The Fender Blues Junior (BJ) is probably Fender’s best-selling valve amplifier.
However, it does have a number of poor design features, not least the layout of the PCB on which the EL84 power valves are mounted.
The EL84 is a well-designed valve as the high voltages on the anode (pin 7) and screen grid (pin 9) are located on pins away from the low voltage control grid (pin 2) and the cathode (pin 3).
Fender negate this sensible layout on the left-hand side EL84 looking into the back of the amp by running a PCB track from the grounded cathode to the ribbon cable from the bottom PCB to the main PCB via the gap between pins 6 and 7, i.e. right next to the high voltage anode connection on pin 7, which the EL84 was designed to avoid.
This is a mistake, and we have seen many Blue Junior PCBs damaged due to a short from the anode to ground via this trace.
Fender have addressed this problem in the current Mk IV version of the BJ by slashing the PCB trace from pin 3 and hard wiring pin 3 directly to the ribbon cable. We now perform this mod on all BJs we get in.
Regardless, given that there are plenty of BJs out there with the older style PCB, so we designed a replacement PCB that avoids this problem.
Our PCB is made from 1.6 mm FR4 (this is a better material than the synthetic resin bonded paper used in the BJ), 2 oz copper traces (1 oz is standard), double-sided, and plated through holes.
The PCB is designed to accommodate Belton noval sockets, which are used in some versions of the Blues Junior, and are very good quality sockets. You can recycle the sockets from your old PCB. Sometimes if a valve short occurs one of the power valve sockets will be damaged, and we can supply replacements.
We have added pads so the filament wiring to the pre-amp valves is hard wired off the PCB with tightly twisted wire, which should help to reduce noise. Again we can supply the PCB with this wire in place.
Additionally, there are pads for 2 snubber capacitors (C1 and C2). Fitting these will eliminate the tendency for the BJ to oscillate at around 50 kHz. This of course not audible, but stresses the valves and the filter caps. We recommend 220 pF, and can supply these. Alternatively if you are removing the main PCB, you can change the 47 pF capacitor across the phase inverter to 220 pF; this is designated C33 or C14 depending on which version of the BJ you have.