Cornell Plexi 18 User manual

© 1995-2007 CORNELL Musical Equipment Design & Development 1
Plexi 18/20
Owner’s Manual
DC Developments
Musical Equipment Design & Development
Unit 13 Rosshill Industrial Park, Southend-on-sea, Essex SS2 5PZ England
Tel/Fax [phone]
www.dc-developments.com

© 1995-2007 CORNELL Musical Equipment Design & Development 2
CONTENTS
Introduction 3
The Cornell Plexi Sound 3
Construction and Design 4
The Pre-amplifier
The Output Stage
Controls 5
Specifications 6
Valve Replacement 7
Please read all instructions before operating the amp!
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 earthed. Use the correct mains cord.
If you require any further information regarding the Plexi 18/20 amplifier, or any
other CORNELL products, please do not hesitate in contacting us
(details on front page).

© 1995-2007 CORNELL Musical Equipment Design & Development 3
Introduction
Thank you for choosing a Cornell Plexi 18/20 amplifier.
Your amplifier employs valve technology, and is hand built to the highest standards.
Only top quality components are used in our amplifiers, so that you can enjoy years
of trouble-free use.
Please take time to read this manual, to ensure that you get the best from your
Plexi 18/20 amplifier.
Most guitarists of today will know some amplifier history, and the name ‘Plexi’ must
be one of the best known. Plexi is a nickname for an amplifier produced between
1963 and 1969, and consisted of a range of three amplifiers of 20, 50 and 100
Watts. These amplifiers remain a legend, which is reflected in their second-hand
value.
The Cornell Plexi amplifiers are not one-hundred-percent copies of the old 1960s
models - they are more like one-hundred-and-ten-percent copies !
Research has shown us that not all amplifiers made at this time were the same.
Minor changes - not just in a visual sense, but also in the basic circuit design -
appear between batches made at the factory. We have taken the basic design to
its finest point, tailoring the components and adjusting the circuit to produce the
ultimate Plexi sound.
The Cornell Plexi amplifier is not just a good-looking reproduction antique to be
admired; it reproduces that classic sound and can be gigged and gigged, and
gigged.
Cornell Plexi Sound
The Cornell Plexi amplifier has a great tone that is instantly recognised, and often
emulated. Some boffins will tell you that its tone is due to its simplicity of design,
and this is the result of having a minimum number of components in the signal path.
Amplifiers of today offer a wide range of tones and sounds, resulting in a mass of
controls that can confuse even the most enthusiastic fiddler. The result of this
complexity is that most modern amplifiers sound the same, with a noticeable loss of
natural tone - tone that can only be reproduced by simple design and point-to-point
wiring.
Cornell ‘Cream Box’
Part of the Cornell Plexi sound is due to our little ‘cream box’. We can’t tell you
what’s inside - it’s our little secret. We think it makes our Plexi 18/20 sound rather
special. But take a listen and judge for yourself.

© 1995-2007 CORNELL Musical Equipment Design & Development 4
Construction and Design
Virtually all modern amplifiers are built using printed circuit boards, which helps to
speed up production and keep down costs. There is, however, one major draw-
back with printed circuits - conductor tracks that connect components together often
run very close to one another, creating a small capacitance. The result of this in a
guitar amplifier is a substantial loss in both high frequency response and harmonic
richness.
Cornell amplifiers are designed to avoid this by having point-to-point wiring through-
out. Components are mounted onto a specially designed component board which
is then sealed, using a conformal coating to prevent noise and the reduction of
component life caused by moisture penetration.
The hand wiring, carried out by our highly-trained staff, takes time but ensures that
each of our amplifiers performs to the highest standards of reliability and sound.
The stainless steel chassis holds our specially-designed transformers, and chassis-
mounted (rather than circuit-board mounted) valves (tubes).
Such attention to detail ensures a very close reproduction of the full harmonic
spectrum of your guitar’s pickups, resulting in a pure, natural sound that will turn
heads at any venue and/or that records superbly.
The Pre-amplifier
The Plexi 18/20 is designed to be simple in use, and yet produce the quality of
sounds that the musician of today expects from a professional amplifier.
There are two independent channels, each having its own volume control which
controls the overall loudness of the amplifier. The tone controls are designed to
work over the entire frequency range of your guitar, allowing increased amounts of
low and high frequency adjustment.
Channel Two has a brighter tone than that of that of Channel One.
The Output Stage
The Plexi 18/20 amplifier has Class A output design, achieved by means of fixed
bias resistors. The resistance values are selected to complement exactly the char-
acteristics of the output valve. Cathode bias resistors help to stabilise current flow
throughout the lifetime of the valve. The voltage across each bias resistor will vary
with the valve’s characteristics. When these voltages are equal, each valve is
drawing the same amount of current and the valve set is ‘matched’, resulting in
maximum output and long life.

© 1995-2007 CORNELL Musical Equipment Design & Development 5
Controls
1. Input - Hi & Lo [applies to each channel]
Both inputs offer high impedance connections for instruments.
The Hi input provides 6dB more gain than the Lo input.
2. Volume
Adjusts the gain of the pre-amp and is used to set the overall volume of the
amplifier.
3. Tone
Adjusts the amount of high and low frequency boost.
4. Channel 2
The controls are the same as for Channel 1.
The overall tone of Channel 2 is brighter than Channel 1.
5. 18/20 Switch
This changes the rectifier from valve (18) to semiconductor (20). The valve
rectifier gives a bluesy ‘sag’ in output, while the semiconductor rectifier has a
faster response and less sag, improving transient attack.
6. Low-High Switch
This reduces the maximum power from 20 Watts in the ‘high’ position, to 5W
in the ‘low’ position.
7. Output Connectors
The two output jacks are wired in parallel. The output impedance switch
allows an 8 Ohm or 4 Ohm load to be used.
8. Mains Indicator
When illuminated, this shows that the valve heaters are on, and that there is
power to the amplifier.
9. Mains Fuse
The fuseholder contains the mains fuse for the amplifier. The mains fuse
protects the amplifier from electrical faults.
Replace a failed fuse only with the same type and rating.
Do not operate the mains switch without a 30 second delay between
operations. There is an internal thermistor to reduce mains surges. This
protects the amp and helps to ensure a long life for the components. The
thermistor works by introducing a resistance in the circuit when the power is
switched on. Within a few seconds the thermistor heats up and its resistance
reduces, allowing the full current to flow into the mains transformer. After
switching off the amp, it takes a few seconds for the thermistor to cool down
and to return to its initial, protective resistance value.

© 1995-2007 CORNELL Musical Equipment Design & Development 6
Specifications
POWER ..…..................................... 230 Volt AC, 50Hz, 80 Watts
115 Volt AC, 60Hz, 80 Watts
FUSES ……………. ........................ 600mA Slo-blow for the 230v model
1 Amp, Slo-blow for the 115v model
INPUT IMPEDANCE ....................... Hi - 2.2 MegOhm: Lo - 94 kOhm
CONTROLS ..................................... Channel 1: Volume, Tone
Channel 2: Volume, Tone
OUTPUT …………… ...................... High: 18/20 Watts
Low: 4.5/5 Watts
CABINET .......................................... Pine shell & plywood baffle. Green covering.
SIZE Head ..................................... Height: 7.75in (195mm)
Width: 19in (480mm)
Depth: 8.5in (215mm)
Combo …….......................... Height: 20in (510mm)
Width: 24in (610mm)
Depth: 9in (230mm)
WEIGHT Head ……........................ 22 lbs / 10 Kg
Combo 44 lbs / 20 Kg
OPTIONS (on request) .................. Choice of speakers, colours

© 1995-2007 CORNELL Musical Equipment Design & Development 7
Valve Replacement
Valve life varies, depending upon playing style and application. Power valves have
a much shorter life than pre-amp valves, which can last for years. In order to
maintain the optimum performance of your amplifier, the valves should only be
replaced with the same types of valve, as specified below.
The Plexi 18/20 amplifier has two pre-amp valves (12AX7/ECC83), two power
valves (EL84) and a rectifier valve (GZ34).
Warning
When changing valves, ALWAYS unplug the amplifier, and remember that the
VALVES ARE HOT! Wait for the valves to cool, or replace the valves when
the amplifier has not been used for several hours.
Valve Line-up
Mains
Transformer Output
Transformer
EL84 EL84 ECC83
Front panel
ECC83
GZ34