Hoffman
Board Kit Instructions
This
kit should contain, depending on the kit.
1- Set of instructions (You are reading them now).
1 - Layout diagram.
4 - #12A 1Ú4 input jacks.
4 - 68k input jack resistors.
2 - 1meg input jack resistors.
1 - Circuit board with all mounting hardware (standoffs, screws,
nuts)
1 - Pot harness assembly.
2 - 1.5k Fender or 2 - 5.6k Marshall or 4 - 1.5k Vox grid resistors.
2- 6.8k mid control resistors on AB763 kit
1 – 220k reverb jack resistor on AB763 and Reverb unit
kits.
4 - Round terminal lugs for chassis ground connections.
Assorted green, red, black and white wire.
Shielded cable for input jacks.
Heat shrink tubing for shielded cable
WARNING: DO NOT WORK ON ANY AMPLIFIER
WITH THE AC PLUG PLUGGED INTO THE WALL SOCKET! ALWAYS DISCHARGE
THE FILTER CAPS BEFORE WORKING ON AN AMPLIFIER!
IF YOU ARE UNSURE OF HOW TO DO SOMETHING PLEASE TAKE IT TO A
TECHNICIAN!
NOTES:
* When you see this on the layout diagram 820/220. It means
there is an 820 ohm resistor with a 220uf cap in parallel across
the resistor.
* V12= Valve 1, pin 2
* The numbers on the pots go to the same number on the board.
Example: There is number 1 on volume pot. Find the number 1
on the board and connect the two points with a wire. Installation
instructions for add on boards.
1. Place the circuit board inside the amplifier where it will
be installed and use a sharp object or a pencil to mark the
location of the mounting holes. Make sure to look under the
chassis to make sure that you are not going to be drilling into
an object such as a transformer on the other side of the chassis.
If the mounting holes happen to be where a transformer is, then
you can either move the transformer or shim up the transformer
using washers so that transformer is not setting on the screw
heads of the circuit board mounting screws. You can also make
a new mounting hole in the circuit board and move the mounting
point.
2. Remove the board and use a punch to mark where the holes
will be drilled. Drill the mounting holes using a 11/64 drill
bit. The bit should be just slightly larger than the mounting
screws, so it will be easier to position the board.
3. Mount the board into the amplifier using the supplied circuit
board standoffs between the chassis and the board. If possible
keep the heads of the screws on the chassis side and the nuts
and washers on the inside of the chassis. If you are also installing
transformers, you may want to install them first if any screw
heads are going to be under the board.
4. Face the front of the amp towards you just like the wiring
diagram and start with the first pre-amp tube. On the pre-amp
tubes you should do each pin in a counter-clockwise logical
sequence. Use the following wire colors when wiring the amp.
(PIN
3, CATHODE 1, V1A -- BLACK WIRE) (PIN 8, CATHODE 2, V1B -- BLACK
WIRE)
(PIN 2, GRID 1, V1A ------ WHITE WIRE) (PIN 7, GRID 2, V1B ------
WHITE WIRE)
(PIN 1, PLATE 1, V1A --------- RED WIRE) (PIN 6, PLATE 2, V1B
--------- RED WIRE).
HEATER NOTES: Pins 4 and 5 are connected together and are one
side of the 6.3 volt ac heaters. Pin 9 is the other side if
the heaters. The heater wires should already be wired up on
your amp. If you are building from scratch you will have to
run your heater wires in a serial twisted pair manner to each
tube socket. Use green 18 gauge stranded wire for the power
tubes and 20 gauge solid for the pre-amp tubes. Run the twisted
pair heater wires about an inch over the top of the tube sockets
and come straight down to the pins. Some amp builders like to
keep the pins the same, socket to socket. For example, pin 9
on the first pre-amp tube would continue on to pin 9 of the
next tube and so on down the line. The power tube heater pins
are 2 and 7, and you can do the same thing when you wire them.
5. Attach each wire to a turret lug first and then to the tube
socket pin last. Starting with PIN 3 of the first pre-amp tube,
strip enough of the black hook up wire so that it can wrap around
the base of the turret lug. If you make a half loop around a
piece of tubing or something roughly the same size as the lug
it is easier to form the wire around the base of the lug. Solder
the wire to the turret lug with enough heat to solder the wire
correctly, but not so much heat that you unsolder the components
from the lugs. Then lay the wire out and run it up to the pin
on the tube socket. The wires should not be over the top of
the tube socket. They should go around the socket on their way
to the pin they will be connected to. The heater wires going
to PINS 4, 5 and 9 usually are suspended in the air above the
socket and you do not want to be close to those wires. Cut the
wire to the correct length. The wires should not be too long
or too short. They should be as short as possible with just
a little curve for flexibility. Strip the wire and solder it
to the correct tube socket pin. Check the lay out diagram often
to see if you are connecting everything correctly. You may want
to keep the red wires, going to PINS 1 and 6, away from the
black and white wires. You can suspend the red wires above the
other wires or the other way around but they should not be laid
next to or parallel to the black and white wires. The red plate
wires contain the high voltage and may induce noise into the
other wires. Solder all the pre-amp tubes first and then go
on to the power tubes. (TIP: If the solder on the top of the
turret lug gets sucked down into the lug, wait until the lug
has cooled down and then return to the lug to add a little bit
of solder around the hole in the lug where the components are
inserted. Never keep adding solder to a hot lug that keeps sucking
it in. First, you are probably heating up the component too
much and secondly the lug may be dripping solder out the bottom.
This may lead to a solder blob that could touch the metal chassis
and short out the board. Solder all the wires to the tube sockets
and then come back and resolder the tops of the lugs all at
once after everything has cooled down.)
POWER TUBE NOTES: The power tubes cathode, PIN 8 and PIN 1 (BLACK)
are connected together and then soldered to the 1 ohm resistors
on the board. The current of each power tube can now be checked
across each 1 ohm resistor. Also with PIN 1 and 8 tied together
you could use EL34 or 6550 power tubes if your power transformer
can handle it. PIN 3, the plate, should already be connected
to the output transformer. PIN 4 is the screen (red wire) and
it is connected to the (1K/5W or 470 ohm/3W) screen grid resistors
on the board. PIN 5 is the grid (white wire) and it is connected
to a (5.6K or 1.5K) resistor that is soldered to PIN 6. PIN
6 is then connected to the junction of the 220K resistor and
the .022 capacitor on the board. PINS 2 and 7 are the heaters.
6. Install the pots into the front panel. Install the jacks
into the front panel. Connect the bare ground buss wire from
the pots to the ground of the jacks. Connect the black ground
wire from the left end of the circuit board to the bare ground
buss that runs along the backs of the pots and goes to the jacks.
Install the two 68K and one 1M resistors on each set of jacks.
The shielded cable should be soldered to the input jacks and
then go to the first tube to pins 2 and 7. Cut the cable to
the correct length by routing it directly from the two 68K resistors
to the tube socket. Leave just a little slack in the shielded
cable. Strip the cable and connect the center wire to the junction
of the two 68K resistors. Connect the shield of the cable to
the ground buss on one of the jacks. Strip the tube socket end
and remove the outer shield. You will only connect the center
wire to the tube socket pin. Use some heat shrink to cover the
outer shield on the tube socket end so it does not touch anything
and cause problems. Shrink the heat shrink with the barrel of
your soldering iron.
7. Connect each pot connection using the white wire. Strip one
end of the white hook up wire and solder it to the pot tab and
then find the location on the board where the wire will be attached.
Cut the wire as short as possible leaving just a little play
in the wire. Strip the end of the wire and insert it into the
hole in the top of the lug. The wire will be in the hole with
the component. Solder the top of the lug carefully using just
enough heat. A sharp soldering iron tip is a big help when doing
fine soldering like this.
8. Hook up the feedback wire. Soldered a white wire to one end
of the FEEDBACK RESISTOR and solder the other end to the speaker
tap you are going to use. (8 ohm in MARSHALL'S)
9. Find the black ground wire at the right end of the board
and connect this wire to the chassis ground. If the amp has
a center tap, connect the ground wire to the chassis where the
center tap of the power transformer is connected.
10. Connect each filter cap positive end to the correct location
along the power supply rail. This rail is along the front edge
of the board closest to the pots. Look at the layout diagram
to see where each filter cap should be connected. Wrap the wire
around each turret lug and solder it. The filter cap grounds
should technically be connected as close as possible to the
cathode of the tube that they go to, but this is usually not
possible. In FENDER style amps the filter caps are in a can
on the back of the amp and you can use the stock method of filter
cap grounding. In MARSHALL style amps each cap is grounded to
the chassis and flows through the chassis back to the center
tap.
11. If your power transformer doesn't have a heater center tap
then solder a green wire from each heater wire on the first
power tube (PINS 2 AND 7) and connect the two wires to the two
100 ohm resistors on the right end of the circuit board. If
you are running your own heater wires, then solder the heater
wires from the power transformer to the two 100 ohm resistor
lugs and then connect all the heaters starting from the circuit
board. The two 100 ohm resistors make an artificial ground to
prevent hum.
12. Connect the bias tap from the transformer to the bias range
resistor on the board. This resistor may have to be changed
up or down in value to get a correct bias range.
13. Connect the high voltage wires from the power transformer
to the two diodes on the board if your board has an on the board
rectifier. If you have a tube rectifier, pin 8 of the rectifier
tube should go to the standby switch first and then from the
standby switch to the B+ point on the power rail. The MARSHALL
board is set up to use a half wave rectifier like the original
amps. Some MARSHALL'S and other amps use a bridge rectifier
instead. If your power transformer uses a bridge rectifier we
can supply you with a separate bridge that you can bolt down
on the chassis, or on the board.
14. Connect the choke from the B+ to the next lug on the power
rail to bridge the gap between B+ and the screens.
15. After all connections have been made and double checked,
(please take the time to double check all connections). Power
up the amp with no tubes in the sockets. Using a multi-meter
set on DC volts, check to see if pins 1 and 6 on the pre-amp
tubes have high voltage. Check pins 3 and 4 on the power tubes
to see if they have high voltage. Check pins 5 on the power
tubes to see if they have negative voltage. Turn the bias pot
so that pin 5 has the most negative voltage that the bias circuit
can produce. This is done so that the power tubes will be over
biased when you first turn on the amp with the tubes inserted.
You should have at least negative 50 or -50 volts available
in the bias circuit. If not, the bias range resistor in the
bias circuit cuts down the AC voltage coming from the power
transformer and you will have to raise or lower the value to
get a correct bias range. Switch your meter to AC volts and
make sure you have roughly 6.3 volts of heater voltage across
the tube sockets. If all the voltages look correct, turn off
the amplifier.
16. Insert all tubes into their sockets. Power up the amp and
let it warm up for 30 seconds. Connect your multi meter across
one of the 1 ohm power tube cathode resistors. (positive closest
to the tubes, negative closest to the front of the amp.) Set
your meter on DC milivolts. You are going to watch the power
tube current as you turn on the standby switch. Be prepared
to quickly turn the standby switch off if the current is too
high on the power tubes. Turn on the standby switch and watch
the meter. If the current is below 50 milliamps (milivolts converts
directly to milliamps) then you can bias the amp to where ever
you are going to bias it. Between 30 and 40 milliamps is good.
If the current rushes up to above 50 milliamps you may not have
enough bias voltage and the bias range resistor in the bias
circuit may have to be lowered in value.
PERFORMANCE MODS. Most of the kits are true to the original
schematics, but there are a few things that have been done to
some of the kits that are improvements. Some of the boards include
better bias systems because some of the old amps did not have
bias pots.
Some kits have the 1 ohm cathode resistors on the mother board
which makes biasing a breeze.
The main filter caps shown along the front edge of the layout
diagram are only suggestions. You can use whatever values you
prefer.
Any of the kits that have four inputs or two channels can be
bridged internally at the first pre-amp tube. If you join pins
two and seven together and just use one pair of input jacks,
you can use the two volume controls as mix controls. You can
then mix in a bit of the bass heavy channel with the brighter
channel without using a external guitar cable between the two
sets of input jacks.
Some of the kits have channels that are bright channels. They
are way too bright for my personal taste and I like to remove
the bright caps that jumper across the volume pot. The plexi
kits have a bright bypass cap (500p) on the channel mixing resistors,
(470k) on the main board. You can decide if you like it there
or you can remove it.
If you do not need two channels on your amp then you can use
the second half of the first pre-amp tube as an extra stage
of gain. I used to do this on HOFFMAN amps and it seems to be
quite useful and makes the amp more versatile. If you use a
DPDT mini toggle switch there is no channel switching tone killers
in line with the tone path and the amp goes back to stock with
the flick of a switch. The layout diagram is on the web site
in the info section. This is an unsupported mod, please do not
ask me to help you get it working, it does work.
You can increase the tremolo strength on the AB763 board by
finding the 470K resistor that leads to two .1 caps. This resistor
is in the lower right part of the tremolo circuit on the layout
diagram. Change the 470K to a 220K.
