100 watt amp kit step by step

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100 watt amp kit step by step
I NSTALLAT I ON GUI DE
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METROPOULOS AMPLIFICATION
1045 ADAMS RD.
BURTON, MI 48509
810 614 3905
www.metroamp.com
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I NSTA L L ATI ON G UI D E
STEP 1
The first step is to take inventory of all the parts in your kit. You will
find a packing list included with your documentation.
Next, you’ll want to set up a workspace and start assembling. A good
way to start is to glue the plexiglass panels in place. Some clamps go
a long way for holding them in place.
Be sure to give them ample time to dry before continuing.
You might even cover them with masking tape to avoid scratching them
during assembly of your amp.
I NSTA L L ATI ON GU I DE
STEP 2
With your panels in place,you can start installing the transformers
and choke. The mounting holes should line up and wires should only
pass through holes that have rubber gromets installed.
A
Power and output trannies are secured using the large brass bolts
with lock nuts. And the choke uses the medium brass hardware.
Be sure to mount the OT rotated 90 degrees to the PT for
hum cancellation.
B
Once installed I use small spring clamps to hold the wires together
and out of my way.
A
B
I N STAL L ATI O N GU I DE
STEP 3
Next, the tube sockets get installed. The output tube sockets each get
a ground lug on the bolt closest to the rear of the chassis. Be sure that
the notch near pin 1 points towards the the rear, as well.
Don’t forget to install the tube retainers on top of the chassis.
The output tube sockets use the medium brass bolts and the preamp
sockets use the small brass bolts.
I N STAL L ATI O N GU I DE
STEP 4
A
Next, install the filter caps into their mounting brackets. Be sure
the negative terminal is in line with a mounting tab on the bracket
as several of them will attatch to ground lugs.
B
The caps mount with medium brass bolts and all but two get
ground lugs. Those two are shown below and are mounted closest
to holes for the switches and indicator lamp. The cap nearest the
fuse holders (top right) will get an extra double hole ground lug
for transformer wires.
C
Two of the caps also have a 56K 2 watt resistor mounted between
their two positive terminals and ground terminal (shown below).
These resistors and the ground connections should be soldered
now. Grounds are made with 18 gauge buss wire.
B
A
C
I NSTAL L ATI ON GUI DE
STEP 5
A
1 ohm resistors mounted on the output tube sockets are an easy
way to measure and adjust bias. One should be wired on each
socket between pins 8 and 1. The extra lead from pin 1 shouldgo
to the ground lug.
B
Next locate and twist tightly the two black wires from the PT.
These are the heater wires. They attatch to pins 2 and 7 of the
output tube socket farthest from the preamp tube sockets. The
green wire is the center tap for the heater winding which attatches
to the ground lug on the nearby filter cap.
C
A second green wire is the bias winding common tap, It should
also be grounded on a lug. The 220V and 240V taps aren’t being
used in this amp so they’ve been cut short and had shrink tube
applied as an insulator. They are neatly cable tied with the other
remaining wires.
B
C
A
I NSTAL L ATI ON GUI DE
STEP 6
This is one of the most tedius parts of the whole job: wiring the heater
connections. But it’s worth doing well for better hum rejection. Not
to mention looking pro. The wires (red and black) should be twisted
tightly and only as long as necessary.
A
The output sockets will be wired in parallel to pins 2 and 7.
B
The preamp sockets will be wired in parallel to pins (4 and 5
together) and 9.
A
Take your time, twist tight and remember that doing a good job will
reduce noise in your amp significantly.
B
I NSTAL L ATI ON GUI DE
STEP 7
A
Next you can mount and wire the impedance selector switch. It
mounts with the small brass bolts. You’ll want to add a purple
wire to the wires from the OT and cable tie them.
FOR 100 WATTS THE COLOR CODE IS:
Brown – common
Yellow – 8 ohms
Black – 4 ohms
Green – 16 ohms.
The purple wire is the negative feedback wire and normally attatches to
the 8 ohm tap in amps with EL-34’s. You can try it on any of the taps.
B
Now, using the yellow wire, connect pins 6 of the output sockets
to each other. Also connect a length of wire to the socket closest
to the preamp sockets. This will go to the circuit board.
B
C
Locate two 1.5k resistors (brown-green-red) and
extend one of each of their leads with buss wire
and cover them with the insulating material. One
of these will go between pins 5 of each pair of
output sockets. The left two and the right two.
C
A
I NSTAL L ATI ON GUI DE
STEP 8
A
Locate the (4) 1K 5 watt resistors. They mount on the output tube
sockets, between pins 4 and 6.
B
Next, mount the speaker output jacks. The brown wire from the
OT attatches to the terminal closest to the chassis. So does the
black wire (red arrow) which connects to a ground lug on an
output tube socket mounting bolt.
C
The terminals on the jacks all get connected with 18 gauge buss
wire. I also extend the buss wire from the terminals farthest from
the chassis, over to the impedence selector switch. Some insulation
here is a nice touch.
A
C
B
B
I NSTAL L ATI ON GUI DE
STEP 9
A
Now, install the (2) toggle switches. The red
wire from the PT attatches to the lower two
terminals of the power switch. The yellow wire
from the PT attatches to, and connects together,
the positive terminlas of the filter cap closest to
it. The orange wire attatches to the mains fuse
holder (4 amp fuse).
B
The blue and grey wires get twisted together
and attatch to the lower two terminals of the
standby switch. At this point all then PT wires
should be attatched except the white bias tap
wire which connects to the circuit board.
A
B
A
C Next, using red wire, make jumpers bewteen
the filter caps as shown. Also, run red wire from
the filter cap nearest the power switch, to the 1
amp fuse holder. And run a red wire over to
where the circuit board will mount. Make sure
to leave some extra wire.
C
You’ll need to run a length of yellow wire
from the filter cap with the 56K resistor, too.
Cable tie these together.
I N S TAL L AT I O N GU I DE
STEP 10
A
A
Attatch red and black wires to the indicator lamp and install it.
Check to see if you have a 120VAC or 6.3VAC lamp. The 120V
lamp will be wired as in the pics. The 6.3V lamp is wired to the
heater connections on the last output tube socket.
Twist the wires tightly. One wire attatches to the lower power
awitch terminals with the red wire from the PT. The other wire
attatches to the mains fuse holder with the orange PT wire.
Cut two lengths of blue wire and solder them to the upper
terminals of the standby switch. Twist them and run them over
to where they will later be wired to the circuit board.
Solder a length of blue wire to the filter cap near the fuse holders.
Run it over towards where the circuit board will mount and, like
the other wires, leave plenty of extra.
B
Attatch a length of red wire to the high voltage (HT, 1 amp) fuse
holder and run it over with the blue wire. Cable tie these wires
where ever necessary. They should now look like a wiring harness,
as in the pictures.
B
A
I N S TAL L AT I O N GU I DE
STEP 11
Now locate the 6 pots. Each one should be
installed with a lock washer on the inside of the
chassis and a flat washer and nut on the outside.
Their values from left to right are:
PRESENCE
BASS
MIDDLE
TREBLE
VOL1
VOL2
5K linear
1 meg audio
25k linear
250k linear
1 meg audio
1 meg audio
The bodies of the pots should be connected with
a piece of 18 gauge buss wire. A buss wire should
also connect terminal 3 (farthest left on each pot)
to the ground buss for the volume controls and
middle tone pot.
A .1uf 100V cap is mounted on the presence
pot, with one lead connecting to pin 2 (center
terminal) and the other lead conneting to the
ground buss and terminal 3.
A black ground wire attatches from the ground
buss to the ground lug on the filter cap near the
output tube sockets. Another connects the
ground buss to the ground lug on the filter cap
near the preamp tubes sockets.
You could also attatch a 500pf “bright” cap across
pins 1 and 2 of the ch 1 vol. control at this time.
I prefer to wait so I can tweak the amp with the
perfect value cap, which usually ends up between
100-500 pico farads (pf).
I N S TAL L AT I O N GU I DE
STEP 12
A
A
You should assemble the pairs of input jacks before mounting
them. Follow the diagram (above left) and install the 1meg resistor,
ground buss wire and crossover wire. Green wires will also attatch
to the terminals I’ve labled as “high input” and “low input”. This
coresponds to the high and low sesitivty inputs for each channel.
The jack with the 1 meg resistor is the high input. This can get
confusing when the chassis is turned upside down.
B
Ground wires should be attatched to the lower right terminal
(with the 1 meg and buss wire) of each input assembly and ran
to the ground buss on the pots.
C
Next, red wires need to be soldered to the positive terminals of
the filter cap near the preamp tubes. They need to be left approx.
8 inches long for installation to the circuit board later.
B
C
A
HI I NPUT
L OW I NPUT
I N S TAL L AT I O N GU I DE
STEP 13
A
Next, install the circuit board mounting bolts,
there are six. You should check their alignment
with the circuit board before tightening them.
When the fit is right, you can start on the fun
part…loading the board, right?
Actually…it’s easier to wire the board before
loading it. But don’t worry, you’re getting close.
Included with these instructions will be a full page
diagram. You’ll want to refer to it for the wiring of
the buss wire and insulation. They should go on the
board before anything else.
B
Once installed, your board should look like the
one below.
B
A
I N S TAL L AT I O N GU I DE
STEP 14
A
A
Now you can start wiring the board into the amp according to
the diagram. It’s easiest to start with the wires you ran in the power
supply section. From there you can work your way to the left. I
prefer to wire the power supply wires, followed by the wires on
the input side of the board and finally the wires to the preamp
sockets.
This step is the most time consuming, and it’s worth taking your
time and making nice solid connections between the wires and
terminals.
A helpfull tip for wrapping the wire around the terminal is to strip
about a 1/4” of insulation, then apply a small amount of solder to the
end so the bare wires stay together. Now bend the wire around the
terminal with needle-nose pliers and apply solder.
B
I also find it easier to connect the wire to the terminal before
feeding it through the hole in the board. It’s a pain to do right,
but nicely wrapped connections make for a great looking amp.
Don’t be afraid to clip off the end and start over on a wire just
won’t cooperate.
B
A
I N S TAL L AT I O N GU I DE
STEP 15
Now, finally, comes the fun part. Installing the components on the
board. Just like the wiring, attention to detail here makes all the
difference. It’s worth taking the time to straighten all the component
leads with needle-nose pliers. Next, hold the component in position
over the terminals it attatches to, with the pliers grab one of the leads
where a bend should go. Bend the lead and cut it off so about an
1/8” is left to go in the terminal. Put the component back in place
and repeat the bending and cutting. It should now fit perfectly with
straight leads going to each mounting point.
Be sure to observe the polarity of the diodes and electrolytic caps
(10uf bias caps and 250uf cathode cap). The positive side of the bias
caps actually go to ground near the bias pot.
Don’t forget to install a bright cap on the channel II volume pot, between
pins 1 and 2.
A
You also need to install a 100K resistor on the second preamp tube
socket. One lead should connect pins 1 and 7, and the other end
should attatch to pin 6.
When you’ve gotten this far, take a break and congratulate yourself
however you see fit. You’re so close to firing up your amp that your
neighbors are getting nervous.
A
I N S TAL L AT I O N GU I DE
STEP 16
The final thing to install is the AC cord and strain
relief. Strip 6-8 inches of insulation from the end
of the cord and run in through the hole in the
chassis. Make sure there’s anough wire inside to
reach the poswer switch. Wrap the strain relief
around the cable and compress it using pliers. You
should be able to push it into the hole.
The white wire attatches to the open terminal on
the mains fuse holder. The green wire gets a ground
lug and needs to be bolted to the chassis with it’s
own bolt to meet code. Finally, the black wire runs
to the power switch and attatches to the the top
two terminals.
Install the fuses (4A main and 1A HT) and look
over your work. Take some time to double check
any components or wiring you’re not sure about.
It’s better to catch something before powering up
the amp.
If you’re satisfied and with no tubes installed, plug
in the AC cord, cross your fingers (just kidding)
and turn on just the power switch. Assuming there’s
no explosions, you can take some voltage readings.
Set your meter to AC volts on the 10 volts or higher
range. Put the black lead in one of the chassis
mounting holes and touch the red lead to pins 2
and 7 of each of the output tube sockets. You should
read approx 3.25 VAC. You should get the
same reading on pins 4, 5 and 9 of the preamp
tube sockets.
If all is well, set you meter to DC volts on the 100
volts or higher range. Touch the red lead to pin 5 of
each of the output tube sockets, you should read
anywhere between -30 and -50 VDC. Adjusting the
bias pot should change this reading.
If you don’t get these readings, check the black wires
from the PT for the heater voltage and the white
wire from the PT for the negaitve bias voltage. Make
sure these are right before moving on.
I N S TAL L AT I O N GU I DE
STEP 17
I like put my meter in the extra
space in the chassis. Notice the
black lead in the mounting hole.
This allows you to put only one
hand in the amp at a time. The
other hand should go in your
pocket to avoid touching any
part of the amp that might use
your body to complete a circuit.
With no tubes installed yet, and
if your readings are correct so
far, turn on the standby switch.
With your meter set to the
highest DC volts setting,
measure the voltage on pin 3 of
each output socket and on each
side of the 1k 5 watt resistor.
Be extremely carefull as these are the highest voltages in the amp.
Your readings should be in the 450-500 volt range.
If so, turn the amp off and install the preamp tubes. Turn on the power switch with the standby off and
watch to see that the 12ax7’s light up. When they do, flip the standby on. You can now reference your
readings to the included voltage chart. Keep in mind that your readings may vary from the chart by 1020%. The important thing is that the readings are proportionate to those listed.
For example, if your highest voltage reading is 20 volts less than the listed voltage, that may be a 5%
difference. All of your readings should be approx 5% less than those on the list.
When you’re happy with your maesurements, turn the power switch off but leave the standby switch on.
This will allow the filter caps to drain their stored voltage. Now install the EL-34’s, turn the bias pot all
the way towards the front of the amp, plug in a speaker and turn the standby switch to off. Flip the power
switch on and let the EL-34’s heat up.
I N S TAL L AT I O N GU I DE
STEP 18
Now turn the standby on. Slowly bring up the
volume control and listen for any squealing,
humming or otherwise unnatural sounds. If there
are none, turn the volume back down. Set your
meter to DC millivolts and touch the red lead to pin
8 (with the 1 ohm resistor) of any of the output
sockets. This will tell you your bias current, which
should be quite low. Next turn the bias pot up about
half way and measure on pin 8 again. The reading
should be much higher. Continue this process until
you read 35mV. That indicates that the tube in that
socket is properly biased at 35mA. Check the other
tubes in the same way. They should vary by a few
mV’s, but normally not more than 5mV’s. You should
be able to find a setting that puts all the tubes between
30 and 40ma’s.
Take some time to play the amp now, test each input,
make sure the controls all do what they’re supposed
to. Watch the ouput tubes to make sure they don’t
ever glow red. If they do, increase the bias voltage.
After playing the amp for a while, recheck the bias
as it will drift.
If all is well, CONGRATULATIONS! Next, you can
start to tweak the amp to your liking. Please visit the
forums at: www.metroamp.com/forum to share your
experience and for tech support and mod suggestions.

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