Wed Apr 7  06:59
                                     Dave Stork
                                    dstork@ibm.net


A friend stopped by and asked for a simple, reversible mod
to juice up the preamp in his Blues Jr. and increase the
reverb depth slightly. He's an apartment dweller and he
wants to be able to get a hotter crunch tone at low volume
without ruining the amp's ability to produce a good clean
tone. Here's what I came up with on short notice:

Change R3, R10, R30 and R39 to 150K.

Change R5 and R44 to 220K. 

Add 22uF/25V across R38.

After the mod, the preamp produced a nice bluesy crunch
even with single coils. The dynamic character is good... it
cleans up nicely as you let up on the pick or turn down the
guitar volume. The increase in reverb depth is subtle, but I
didn't want to increase R44 beyond 220K let the reverb
driver be driven into clipping. 

Blues Jr. owners who are looking to try something different
might want to give this mod a try. It's easy to do (if you're
used to working with PC boards) and is easily reversed if
you don't like it.

When posting critiques, please bear in mind that this mod
was requested, designed, installed and tested in the space
of one hour :-) 


Dave:

    If it's okay with you I'd like to add your mods to a
blues_jr.txt file to go with the schematic on my site. (I may
dig through the archives here to add other suggestions, too.)

    BTW I think you were going to post some mods you had
done to a Pro Jr awhile back. Maybe you can post them to a
new thread when you get time to dig them up...

Thanks!

Steve Ahola

P.S. I just noticed that the Blues Jr has an extra triode
(V2A)- could probably have a lot of fun experimenting with
that! Like adding a cathode follower before the tone stack,
or an extra gain stage like a Boogie Mark I...

    For the benefit of others looking for these resistors in the
schematic...

R3 - Rp for V1B (100k to 150k)
R10 - Rp for V1A (100k to 150k)
R30 - Series resistor on Treble wiper (10k to 150k)
R39 - Rp for V2B (100k to 150k)
R5 - Resistor to ground from Volume wiper (130k to 220k)
R44 - Resistor to ground on input of reverb IC (120k to
220k)
R38 - Rk for V2B (add 22uF/25V Ck cap)

    Quick'n'dirty ain't bad! Thanks again! 

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And here are some Blues Jr. tips from Randy of Tone Lizard Amplifiers:

1)Of course increasing R4 might send the reverb driver into clipping.Do
you want more reverb depth? Increase R50. Try 390K or 470K.
2)Put in a 250K master (use the part # for the treble control). You can
debate about removing R40.
3)Jumper R18. Debate about the value of R17.

The phase inverter looks like a spot for a few other ideas:
4)Reduce R13 to 10K
5)Reduce R16 to 100 ohms
6)R26 is the presence control as it were. Play with the value or!
short it out for full effect.

I guarantee these will work, and better. Please, this is no battle of
the techs, just a tip from someone who has 'been there, done that!'

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