Steve,
Here's an article I wrote on how to put a bias control into a Pro Jr. 
These amps have been very popular and there are alot of them in the 
field. I been un-doing soldering of added-on fixed resistors (for bias 
reasons) on these and the circuitboards cannot stand multiple soldering 
and de-soldering for everytime a new set of tubes are fit. So here's the 
fix. You may, if you choose, post this on your site.

Thanks, and regards,

Bob M.,
from the Ampage

P.S. Happy New Year!



Fender Pro Jr.  -  Adding a bias pot

This is easy and it takes about 1 hour. Every pair of EL84's I plugged 
in to my Pro Jr. gave me such drastically different mA readings, I 
decided that the work was worth it.

1).   Get a 10K cermet pot (long rectangular type), 15~20 turns. Get a 
12K resistor and a 30K resistor (both 1/2 watt). If you cannot find a 
30K, you can get by without it.
2).   Un-screw the circuitboard (from now on, we'll call it the C.B.) 
and remove any wires from lugs so you can give yourself access to the 
underside of the C.B. (solder side). Write down which color wires go to 
which terminals (they're all marked).
3).   Remove (de-solder) R29 (15K) and R30 (33K) from the C.B. Clean the 
C.B. pads well on the underside for the new parts.
4).   Install one end of the 12K resistor at the ground side hole of R29 
so the resistor is standing up. That is the hole that is farthest from 
the edge of the C.B.
Double check with your ohm meter to make sure that it is ground.
5).   Solder the other end of this 12K resistor to the bottom lug of the 
cermet pot (the adjusting screw for the pot is on the top).
6).   Install one end of the 30K resistor at R30, in the hole nearest 
the edge of the C.B., again, standing the resistor straight up.
7).   Solder the other end of this 30K resistor to the top lug of the pot.
If you cannot locate a 30K resistor, desolder one end of the stock 33k 
resistor and connect the other end to the pot, as described. You may 
have to lengthen the lead at the desoldered end, using a bit of wire if 
you use the stock 33K resistor.
8).   Solder a wire to the wiper (middle) lug of the pot and connect 
that wire to the other hole of R29, the one nearest to the C.B. edge.

Construction tip:  I connected the resistors and wire to the pot first, 
using heat shrink whenever possible, and had a 'practice run' to make 
sure everything fits and lines up just right, installing the pot, on 
end, between the holes of R29. When you're ready for mounting, put a dot 
of crazy glue on the bottom of the pot, stick the resistor and wire 
leads thru their respective holes and then hold the pot in place for 
about a minute so the glue has a chance to set. You can then flip the 
board and solder the leads to their pads. Clip off the excess lead.

9).   Clip your ohm meter between the DC side of the bias diode and 
ground. Using a small screwdriver set the pot to read at 15K ohms. This 
is the exact point of adjustment on the pot that emulates the stock Pro 
Jr. bias.

Notes:  I started out with 10K (for R29) and 27K (for R30) but I noticed 
that my now variable bias gave me alot of 'hot' bias setting and less of 
the 'cold' bias settings, so I modified the parts values as above. 
Here's what I originally got:
Bias pot variation (at Pin 2 - grid of power tubes): Voltage, -12.80 to 
-6.90.
Amperage: 14.0 mA to way over 50 mA (I stopped turning).
The schematic says bias should be set to -10.4 Vdc at point Z. When I 
set point Z to -10.4 Vdc on the bias pot, I read -10.17 at the grid, pin 2.