good sounding blues jr mod

[Andy Le Blanc]

A client had me mod his blues jr. He gave me the amp and a wad of
print off the internet. He was very interested in vox voicing and had all ready
replaced the speaker with a GT , EV clone. The problem with an extensive
re-capping and re-voicing is that it does nothing to make the power side
anything like a vox. The blues jr has fixed bias, feedback, and a fender inverter.
Its not a vox, and the investment to re-manufacture one (not a bad idea)
can prohibitive. What seemed the best solution was (per-online sources)
to use a 5751 in place of the first 7025 and modding the mid function in the tone stack.
The suggested extensive recap, replaces cap types and values, changes the "bright",
tone caps, slope resistor, feedback ratio, etc.
But with the way the guy plays and his speaker choice, that would not be a
good thing. So, to get the top end detail, I tried a Hi-Fi trick of adding a poly
to bypass the last filter in the PS that supplies the pre (I didn't choose to
add one to the cathode of the first gain stage as I have seen it done
because it would also mod the fat switch function), this combination seems to make
for a tone that has more detail with out being too ice picky
with the much more efficient speaker.

[Structo]

I think I have heard of that trick before.

So your saying just bypassing the Preamp filter cap that it added top end?

What value is the preamp filter in that amp?

What value poly cap did you end up with?

Thanks, Tom

[Andy Le Blanc]

It's a green board, the last filter is a 22uf/500v. I grabbed a .02 but its to taste,
anything .001 to 1.0 and you can mix types. This is a "detail", not a bright mod.
You have to be careful, to much detail in a guitar amp isn't a good thing.
It appears as brightness but doesn't alter the signal path. Too much and the
amp starts to get over tone-ish .

[Andy Le Blanc]

The fellow was very happy tonight, the amp had plenty right next to a drum kit.
Nice tone, wasn't bad to run FOH around, nice presentation without being
overly bright. Totally different beasty, good cleans and treated his pedal
board nicely.

[echuta13]

I guess the general concept is that the film caps lower ESR (and smaller size) helps with transient response? Neat to hear it has a noticeable effect.