Fender Pro Junior - Excessive Hum Modifications - Rebuild Repair

The Fender Pro Junior is a great little amp with plenty of volume, plenty of tone and usually plenty off excessive hum with the input volume down (nothing plugged in).  It helps to get rid of the stock tubes and replace them with some NOS ones.  The 12AX7s can be rolled to hear which ones sound best to you.  I happen to use NOS Sylvania 6BQ5s and Tung Sol 12AX7s.  Here are my mods.  Use at your own risk. 

Start by clipping the wire ties that hold the transformer secondary wires together. Unsolder the two green filament leads from the tube circuit board, carefully twisting them and resoldering them.  Keep them away from the other wires as pictured.

Add a 10 mfd @ 25 VDC electrolytic cathode bypass capacitor across resistor R4.
Add a 10 mfd @ 25 VDC electrolytic cathode bypass capacitor across resistor R11.

The back chassis cover is held on by six screws.  The top center one is too long and makes a connection to a potentiometer shield, causing hum.  You can either cut the screw shorter so it makes no contact with that shield or leave it out.

   

No more hum!