Stage one and two are in the capacitor can on the back of the
chassis in the Bassman reissue. This can has four caps inside but there are only two
stages of power supply filtration in this can. You will see two 100uf/350v caps (stage
one) that are joined together in Series, not parallel and two 47uf/350v caps (stage
two) joined together in series. When you put two caps together in series, the
capacitance is divided by two, and the voltage is added together. If you picture two caps
stacked up like two flashlight batteries so that the positive of the bottom cap is
connected to the negative of the top cap, that is series. You use the negative of the
bottom cap and the positive of the top cap for the electrical connections.
So we have stage one, which is two 100uf caps, divided by two = 50uf of total
capacitance and a voltage rating of 350v + 350v = 700volts. Stage one has a red wired
soldered to the pc board. The red wire is connected to the positive end of the top
100uf/350v cap in the series stack. The red wire then goes through a hole in the chassis
and ends up inside the amp connected to stage one on the circuit board.
Stage two: We also have stage two in this can which is the two 47uf/350v
caps in series, divided by two = 23.5uf with a total voltage rating of 700 volts. Stage
two has an orange wire soldered to the pc board. The orange wire is connected to the
positive end of the top 47uf/350v cap in the series stack. The orange wire then goes
through a hole in the chassis and ends up inside the amp connected to stage two on the
circuit board.
Both of these series arrangements for stage one and two have 220k bleeder
resistors across the leads of each cap to balance the voltage and bleed down the voltage
after the amp has been shut down. Leave all of this arrangement in the cap can alone, it
is perfectly useable and just fine as a far as power supply filtration goes.
There are a couple changes that I like to do to the wires leaving the filter cap
pc board. I like to run new stranded 18 gauge wires in place of the stock red, orange and
black wires that leave this board. You could use the existing red and orange wires if they
will reach the lugs on the board where they need to be connected. They are a little stiff
and hard to work with, but they will work.
The black ground wires are not long enough to reach the power transformer
grounding point and it is a good idea to run your grounds like I have explained below.
New black ground wires: Unscrew the four screws holding this pc board in place
and turn it upside down so you can work on it. Make sure you have drained the capacitors
first by touching the red and orange wires to the black wires. If you touch all four wires
together for 30 seconds, you will be safe and the caps will have bled down enough so as
not to shock you.
You are going to need one piece of black 18ga stranded wire that is long enough
to go from the cap can pc board, through the hole in the chassis and end up over by the
power transformer. This wire will be grounded on the same power transformer bolt as the
other major grounds in the amp.
Find the spot where the two black wires are joined to the cap can pc board and
remove the two black wires. Take your piece of black 18ga stranded wire and strip enough
of the end of the wire so that you can bridge across the gap and connect the wire end to
both places on the pc board where the old black wires were. You can just solder the wire
flush to the surface of the copper pc board traces. You may have to scrape the copper
clean to get a good solder connection. You do not have to stuff the new black wire into a
hole, just laying it on the copper surface and soldering it to the surface will work just
fine. Ok, now you should have just one black wire that is connected to both grounds on the
cap can pc board. The other end of this black wire gets a ring terminal crimped and
soldered to it. It is bolted down to the chassis on the same power transformer bolt as the
power cord green wire, the black wire from the end of the board kit and the red/yellow
center tap wire from the power transformer.
These are the major grounds of the amp and it is a good idea to put a soldered
ring terminal on all the wires and use one of the power transformer bolts to make a good
chassis/ground connection. Do not try to solder wires to a chassis, always crimp and
solder a ring terminal and bolt down your major grounds.
So basically you just replaced two separate ground wires with one big one. Stage
one and two will share a common ground wire. This method has been tried and tested and
makes for a very quiet amplifier, please do not try and re-invent the wheel here.
While I am on the subject of grounds, do not try any of that star ground crap
that some techs preach. The star grounds create more problems than they fix. This method I
am describing here has been proven to produce a very quiet amplifier as far as ground hum
is concerned.
Now that we have our grounds for stage one and two taken care of we need to take
care of the two positive wires for stage one and stage two. I replace both the red and
orange wires with a new piece of stranded 18ga red wire. If you are using the old red and
orange wire, you can skip this section and go to Stage three and four.
New red and orange positive wires: You will be replacing the red and orange
wires with two red 18ga stranded wires. You will need to figure out how long each piece of
red 18ga stranded wire should be and then solder them in place of the red and orange
wires. The red wire is stage one and the orange wire is stage two of the power supply
filtration. The red wire is connected to the positive end of the two 100uf caps and the
orange wire is connected to the positive end of the two 47uf caps. Look at the layout
diagram and find the turret lugs for stage one and two and solder the wires to the correct
spot. Flip the cap can pc board over and bolt it down to the chassis. Put the cover back
on and screw it down. You have just finished stage one and two of the power supply
filtration circuit.
Stage three and four: Stage three and four filter caps are on the main pc board
in the Bassman reissue. There should only be two 22uf/500v caps on that board, but there
are 3 for some unknown reason. We only need two more caps for stage three and four and you
can do one of two things here. You can remove two of the 22uf/500v caps from the main pc
board and reuse them or you can use two new caps. I reuse two of the original caps, but
the leads are kind of short. New caps are nicer because you have longer leads, but the old
caps work just fine unless they are 15 years old. Get new caps if the caps are really old.
The best way that I have found to mount stage three and four caps is this. Your
main board kit circuit board should already be installed and all the wires in the amp
should be installed except the wires that lead to the pots. Install the pot harness, but
do not install the pot wires yet. Take the two 22uf/500v caps and make a little hook at
the very end of the negative wire. This hook will hook onto the pot harness ground buss
wire that runs down the back of the pots.
Put the two 22uf/500v caps down inside the amp, underneath the pot harness so
that they are parallel to the front pot panel and as close to the front metal panel as is
possible. Make sure the caps are not near the high voltage power supply rail that runs
down the front of the circuit board. The caps are sitting on the blank area of the circuit
board, towards the pots side of the chassis. If you move the caps around, you will find a
spot that lets you connect the positive ends to the correct lugs on the circuit board and
the negative ends will be facing each other. In other words, the positive ends are far
apart from each other and both caps are in a straight line like a couple flashlight
batteries.
Face the two negative cap ends towards each other and wrap the hook onto the pot
ground buss wire. Solder the negative ends in place. I like to take a piece of the PVC
coating off the red 18ga wire and slide it onto the positive ends of each 22uf/500v cap.
This lets you know that the wire is hot and to keep your fingers away and keeps you from
accidentally touching the positive end of the cap. Make a small hook on the positive end
of each cap and solder each cap to stage three and stage four on the circuit board.
You will notice that stage three and four each have several lugs that the
22uf/500v caps can be soldered to, you do not have to solder it to the lug that you see on
the layout diagram. If three lugs are all connected in a row with a piece of buss wire,
then any of those lugs will work. That is called a buss and you can figure out which lugs
work best for the stage three and four capacitors.
Stage three and four capacitors do not have very long leads and this is a good
thing. The ground noise level is very low after doing the capacitors and grounds the way I
have described.
One last ground tip: The ground wire coming from the preamp end of the board kit
should be soldered to the pot buss wire or to a input jack ground tab. If you use an input
jack for the grounding connection, make sure the jack is making good contact with the
chassis. I prefer to solder this ground wire to the pot buss harness wire. Again, do not
do any star grounds; you will have problems and ground noise. I have removed many star
grounds on amps over the years because people were told "This is great, you must do
it". There is more resistance in a piece of wire than there is in a good chassis
ground. Chassis grounds have always worked well if done properly. Look how many millions
of amps were built that way and work just fine. The only reason you see some Fenders
having problems is because the brass ground plate under the pots corrodes and is not
making good contact with the chassis. I prefer not to use the brass plates and do it like
Marshall did it, solder a buss wire down the back of the pots and let all the pots make a
ground connection to the chassis. The ground currents make their way through the chassis
and back to the main grounding point at the power transformer. You should have four ground
wires connected to ground through a bolt on the power transformer. The power cord green
wire, the black wire from the end of the board kit, the red/yellow center tap wire from
the power transformer and the black wire from the filter cap can.
