Super Champ XD harsh treble--any popular mods?

[Mr Johnny Birchwood]

Hey guys, got an inquiry about a Super Champ XD with a very harsh and undesirable treble response. Owner is curious if there are any modifications that would tame the treble. I am reluctant to mess with the solid state preamp and as far as I can tell, tone shaping is done through the DSP. Besides replacing the stock chinese mfds tubes with JJ's I'm not sure what else I could do. Are there any well-known or published mods for the Super Champ XD that would tame the treble?

thanx

[soundguruman]

Change the value of the bias filter capacitors...you will be surprised.

[Mr Johnny Birchwood]

Go up or down from the 100uf 50v?

[soundguruman]

Go up or down from the 100uf 50v?

John Birchwood, don't I know you?
My first thought, off the cuff, would be to increase C42 to 2.2 uF and delete C 37.
Fender Super-Champ_XD_schematic.pdf
The 10K on the input jack, I would change to 68K.
On the bias, I would try changing C 17 to 150 or 200 uF. The bias circuit can be tweaked, to produce a kind of low frequency resonance on the grid of the output tubes.
Anyhow, I kind of tend to modify the bias circuit. You will want to experiment with that for a solution.
Personally, I get rid of all "illinois electrolytics." because I think they make the amp sound like crap. I use F&T, and I notice it really helps smooth out the harshness.
As far as the speaker is concerned, are you sure the voice coil is free, are you sure the speaker coil is not rubbing against the magnet, when the speaker moves? Remove and inspect the speaker.
Just using a "better" speaker might help a lot.
Make sure the bias of the output tubes is HOT enough. Make sure there is no crossover distortion on the output. HEAT up those tubes, and get rid of distortion. It's important to do that.
Then, as you can, change C 38 to a larger value .003 instead of .001 for example, and it will help balance the frequency response of that first input stage. .001 may not be enough to roll off the highs.

[Mr Johnny Birchwood]

Mr Johnny Birchwood is a stage name, not my real name. An amalgamation of Jimmy Driftwood and Johnny Horton and that I use when playing with a local honky-tonk band. I don't use my real name on here lest a customer stumble upon this forum and find out how little I actually know...

I don't have the amp in front of me, got an email inquiry from a guy who lives out of town. I just wanted to get some ideas before I tell him whether or not I will mod his amp. Thanks for the suggestions. He has already replaced the speaker for an unknown 12-inch and replaced the power transformer with an Allen model. I don't know if the tfx was replaced due to failure or not. I would have to replace those inferior stock tubes and bias properly before considering what, if any, mods to perform. I am very appreciative of the information.

[soundguruman]

Mr Johnny Birchwood is a stage name, not my real name. An amalgamation of Jimmy Driftwood and Johnny Horton and that I use when playing with a local honky-tonk band. I don't use my real name on here lest a customer stumble upon this forum and find out how little I actually know...

I don't have the amp in front of me, got an email inquiry from a guy who lives out of town. I just wanted to get some ideas before I tell him whether or not I will mod his amp. Thanks for the suggestions. He has already replaced the speaker for an unknown 12-inch and replaced the power transformer with an Allen model. I don't know if the tfx was replaced due to failure or not. I would have to replace those inferior stock tubes and bias properly before considering what, if any, mods to perform. I am very appreciative of the information.

Yes, it sounds like the bias is all goofed up. But I can't help modding. I always want MORE of something.
Yes, the "harshness" in the treble is probably crossover distortion, from improper biasing.