Cornell Plexi 18/20 Combo

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Cornell Plexi 18/20 Combo

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EL84 Tube Amp Celestion 12" Speaker


The Cornell-Plexi 18/20 Combo is made to the basic circuit of the PA20 Plexi panel which was produced in the 60s. Additional Cornell design features like the switchable valve rectification and power reducer are fitted as standard.


Complementary push-pull class “A” output has been used in many renowned amplifiers throughout the history of guitar amplification. The problem with this basic circuit, which has remained common to all manufacturers with some minor differences, was one of unequal current flow through each output valve and unacceptable crossover distortion. At high output levels the problem is accentuated and results in a fuzzy sound.


We have corrected the design and resolved the problem by balancing the output and reducing the crossover distortion. This results in a much smoother sound at high output levels. The output having very little crossover distortion. The difference can be seen and measured on an oscilloscope.


We have encapsulated the components and called it the Cornell ‘Cream Box’ which describes the output tone. We now use this circuit in all of our complementary Class “A” push-pull output designed amplifiers.



Channel One:

Hi & Lo inputs




Channel Two:

Hi & Lo inputs.





18/20 (Rectifier)

Mains (On/Off)


Class A Push pull Cathode bias.


Valve Complement:

2 x ECC83

2 x EL84

1 x GZ34



High 20 watts

Low 5 watts

8 & 4 ohms jacks


Supplied with one 12 inch, or two 10 inch Celestion speakers


Contains Cornell 'Cream Box'

Additional Information

Additional Information

Model Plexi 18/20 Combo
Power (W) 20

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