My Fender Blues Junior is from 2003 I think (MIM cream board) so I use pages 2 and 3 of this document : Fender Blues Junior Schematics
Bias is set far too hot by Fender. I changed R52 with a 100k variable resistor (I think a 50k would have done the job, but I had a 100k in my spare box …)
EL84 max Pa is 12W.
I measured 345V on the plates (and 310V for Vg2) of the EL84. A little high as “limiting values” in datasheet are 300V max for anode voltage and 300V for Vg2. So I used a pair of 6P14P-EV (EL84M equivalent, 400V anode max, 14W max for Pa).
Ig2 was ~2mA, so I set Ik to 26mA. So Ia = 24mA ; 24mA x 345V = 8.3W = 70% x 12W
Power supply caps mod
I used F&T 100uF for C25, 47uF for C26, 22uF for C27 and C28, and I replaced C24 with an Orange Drop 716p (0.1uF) and C29 with a high quality polyester (0.01uF – Orange Drop was to big to fit …)
Other caps mods
I replaced signal with high quality Orange Drop polypropylene or tubular polyester caps (OD because in a Fender amp, I like the chime … and tubular polyester when OD where to big to fit …). I also replaced many ceramic caps with silver mica caps. Be careful to keep C14 (47pF) as a ceramic 1kV because it is used to prevent oscillations …
Tone stack and Cathode Follower mod
I replaced the 0.022uF bass cap in the TS by a 0.1uF orange drop for a better bass response :
As V2A does nothing, I think it’s a good idea to implement a cathode follower to drive the tone stack, as described in http://billmaudio.com/wp/?page_id=141
Billm proposes to use a 12DW7 (a 1/2 12AX7 and a 1/2 12AU7 for the cathode follower). An exciting idea as 12AU7 are good for cathode follower, but I prefered use a 12AX7 as the schematic mimics a Fender Bassman 5F6 stage pre-TS … did you say mojo ?
I decided to replace the output transformer with a Mercury Magnetics.
Note the difference in size :
A Celestion Greenback is my choice to achieve the modifications :
So, from this :