Jim S.
JimSxxxxx@xxxxxxxx.xxx.net
6/30/98
11:02AM


No, you don't need to remove the PC board to
add a bias pot, but the job is tricky. 

First, you need to obtain a 50K 20-turn cermet
trimmer potentiometer. This is a very small part.
It usually looks like a blue plastic rectangular box,
with a tiny adjustment screw coming out of one
end and three thin (and easily broken) terminals
coming out of an adjacent longer side. Any good
electronics supplier should carry these. 

Next, you need to locate the fixed resistor (I
believe it's 22K) in the bias circuit that will get
replaced by the cermet pot. You'll need to have a
schematic and/or PC board layout diagram to
help you find it. Once you find the resistor, clip it
out, leaving as much lead length still attached to the
board as possible, since you'll be soldering wires
to them. Use needle-nose pliers to bend up those
leads so that they're sticking straight up from
the PC board, but be careful not to break them
or pull them out. 

Find a suitable empty area on the PC board where
you will be super-gluing the cermet pot (with the
screw pointing out at you and the terminals sticking
out to the side). Estimate how long wires need to be
to connect the remaning resistor leads to the
cermet pot and add another inch or two for
good measure. Cut two wires to length and solder
them to the middle terminal and one of the
end terminals of the cermet pot. Super-glue the
pot to the PC board. Route the wires over to where
you cut out the resistor. Cut the wires to the exact
length required to reach the leads and strip the
wires. Now here's the trickiest part - solder the
wires to the leftover resistor leads, but do it
quickly so that you don't melt the solder connection
holding those leads to the traces underneath the
board. (Otherwise, you WILL have to remove the
PC board to resolder these connections). Try
not to keep the tip of the soldering iron on the
connection for more than a second or so. 

Biasing should be easy and accurate once you've
installed the pot. Those 20 turns let you really zero in
on an exact reading. Take your time with this mod,
and it should work out well. 


Here's another mod I think is worth trying out in
reissue Bassmans: The tone stack in the reissues
uses .1u and .022u caps with a 100K slope
resistor. Most players prefer the sound of two
.022u caps and a 56K slope resistor - the bass is
response clearer and less "tubby" sounding.
Fortunately, these components are easily
accessible because they're mounted on the same PC
board strip that the pots are soldered to. To get at
the trace side of this board, unscrew all of the
pot nuts and gently pull the board out and turn it
over. 
Unsolder and remove the .1u cap and the 100K
slope resistor. Replace them with a .022u cap
(630V) and a 56K 1/2W resistor. Put the board
back in place, feeding all 6 pot shafts back through
the chassis holes and replacing the nuts. Enjoy
the improved tonal response. 

The above two mods (along with switching to a
5AR4 or 5U4 tube rectifier) are, IMO, a
cheap way to make a big improvement in the sound
of the reissue Bassmans.
And they don't involve replacing speakers,
transformers, or circuit boards.

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