VACUUM TUBE BUILDER TIPS
Building vacuum tube circuits requires some special knowledge. Probably the most important is working with high voltage. Voltages used in vacuum tube audio circuits can be as high as 500 volts. Besides being careful what you touch, special care must be taken when wiring circuits.
The wire you use must have insulation rated at least 600 volts. Pre-amplifiers, voltage amplifiers and other low level audio circuits that do not involve power amplifiers do not draw a lot of current, 22 - 26 gauge wire rated at 600 volts will work fine. Some of the new Teflon insulated wire available works well but is more costly than standard wire.
Some colors were considered standard when wiring vacuum tube circuits so that at a glance you would know what some circuits were. Here is a list of colors.
BLACK = GROUND
YELLOW = FILAMENT
RED = HIGH VOLTAGE
BLUE = PLATE CIRCUIT
GREEN = CONTROL GRID CIRCUIT
AVAILABLE FROM
ALLIED ELECTRONICS
26 GAUGE WIRE
BELDON 600V
TEFLON INSULATION
(last update 11/2007)
STOCK #
214-1080 - VIOLET
214-1081 - WHITE
214-1082 - BLACK
214-1083 - RED
214-1084 - GREEN
214-1085 - YELLOW
214-1086 - BLUE
214-1087 - ORANGE
214-1088 - SLATE
214-1089 - BROWN
You must be able to read and follow circuit drawings and know which way polarized capacitors are put in a circuit. If you are new at this it will take awhile to get the hang of it. If in doubt about something, ask someone who knows.
PICTURED ABOVE, SCHEMATIC DRAWING ON LEFT AND SAME CIRCUIT ON RIGHT
PICTURED ABOVE, DOTS SHOW WHERE CONNECTIONS ARE MADE BETWEEN COMPONENTS
BLUE ARROWS POINT TO AREAS WHERE CIRCUITS CROSS BUT DO NOT CONNECT ( NO DOT )
USING TWISTED PAIR FILAMENT WIRING
THE FILAMENT WIRING FROM TUBE SOCKET TO TUBE SOCKET IS NORMALLY DONE WITH A TWISTED PAIR OF WIRES. THE THEORY FOR DOING THIS IS THAT THE AC FILAMENT VOLTAGE HUM LEAKAGE WILL CANCEL ALONG THE TWIST AND REDUCE HUM PICKUP IN CRITICAL STAGES. ALTHOUGH THIS IS TRUE TO SOME EXTENT, EXPERIENCE HAS SHOWN IT ONLY SLIGHTLY REDUCES HUM PICKUP. THE BEST PRACTICE IS TO USE DC VOLTAGE FOR THE FILAMENTS OF ALL TUBES EXCEPT THE RECTIFIER OR POWER OUTPUT TUBES. WITH DC YOU DO NOT HAVE TO TWIST FILAMENT WIRES AND YOU CAN IMPROVE HUM AND NOISE FIGURES BY 5 TO 10db.
Capacitor values are usually in MFD ( microfarad )
( microfarad. A unit of capacitance; one-millionth of a farad )
For values less than .001MFD values may be in MMFD ( micro-microfarad )
Another notation is PF ( picofarad ) and is the same as MMFD
EXAMPLES
331 = .00033MFD = 330PF
101 = .00010MFD = 100PF
Values below 1000PF may be printed in actual value or number code
EXAMPLE
470 ( number code ) = 47PF
or
it may be marked at value 47
TUBE SOCKETS
Viewing a tube socket from the bottom ( terminal side )
count pins clockwise, pin 1 is just left of the open space.
WIRE STRIPPERS
Wire strippers like below are not expensive and well worth getting
Wiring in a home built vacuum tube project is usually point to point. This is where components go from terminal to terminal. All the wiring must be hand soldered and soldering must be done properly. The solder must flow evenly and be smooth.
HERE'S HOW TO DO IT...
Use long nose, needle nose, pliers and neatly secure wire around terminal.
ORIGINAL ILLUSTRATION COURTESY OF KNIGHT KIT 1. Make good mechanical connections and keep resistor and capacitor leads as short as possible, unless specified otherwise.
2. You will need at least a 25 watt soldering iron, a 40 watt iron would be better. Coat the tip of the iron with solder. Then firmly press the flat side of the tip against the parts to be soldered together. Keep the iron there while you......
3. Apply the solder between the metal to be soldered and the iron tip. Use only enough solder to flow over all surfaces of the connection. Remove the iron.
DO NOT MOVE PARTS UNTIL THE SOLDER HARDENS. If you accidentally move the wires as the solder is hardening, apply your iron and reheat.
4. Compare your soldering with the pictures on this page. You have a good connection if your solder has flowed over all surfaces to be connected, following the shape of the surfaces. It should appear smooth and bright and all wires in the connection should be well soldered.
You have not used enough heat: if your connection is rough and flaky-looking, or if the solder has formed a round ball instead of spreading.
REMEMBER, LARGER METAL SURFACES TAKE A LONGER TIME TO HEAT.
There are different types of solder available. You need an electronic type solder that has flux inside the solder. The type of flux is important.
NEVER USE ACID CORE SOLDER. Solder with acid flux will eat into your connections and ruin electronics. The most reliable is rosin core solder. Rosin flux left on a connection is the least destructive. There is solder with organic flux available now but organic flux MUST be completely cleaned off your connections or it will corrode connections over time. Organic flux cleans with water but it is impossible to get all the flux off on point-to-point wiring used in a vacuum tube chassis. 
Using lead-free solder is up to you. However, when using solder with lead, you should wash your hands good after soldering. You can use thin gloves to be extra safe. More of a hazard than working with lead based solder could be breathing in flux fumes. Have a small fan blow the flux smoke away from you, and anyone else nearby, while soldering.
Because of the high voltages, bare (uninsulated) component wire leads must be dressed, or placed, so they are not near other bare wires or terminals. Wire leads connected to terminals should not have wire ends sticking out, wrap wire ends close against terminals.
TUBINGPutting tubing on bare component leads is a good idea. Tubing protects the bare wires from shorting and improves the looks of the wiring. Using heat shrink tubing works well, the tubing is heated and shrinks tight around the wire. Heat shrink tubing will not melt like some plastic tubing does at soldered connections. For most component bare leads, 1/16 inch tubing usually works well.
You can use a lighter or a heat gun to heat shrink tubing
( a heat gun like shown is handy if you can afford one )
Laying out the chassis for drilling and punching requires care. Make sure you use a chassis large enough for your project so components are not jammed in a tight space. Some vacuum tube components such as power transformers are heavy. It's best to mount heavy components in a corner of the chassis where the chassis is stronger. Usually, you would have your power supply and input pre-amplifier stages as far apart as possible to avoid hum pickup.
You will no doubt be drilling and punching your own chassis. There are chassis punches available for punching out holes for tube sockets. These punches are costly but are a lot easier and neater than filing out large holes. Greenlee is one manufacture of punches.
A handy item to have is a tapered reamer for those larger holes if your drill only goes up to 1/4" or 3/8" drill bits.
Also good for making bigger holes is a stepped drill
You will need something to deburr holes, remove the sharp edges after you make a hole.
( for small holes you can also use a larger drill bit to deburr )
BUILDING THE CHASSIS
Finding the correct size chassis:
For vacuum tube components you will most always need at least a 2 inch high chassis, a 3 inch high chassis might be preferred depending on the largest part going inside the chassis. You will need room for power supply parts, power transformer, rectifier tube, filter capacitors, filter choke or power resistors and don't forget a fuse holder. The power transformer usually mounts on the top of the chassis.
The rest of the room in the chassis is for your amplifier components. You will need room for tube sockets, terminal strips, input output connectors (usually in the rear side of the chassis), volume and other controls (usually in the front side of the chassis) and all the resistors and capacitors, etc.
Laying out the chassis:
Take the components that are to be mounted on top of the chassis and place them where you think they will work best. Putting masking tape on the top of the chassis will help with marking drill hole points.
marking the chassis for holes
this chassis has protective plastic on it for marking holes
The best location for the power transformer is in a rear corner of the chassis. The AC cord and fuse holder should also be in the rear corner (back side) of the chassis by the power transformer. All the AC wiring should be kept as far away from amplifying circuits as possible. Keep in mind that you will need room under the chassis for the parts that connect between tube sockets and terminal strips so space your tube sockets and terminal strips. Holes that wires will pass through, AC cord, transformer wires, etc, will need a rubber grommet to prevent the bare chassis from cutting into wire insulation.
Once you have the layout planned, take a pencil and mark drill hole points. For tube socket holes, only mark the big hole for the socket. The little tube socket mounting holes will be marked after you punch out the large tube socket hole.
chassis marked for holes
Drilling:
At this point it is assumed you know how to use a drill. If you are drilling an alluminum chassis, you could use a hand drill.
First, drill all the hole points with a 1/16" drill, this is the pilot hole and will prevent the larger drills from "walking" as you drill. Now go back and drill the correct size holes where needed. Use sharp drills and do not rush drilling. Taking your time and being careful will give you the best results. The larger holes for rubber grommets and chassis punches can be made by drilling a 1/4" or 3/8" hole and reaming to the needed size with a tappered reamer. After you have drilled your holes, de-burr them.
holes drilled to correct size and de-burred
chassis punch ready to punch out tube socket holes
tube socket holes punched out
marking off tube socket mounting holes
always punch the big hole first, then mark the mounting holes
You do not have to lay everything out at once. You may find it easier to first place your tube sockets and power supply transformer. Then lay out terminal strips and other holes making sure you have allowed enough space for parts to reach and fit between connections.
completed drilled chassis
It will be necessary to have a way to check voltages. A multi-meter will be needed, either analog or digital, to check voltages. Make sure you get a meter that can read down to .1 ( 1/10 ) of a volt DC for reading grid bias voltage. A 0 to 5VDC range should be able to read down to .1VDC
There are mainly two different types of capacitors used in vacuum tube circuits. The capacitors that audio pass through are usually dry type non-polarized capacitors. Non-polarized capacitors can be connected any way in a circuit.
NON-POLARIZED DRY TYPE CAPACITORS
Polarized capacitors, usually electrolytic type, are used in power supply circuits and some other vacuum tube circuits. Polarized capacitors are either marked with one lead positive ( + ) or negative ( - ). The positive lead MUST connect to positive voltage and the negative lead MUST connect to usually ground. The exception to this is in a negative voltage supply. In that case the positive lead would connect to ground and the negative lead to the negative voltage.
All capacitors MUST have a voltage rating higher than the voltage in the circuit the capacitor is going in, at least 25% higher.
ELECTROLYTIC TYPE CAPACITORS
FUSE, always put a fuse in the AC line cord circuit so all transformer primaries are fused. The value of the fuse will depend on the current draw but usually in the 1 to 2 amp range for low level audio amplifiers.
SEE CALCULATING FUSE SIZE IN TECHNICAL INFORMATION
And once again, be careful working with high voltage. Even after the unit is turned off or unplugged, there may be charged capacitors.
