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To follow along with these pages you should go here
and print out the layout diagram for the 5F6A Bassman board.
At this point you should have
all your parts mounted to the circuit board. In the photo above you can see
that all the parts are mounted and soldered. There are some black wires and
screws showing that will be explained further down this page.
Now it's time to clean all the flux/resin
spots off the board again. After soldering the parts onto the board you will
have some new resin spots. I use my handy dental pick to flick off the spots
after they have hardened up a bit then I use a soft bristled paint brush to
sweep the board clean.
Left end of board
Right end of board
I have split the complete board
into two photos for better close ups. You can see a black wire on each end
of the board. These wires are soldered to the two ground busses. The wires
are black 18 gauge stranded wire. The left end ground buss goes to the pot
harness and the right end goes to the transformer chassis ground. These two
wires are explained in the board kit installation instructions which I have
on line at this page.
Schematics and info page
Non reversed layout diagram
Layout diagram reversed
Front of bias circuit board
Rear of bias circuit board
The top left
diagram shows the bias circuit layout diagram area on the 5F6A board.
The
bottom left photo shows the bias circuit on the circuit board.
The top right diagram is the same diagram only a reversed/mirror image.
The
bottom right photo is the back side of the bias circuit board.
On the layout diagram there is a squiggly line that says "under Board". This
is a wire that connects the bias circuit negative bias voltage to the two
220K power tube input grid resistors. It is connected from the bias pot over
to the two 220K resistors. Notice the 220K resistors are joined by a small
buss. The is where the bias wire is soldered. The negative bias voltage
comes from the bias pot then enters the 220K's. When the board is installed
in an amp, the bias voltage will then go to each power tube input grid.
(pin#5).
Some circuit boards have just one wire
under the board like the 5F6A board in this example. Some circuit boards
like the AB763 circuit have several wires under the board. Don't forget to
connect the under board wires. Double check yourself to make sure you are
connecting them properly. You are working on the back of the board and it is
easy to get confused.
This is the mounting hardware that I use on most boards.
It is a 5/8 inch #8 phillips head screw, a 1/4 inch nylon standoff and a #8
keps nut.
On some boards like a Plexi circuit you may want longer
standoffs. On most Marshall chassis the first pre amp power supply capacitor
is under the circuit board. Usually this is a 50uf/50uf/500 volt capacitor.
For mounting hardware I used a 3/4 inch hex threaded standoff, a 3/8 inch
phillips head screw and a 1/2 inch phillips head screw.
Mounting hardware added
The circuit board is now finished. All of our
parts have been mounted, all ground wires and under the board wires have
been installed and all solder resin spots have been removed.
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