Blues Jr. Mod 1a Swap the first preamp 12AX7 with a 12AY7. I tried the 12AT7, didn't make much difference. The others can be just left alone. This sub allows you to screw the amp all the way up without feeding, and you still keep your tone. I would recommend leaving the big guys (EL84) alone.

Blues Jr. Mod 1b My Blues Jr. was a feedback beast until I did some tube swaps. Right now I'm using this settup (as you look at the amp from behind): 12AX7, 12AY7, 12AU7 It's tamed the feedback beast for both my Sure 520D Green Bullet and my Astatic JT-30. Still cranks it out for 15 watts, and I like the tone. Harpin' in Colorado, Ken M.

Blues Jr. Mod 2 from Dave Stork at dstork@ibm.net.
This is a simple, reversible mod to juice up the Blues Jr. preamp and increase the reverb depth slightly at room level. This mod allows hotter crunch tone at low volume without ruining the amp's ability to produce a good clean tone.
  1. Change R3, R10, R30 and R39 to 150K.
  2. Change R5 and R44 to 220K.
  3. 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.
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)

Blues Jr. Mod 2 additional 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.
  4. Reduce R13 to 10K (The phase inverter looks like a spot for a few other ideas !!)
  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.

Pro Jr. tip by Jan van Gastel.
Swapping the two 12AX7 tubes for a 12AU7 (the first one) and a 12AT7 (the second one) the volume knob can be set to about 7-8 now. Before swapping tubes feedback started at 1.5.-2.5.

My additional suggestion: Alternatively, you can use the following swap strategy:
12AY7 1st tube
12AT7 or 12AU7 2nd tube (the AT7 is preferred for stock crystals and the AU7 for hot crystals or magnetic elements).