Cornell Custom 40 User manual

Cornell-Custom 40
and
Cornell-Custom 80
Owner’s Manual
DC Developments
Musical Equipment Design & Development

Contents
2. Introduction
Construction and Design
3. Technical Design
4. Controls
5. Specifications
6. Tube Replacement and Valve Failure
7. Block Diagram
Please read all instructions before operating unit!
If you require any further information regarding this amplifier or
any other
CORNELL product, please do not hesitate to contact us.
WARNING:
TO PREVENT DAMAGE, FIRE OR SHOCK HAZARD, DO NOT EXPOSE THIS
UNIT TO RAIN OR MOISTURE.
NO USER SERVICEABLE PARTS INSIDE, REFER SERVICING TO QUALIFIED
PERSONNEL ONLY. THIS UNIT MUST BE EARTH GROUNDED.
1
! 

Introduction
Thank you for choosing a Cornell amplifier.
Your amplifier employs valve technology and is hand-built to highest of standards.
Only top quality components are used so that you can enjoy years of professional
use.
Please take time to read this manual, to ensure that you get the best from
your amplifier.
Construction and Design
Almost all modern amplifiers are built using printed circuit boards, which contain
conductor tracks that connect components together, these conductor tracks often
run very close to one another. When this happens a small capacitance is produced
between them, the result of which is a substantial loss in both high frequency
response and harmonic richness. The Cornell-Custom 40 and 80 are designed to
avoid this by having specific point to point wiring.
The steel chassis holds specially designed, interleaved transformers, wound with
impregnated paper bobbins to minimise the gap between iron core and windings.
The design ensures a very close reproduction of the full harmonic spectrum of your
guitar’s pick-ups.
The cabinet is of a rigid construction using best quality plywood. The covering is
both heavy duty and burn resistant, The cabinet is handsomely finished and
features original fittings.
Such attention to detail ensures a very close reproduction of the full harmonic
spectrum of the pickups of your guitar. What results is a pure, natural sound that will
turn heads at any venue and records superbly.
2

Technical Design
The Pre-amplifier
The Cornell-Custom 40 and 80 are designed to be simple in use and yet produce
the wide range of sounds that the musician expects from a professional amplifier.
The Bass and Treble controls work over the entire frequency range of your guitar,
allowing increased amounts of low and high frequency adjustment.
The Output Stage
Part of the output transformer of the Cornell-Custom 40 and 80 is a small
independent winding which picks up the signal being produced in the final stage of
the amplification process. As the winding is in the output transformer itself, it will
exactly reproduce the speaker’s signal. This output is used to feed the line output,
reproducing the sound of your guitar as accurately as possible. This output is
isolated from the chassis ensuring that no earth loop can be produced, and thus no
hum, when they are connected to a mixer or power amp.
The Cornell-Custom 40 and 80 amplifier output stage is Class AB1 which means
that the grids of the output valves are controlled by a negative voltage. This is
known as the bias voltage.
The method used in biasing greatly affects the tone of the amplifier. Should the bias
be set too high, cross-over distortion occurs and is constantly present in the sound
of the amplifier, creating a “fuzzy” tone. If set too low, the valves might start to glow
red which will considerably reduce their lives and result in expensive replacements.
It is recommend that a qualified engineer re-bias the output valves when
replacements are first used..
3

Controls
1. Input Hi & Lo.
High impedance input connections for instruments.
Input Hi provides 6db more gain than the Lo input.
2. Volume
Adjusts the overall volume of the amplifier.
3. Bass
Adjusts the amount of low frequency boost.
4. Treble
Adjusts the amount of high frequency boost.
5. Mains Indicator
When illuminated the valve heaters are on and there is power to the amplifier.
6. Standby
Connects power to the valves for operation, use Standby mode for breaks in
usage (e.g. between sets).
7. Output Switch
Output Low position operates the power of the amplifier to one quarter of its
output.
Output High position returns the amplifier to its full output.
8. Line Output
The 3-pin male socket on the base of the chassis next to the output valves is a
600 Ohm line output for use with a mixer or power amplifier.
Pins 2 and 3 are live, and Pin 1 is chassis earth.
9. Impedance Switch
The switch at the base of the chassis next to the line output is an impedance
switch. This switch should be pointing inward for normal use (with 8 Ohms
internal speakers only, or 8 Ohms external speakers with the internal speakers
disconnected); and should be switched to point outward for use when an 8
Ohms extension cabinet is used in parallel with the internal speakers (or some
other combination totalling 4 Ohms). Using the correct impedance selection
enables the amp to deliver maximum power to the speakers.
10. Mains Fuse & Switch
The fuse holder contains the mains fuse for the amplifier and protects the
amplifier from electrical faults.
Do not operate mains switch without a 30 second delay between operations.
An internal Thermistor reduces mains surges which protects the amp and helps to
ensure a long life for the components. It works by introducing a resistance in the
circuit when switched on. Within a few seconds the Thermistor is heated and the
resistance is reduced, allowing the full current flow into the mains transformer.
When switched off it requires a few seconds to cool and return to its initial,
protective resistance value.
Replace a failed fuse only with the same type and rating.
4

Specifications
POWER 115 Volt A-C 60hz120 Watts US Model
230 Volt A-C 50hz 120 Watts GB Model
FUSES Custom 40 1 Amp, 240v Slo-Blow for 230 Volt Model
2 Amp, 120v Slo-Blow for 115 Volt Model
Custom 80 2 Amp, 240v Slo-Blow for 230 Volt Model
4 Amp, 120v Slo-Blow for 115 Volt Model
INPUT IMPEDANCE Input HI 2.2 Meg Ohm.
Input LO 136k Ohm.
CONTROLS Volume, Bass, Treble. (Reverb optional extra)
LINE OUTPUT Coil tapped output transformer
600 Ohms at 0.25 Watts.
OUTPUT Custom 40: 40/10W; Custom 80: 80/20W
SPEAKERS Custom 40 1 X Tone Tubby (8 Ohms)
With 8/4 Ohms impedance selector switch.
Custom 80 2 X Tone Tubby (16 Ohms each in parallel)
with 8/4 impedance selector switch.
CABINET Pine shell & plywood baffle. Tweed covering.
Leather handle. Rubber feet.
Dimensions Custom 40 Height 18ins/450mm (480mm feet to handle)
Width 23 ins/585mm
Depth 9.5 ins/240mm
Custom 80 Height 20ins/510mm (535mm inc. feet & handle)
Width 26ins/660mm
Depth 10ins/254mm
WEIGHT Custom 40 52lbs/23.5kgs
Custom 80 74ibs/35kgs
OPTIONS ON REQUEST Choice of speakers, colours, custom
modifications.
WARNING:
No user serviceable parts inside, refer servicing to qualified personnel only.
This unit must be Earth grounded. Use correct mains cord.
5

6
Tube Replacement
Tube life varies depending upon playing style and application. Power tubes have a
much shorter life than pre-amp tubes which can last for years. To maintain the
optimum performance of your amplifier, the tubes should only be replaced with the
same type as specified on the tube illustration below.
When changing tubes ALWAYS unplug the amplifier and remember that the
TUBES ARE HOT! Wait for the tubes to cool or replace tubes when amplifier has
not been used for several hours.
The Cornell-Custom 40 has three pre-amp valves (12AX7/ECC83) and two power
valves (6L6). The Cornell-Custom 80 has the same pre-amp valves, but four out-
put valves.
Valve Failure
When a valve breaks down its internal elements can short. This can cause major
damage to the internal components of the amplifier and also be a possible fire risk.
There are two fuses which will break under faulty conditions and those are the fuses
mounted on the back panel, although there are others in the chassis. Remember to
replace the fuses with the same type and rating.
Remember – we are always happy to assist in any we can, so please do not
hesitate to contact us.
Chassis Valve Line Up
Mains
Transformer Output
Transformer
EL34 EL34 ECC83 ECC83
Front panel
CAUTION CHASSIS SURFACE HOT
GZ 34 ECC83

BLOCK DIAGRAM
INPUTS
TONE
CONTROLS
Hi
Lo
BASS TREBLE
Output 8 Ohms
Line
VOLUME
2
1
3
Speaker Outputs
Output Transformer
Power Transformer connections
to be altered ONLY by qualified personnel
HT
Bias
Htrs
A
B
C
D
E
F
Fuse Thermistor
1
2
240 Va/c Mains
Connect 1 to A (Blue)
Connect 2 to F (Brown)
Join C & D (Black/Red)
B & E (Yellow/Orange)
- no connection
115 Va/c Mains
Connect 1 to A&D (Blue/Red)
Connect 2 to C&F (Black/Brown)
B&E (Yellow/Orange)
- no connection
100Va/c Mains
Connect 1 to A&D (Blue/Red)
Connect 2 to B&E (Yellow/
Orange)
C&F (Black/Brown)
- no connection
BLUE
YELLOW
BLACK
RED
ORANGE
BROWN
Neutral
Mains
Switch
Live
4 Ohms