Vectronics VEC-1680K User Manual

VEC-1680K Owner’s Manual Vacuum Tube Preamp

INTRODUCTION

General Infor mation: If names like Eric Clapton, Jimi Hend rix, and B.B. King have personal meaning for you, you're gonna love this ki t! Today's solid-state amps can't match the booming sound of the legendary tube-driven powerhouses built by Fender and Marshall. The key to that magic lies with the electron tube-or "valve"--which has much softer saturation characteristics than the sharply clipped response of transistor amplifiers.

With the VEC-1680K in line, your guitar's magnetic pick-ups are terminated with the true ultra-high impedance load they were designed to feed, which unleashes your instrument's natural full-bodied frequency response. And, when you crank up the gain, each note expands in richness with warm even-order harmonics, unlike the more jolting odd-order harmonics generated by transistors. Virtually every great tube guitar amp ever built used a 12AX7 in its preamplifier stage. When you fire up your VEC-1680K, you'll understand why!

Circuitry: The VEC-1680K uses a 12AX7 twin-triode, which is really two individual tubes packaged in one glass envelope. The first stage runs "wide open", with no manual gain control or negative feedback circuitry to load it down. A Gain (or drive) control then regulates how hard you push the preamp's second stage. Drive it lightly, and you get bell-like clarity. Drive it harder, and you'll introduce progressively more fullness and grit! Most of the preamp's distortion and coloring occurs in the second stage. At the preamp's output, the signal is attenuated back down to "guitar" level. The Output control provides added variable attenuation to set the optimum drive level for your power amp. A built-in relay connected to the audio jacks lets you bypass the preamp, or--with the click of a foot switch bring it on-line.

TOOLS AND SUPPLIES

Construction Area: Kit construction requires a clean, smooth, and well-lighted area where you can easily organize and handle small parts without losing them. An inexpensive sheet of white poster board makes an excellent construction surface, while providing protection for the underlying table or desk. Diffused overhead lighting is a plus, and a supplemental high-intensity desk lamp is especially helpful for close-up work. Safety is always important! Use a suitable high-temperature stand for your soldering iron, and keep the work area free of clutter.

Universal Kit-building Tools: No sp ecial tools are required to complete this kit beyond common items normall y used for bench constructio n. We recommend the following:

! Soldering Iron (grounded-tip and temperature-controlled preferred)

VEC-1680K Owner’s Manual Vacuum Tube Preamp

! High-temperature Iron Holde r with Cleaning Sponge ! Solder, 60/40 or 37/63 with rosin or "no-clean" flux (.031" dia. is good

size).

! Needle Nose Pliers or Surgical Hemostats ! Diagonal Cutters or "Nippy Cutters" ! Solder Sucker (squeeze or vacuum pump type), or Desoldering Braid ! Bright Desk Lamp ! Magnifying Glass

BEFORE YOU START BUILDING

Experience shows there are four common mistakes builders make. Avoid these, and your kit will probably work on the first try! Here's what they are:

1. Installing the Wrong Part: It always pays to double-check each step. A 1K

and a 10K resistor may look almost the same, but they may act very differently in an electronic circuit! Same for capacitors--a device marked 102 (or .001 uF) may have very different operating characteristics from one marked 103 (or .01uF).

2. Installing Parts Backwards: Always check the polarity of electrolytic

capacitors to make sure the positive (+) lead goes in the (+) hole on the circuit board. ICs have a notch or dot at one end indicating the correct direction of insertion. Always double-check--especially before applying power to the circuit!

3. Faulty Solder Connections: Inspect for cold-solder joints and solder

bridges. Cold solder joints happen when you don't fully heat the connection-or when metallic corrosion and oxide contaminate a component lead or pad. Solder bridges form when a trail of excess solder shorts pads or tracks together (see solder tips below).

4. Omitting or Misreading a Part: T his is easier to do than you might think!

Always double-check to make sure you completed each step in an assembly sequence.

Soldering Tips: Cleanliness and good heat distribution are the two secrets of professional soldering. Before you install and solder each part, inspect leads or pins for oxidation. If the metal surface is dull, sand with fine emery paper until shiny. Allow the tip of your iron to contact both the lead and pad for about one second (count "one-thousand-one") before feeding solder to the connection. Surfaces must become hot enough for solder to flow smoothly. Feed solder to

VEC-1680K Owner’s Manual Vacuum Tube Preamp

the opposite side of the lead from your iron tip--solder will wick around the lead toward the tip, wetting all exposed surfaces. Apply solder sparingly, and do not touch solder directly to the hot iron tip to promote rapid melting. Keep a damp sponge handy to wipe your so ldering tip on. This removes excess solde r, and keeps the tip properly tinned. If the iron is going to sit idling for long periods, wipe the tip, add some fresh solder, and unplug the iron.

Desoldering Tips: If you make a mistake and need to remove a part, follow these instructions carefully! First, grasp the component with hemostats, needlenose pliers, or your fingers. Heat the pad beneath the lead you intend to extract, and pull gently. The lead should come out. Repeat for the other lead. Solder may fill in behind the lead as you extract it--especially if you are working on a double-sided b o ar d with plat e-thr o ugh hol es. Sho uld this ha pp e n, tr y heat ing the pad again and inserting a common pin into the hole. Solder won't stick to the pin's chromium plating. When the pad cools, remove the pin and insert the correct component. For ICs or multiple-pin parts, use desoldering braid to remove excess solder before attempting to extract the part. Alternatively, a lowcost vacuum-bulb or spring-loaded solder sucker may be used. Parts damaged or severely overheated during extraction should be replaced rather than reinstalled.

Work Habits: Kit construction requires the ability to follow detailed instructions and, in many cases, to perform new and unfamiliar tasks. To avoid making needless mistakes, work for short periods when you're fresh and alert. Recreational construction projects are more informative and more fun when you take your time. Enjoy!

Sorting and Reading Resistors: The electrical value of resistors is indicated by a color code (shown below). You don't have to memorize this code to work with resistors, but you do need to understand how it works:

Resistor Color Code

1st Digit 2nd Digit Multiplier

Tolerence

(gold or silver)

Black = 0 (tens) Brown = 1 (hundreds) Red = 2 (K) Orange = 3 (10K) Yellow = 4 (100K) Green = 5 (1Meg)

Blue = 6

Violet = 7

Gray = 8

White = 9

Silver = 10%

Gold = 5%

When you look at a resistor, check its multiplier code first. Any resistor with a black multiplier band falls between 10 and 99 ohms in value. Brown designates a value between 100 and 999 ohms. Red indicates a value from 1000 to 9999 ohms, which is also expressed as 1.0K to 9.9K. An orange multiplier band designates 10K to 99K, etc. To inventory resistors, first separate them into

VEC-1680K Owner’s Manual Vacuum Tube Preamp

groups by multiplier band (make a pile of 10s, 100s, Ks, 10Ks, etc.). Next, sort each group by specific value (1K, 2.2K, 4.7K, etc). This procedure makes the inventory easier, and also makes locating specific parts more convenient later on during construction. Some builders find it especially helpful to arrange resistors in ascending order along a strip of double-sided tape.

Reading Capacitors: Unlike resistors, capacitors no longer use a color code for value identification. Instead, the value, or a 3-number code, is printed on the body.

Value Code

10 pF = 100 100 pF = 101 1000 pF = 102

.001 uF = 102*

.01 uF = 103 .1 uF = 104

Multilayer

(270 pF)

271

Ceramic Discs

(.001 uF) (.1 uF)

102

104

Electrolytic

1 uF

1uF

35V

As with resistors, it's helpful to sort capacitors by type, and then to arrange them in ascending order of value. Small-value capacitors are characterized in pF (or pico-Farads), while larger values are labeled in uF (or micro-Farads). The transition from pF to uF occurs at 1000 pF (or .001 uF)*. Today, most monolithic and disc-ceramic capacitors are marked with a three-number code. The first two digits indicate a numerical value, while the last digit indicates a multiplier (same as resistors).

Electrolytic capacitors are always marked in uF. Electrolytics are polarized devices and must be oriented correctly during installation. If you become confused by markings on the case, remember the uncut negative lead is slightly shorter than the positive lead.

Danger---High Voltage Warning:

volt wall adapter, a second internal trans former boosts voltages back up to lethal levels to supply plate voltage to the 12AX7 tube! This voltage can painfully shock, injure, or even cause death if you touch it!

tube-type equipment without first turning off and un-plugging the power source from the AC mains!

capacitors before working on tube amplifier circuitry. This added safety procedure will be described in the manual.

Although your pre-amp runs from a 12-

Never work on

Also, you should discharge power supply filter

Testing and Alignment

section of the

PARTS LIST

Your kit should contain all of the parts listed below. Please identify and inventory each item on the checklist before you start building. If any parts are missing or damaged, refer to the manual's warranty section for replacement

VEC-1680K Owner’s Manual Vacuum Tube Preamp

instructions. If you can't positively identify an unfamiliar item on the basis of the information given, set it aside until all other items are checked off. You may then be able to identify it by process of elimination. Finally, your kit will go together more smoothly if parts are organized by type and arranged by value ahead of time. Use this inventory as an opportunity to sort and arrange parts so you can identify and find them quickly.

Qty Part Description Designation VEC P/N

2 100 ohm 2 watt resist or (brn-blk-br n) R12, R13 104-2100

2 3.3K ohm ½ watt resistor (org-org-r ed) R1, R11 101-3330

2 10K ohm ¼ watt resistor (brn-blk-org) R5, R7 100-4100

1 22K ohm ¼ watt resistor (red-red-org) R8 100-4220

2 390K ohm ¼ watt resistor (org-wht-yel) R3, R4 100-5390

2 1M ohm ¼ watt resistor (brn-blk-grn) R2, R6 100-6100

2 500K chassis mounted potentiometer R9, R10 153-5500-1

1 .01 uF disc cerami c capacitor (103) C7 200-2100

2 .1 uF disc cerami c capacitor (104) C10, C11 200-3100-1

2 .22 uF multilayer capacitor (224) C3, C9 220-3220

3 10 uF 350 volt electrolytic capacitor C1, C2, C6 270-5100x-6

3 100 uF 25 volt electrolytic capacitor C4, C5, C8 270-6100-1

1 1N4148 diode D6 300-4148

5 1N4007 diode D1-D5 300-4007

1 12AX7 tube V1 380-12AX7A

1 12.6 volt 1 amp power transformer T1 406-1114

1 Power switch, SPST SW1 507-1028

1 Relay, 2P2T RLY1 408-2042

3 ¼” phone jack J1, J2, J4 601-1005

1 2.1mm power jack J3 601-6121

1 Power Adapter, 12 VAC @ 1 amp 407-1072

1 Tube socket, 9 pin 625-12AX7A

2 Screw, 2-56 652-0375

6 Nut, 2-56 705-0256

6 Screw, self-tapping, type B 654S-0250B-B

6 Screw, 6-32 x ¼” 656-0250

2 Kep Nut, 6-32 705-0632-K

3 Nut, metric 9mm 706-7075

3 Flat washer 710-2550

2 Spacer, ¾” threaded 6- 32 716C-0750

2 Knob 760-0033

1 Shielded wire, RG316 (12”) 878-0316

1 Orange and white twist ed pair (4”) 871-2239

! "

Qty Part Description Designation VEC P/N

2 Hook-up wire (12”) 871-24xx-1200

1 PC board, 2 sided 862-VEC1680

1 Chassis, punched 800-VEC1680

1 Chassis plate 804-VEC1680

4 Stick-on rubber feet 770-1162

VEC-1680K Owner’s Manual Vacuum Tube Preamp

Good

The term solder means to solder the part's leads in place, and to inspect both (or all) solder connections for flaws or solder bridges. Nip off excess protruding leads with a sharp pair of side cutters.

Begin construction by locating the 2-sided PC board. Find the side of the board with long horizontal tracks on it. This will be called the solder-side during construction. Flip it over to the side with no tracks. This will be called the

component side. Unless otherwise indicated, parts will be mounted on the component side (exceptions are three large electrolytic capacitors and two gain

controls to be mounted on the other side). Orient the PC board component-side up. This kit has 11 fixed-value resistors. Mount these now, starting with the smallest

value and moving to the largest. Before mounting each one, carefully bend both leads close to the resistor body to form right-angles, as shown below:

Not Good

Locate two (2) 100 ohm 2 watt resistors (brown-black-brown).

! ! Install a 100 ohm at R12 and solder. ! ! Install a 100 ohm at R13 and solder.

Locate two (2) 3.3K ½ watt resistors (orange-orange-red).

! ! Install a 3.3K at R1 and solder. ! ! Install a 3.3K at R11 and solder.

Locate two (2) 10K ¼ watt resistors (brown-black-orange).

! ! Install a 10K at R5 and solder. ! ! Install a 10K at R7 and solder. ! ! Find a 22K ¼ watt resistor (red-red-orange). Install at R8 and solder.

Locate two (2) 390K ¼ watt resistors (orange-white-yellow).

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