cut more low end and hence sound brighter. Unless you regularly use your "low" input (and who does?) the switch is really unnecessary. To obtain a suitable capacitor, see the input voicing mod "kit" post.

*Since you are essentially bypassing the grid stopper resistor at high frequencies on an older amp, you may need to add a new grid stop resistor. If you don't have trouble with radio frequency interference and it sounds fine, the don't worry about it. The Champion 600 already has an independent grid stopper (the 10K resistor connected to pin 2) so it's not an issue.


Andorephus said...

Hey, I just found your blog last night, and just love it! I've got a Champion 600 which I've been modding, so far i've bypassed the tonestack and just performed your input voicing mod, without a switch, and using a .0068 orange drop. Really neat mod, i'm just gonna use the "low" input now, the low end is cleaned up, more articulate. Well, Keep up the good work on this blog, I'm slowly learning how things work from reading different articles all over the net. Thanks!

--Andy in montreal

akavalve said...

Cool! Glad it worked for you!
And thanks for letting us know.

Fernando said...

Yep, great mod, thank you.
Would be also possible to make the inputs (or just one) more sensitive by lowering both 68k?
Wich is the limit for a 12AX7? (how small the input resistance can be)

akavalve said...

Raising the lower of the 68K resistors (the one not bypassed by the new cap) to 1 Meg will raise the sensitivity of the "low" input by 5 dB. Unfortunately it will also negate the effect of the revoicing capacitor and the "low" input will become almost identical to the "high" input.

I worked out a variation on this mod that will allow you to have a high impedance input on the revoiced "low". The Champion is back with it's owner now so I can't do pics. If you're interested let me know and I can do a post just from the board layout diagram.

But back to your question about sensitivity. Were you asking about equalizing the sensitivities of the "high" and "low" inputs or are you looking for more output from the stage?

You're not going to get a noticeable change in sensitivity by lowering the 68K in series with the 10K grid resistor, which is what I think you're asking. Very little current flows though this resistor which means there is very little voltage dropped across it. I does seem intuitive that lowering or removing this resistance would increase the signal reaching the tube but it's not actually the case. If you follow Ohms Law you see that you would need a significant voltage drop here for a resistor change to make a difference. It's always about the relationship between voltage resistance AND current. If you think about any of them in isolation things will be a good bit more difficult to get a handle on.

If what you do want more gain from the stage John Frondelli's values for stage 1 will work fine.

best, Andy

fernando said...

thank you!
Yes I meant to decrease the input resistances (even on both channels) to overload the first triode sooner. But now I see why I never saw this done on any amp... it won't do anything...

Anonymous said...

Would it be possible to make both inputs brighter with this mod? How would one go about that if so? I really only use the High channel and would like to be able to enhance the highs so it doesn't sound so "woofy"

giambibolla said...

I don't know if I am off topic but I really wish somebody could answer:
I use ONLY low input and swapped the 12AX7 with a lowe gain tube, because I really want a clean tone and the High input distort too early. The question is : what is the difference between the two inputs? Is the tone stack bypassed in the High input?.
thanks to anyone answering.

akavalve said...

In an stock amp there are two resistors that cause a volume drop when you plug into the low input - almost like turning the volume pot down on your guitar.

Plugging into the high input eliminates this volume drop.

Both inputs go through the tone stack.

Clive said...

Hello There,

We have a Fender 600 that goes to a nasty distortion after about 30 mins.
Would that be the power tube?

akavalve said...

Tubes would be a likely culprit and they're the easiest to check. Just swap in a couple new ones and see if the problem goes away.

Anonymous said...

Hi Akavalve!!!
I am loving your blog!!! thanks for sharing your knowledge!
One question:
i am installing a digital reverb circuit in the amp just after the inputs, where would you connect the input of the reveb effect if you want to use both inputs???
Thanks for the help!!!