Plexi 45/50

Owner’s Manual

DC Developments

Musical Equipment Design & Development

Unit 13 Rosshill Industrial Park, Southend-on-sea, Essex SS2 5PZ England

Tel/Fax +44 (0)1702 610964 www.cornellamps.com

© 1995-2007 CORNELL Musical Equipment Design & Development 1

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 45/50 amplifier, or any other CORNELL products, please do not hesitate in contacting us

(details on front page).

© 1995-2007 CORNELL Musical Equipment Design & Development 2

Introduction

Thank you for choosing a Cornell Plexi 45/50 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 troublefree use.

Please take time to read this manual, to ensure that you get the best from your Plexi

45/50 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 secondhand 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.

© 1995-2007 CORNELL Musical Equipment Design & Development 3

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 drawback 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 throughout.

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 chassismounted (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 45/50 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 of that of channel one.

The Output Stage

The Plexi 45/50 is a Class ‘AB’ output design and is achieved by means of a negative voltage applied to the grid of the output valves,

Cathode bias resistors are also employed to stabilise current flow throughout the lifetime of the valve. The voltage across each resistor will vary with the valve characteristics. When these voltages are equal, then 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 4

Controls

1. Input - Hi & Lo

High impedance input connections for instruments

Input Hi provides 6db more gain than the Lo input.

2. Treble Volume

Adjusts the gain of Channel 2.

3. Normal Volume

Adjusts the gain of Channel 1.

4. Bass

Adjusts the amount of low frequency boost.

5. Treble

Adjusts the amount of high frequency.

6. Middle

Adjusts the amount of mid frequency boost.

7. Presence

Adjusts the high frequency feedback from the output stage.

8. Mains Indicator

When illuminated, this indicates that the valve heaters are on, and that there is

power to the amplifier.

9. Standby Switch

When operated, this cuts the HT voltage to the amplifier. Use standby mode for

breaks in usage (eg, between performance sets)

10. 45/50 Switch

This changes the rectifier from valve (45) to semiconductor (50). The valve

rectifier gives a bluesy ‘sag’ in output, while the semiconductor rectifier has a

faster response and less sag, improving transient attack.

11. High/Low Switch

Reduces the output to 15 Watts.

12. 8-16 Switch

Output speaker impedance.

If you have a Cornell 2 x 12 speaker cabinet, set this switch to 16 Ohms.

Should you use two 16 Ohm cabinets, switch to 8 ohms.

13. Mains Fuse

The fuse holder contains the mains fuse to 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 5

Specifications

POWER .............................................. 115 Volts AC, 60Hz, 120 Watts (US model)

230 Volts AC, 50Hz, 120 Watts (GB model)

FUSE .................................................. T2 Amp for the 115 Volt US Model

T1Amp for the 230Volt GB Model

HT 500 mA

INPUT IMPEDANCE ......................... Input Hi 2.2 MegOhm

Input Lo 94 kOhm

CONTROLS ....................................... Normal Channel Volume, Treble Channel Volume,

Bass, Middle,Treble. Presence,

OUTPUT ............................................ 45/50 Watts rms, Class ‘AB’

CABINET ........................................... Plywood (Head)

Pine and Plywood speaker cabinet

DIMENSIONS (Head) ........................ Height: 9in (230mm)

Width: 26in (660mm)

Depth: 9 in (230mm)

WEIGHT (Head) ................................ 37.5 lbs (17Kgs)

OPTIONS (on request) …………...... Colours, Speaker Cabinet.

© 1995-2007 CORNELL Musical Equipment Design & Development 6

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.

To maintain the optimum performance of your amplifier, the valves should only be replaced with the same valve types, as specified in the illustration below.

When changing valves, ALWAYS unplug the amplifier, and remember that the

VALVES ARE HOT ! Wait for the valves to cool, or replace valves when the amplifier has not been used for several hours.

Your Plexi 45/50 amplifier has 6 valves: three pre-amp valves (12AX7/ECC83), two power valves (EL34), and a rectifier valve (GZ34).

Valve Line-up

Front panel

Mains

Transformer

Output

Transformer

GZ 34

EL34 EL34 ECC83

ECC83 ECC83

© 1995-2007 CORNELL Musical Equipment Design & Development 7