[Harp-L] Blues Jr mods

[Victor Creazzi <aerofirewp@xxxxxxxxx>]

 Just reporting back on the Billm Blues Jr. mods.

 Harp content, this amp is a popular harp amp.

  After  researching and studying many "hollow state"
schematics, posts, and books (I'm well into my second
book and planning which one to puchase next, I'm
finding this stuff really interesting)

 I've decided to leave the bias right where it is at.
Every thing I found on Bias adjustment suggests a max
Idle current that would result in 70% of the the tubes
rated dissapation limit for class AB. The factory
setting is at or above 100% max dissapation, however:
(1) The 70% rule makes some assumtions about the power
stage that may not be the case on the Blues Jr. (2) I
have to believe that Fender EEs know what they are
doing in tube amp designs. (3) I am not aware of any
claims that Blues Jr's burn through power tubes at an
abnormally high rate (4) I like to run the pre amp and
the power stage just at the onset of clipping in each
stage, cooling the bias would put power stage clipping
out of reach without lowering the pre-amp setting for
my mic and set up YMMV.

 CAUTION: Tube amps operate at  voltages in the
300-600 range and high voltages may be present EVEN IF
THE AMP IS UNPLUGGED. Working on your amp could leave
you or your amp permanantly DEAD. 
These mods will void your warranty.

 I did decide to modify the tone stack to rid the amp
of it's boxy tone.
First I did the twin stack mod which is very easy and
allows dialing out more of the mids. The resulting
tone stack works somewhat differently in that, if all
of tone knobs are at 1 there is no output. All of the
original tone range is still available just at
different settings (Mids at 3 or 4 is roughly the same
as the stock tone stack with mids all the way off. I
highly reccomend this mod.

 I also added a .068 uf 400 volt capaciter in parallel
to the existing .022uf cap in the bass circut. The
results of this mod are much harder to quantify both
because of the subjectivity involved and because of
the nature of the tone stack where every setting
affects the other two, you definatly get more bass at
lower settings and it seams as though even more of the
mids are cut out at these low settings( I found myself
dailing in a little of the mids (2) to get a more
familiar tone). I also think I can feel more bass
through the floor than I did before the mods, I don't
remember ever feeling this before the mods,
subjectivity again.

This work was done on a green board Blues Jr ~early
2001. I was able to do both mods on the front side of
the board and both mods can very easily be undone by
just clipping them out.

 Special thanks to Billm for his great work on both
the Blues Jr. and Pro Jr. Lots of info on these and
other mods are available on his websites.

Victor


 
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