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6CA7/EL34 Generic

A great many guitar amps and PA amps were, and still are, built around 6CA7/EL34's.




Bottom view. NJ7P Database

Originally designed by Mullard in England as the EL34 these were specifically designed as an audio output valve at a time when ultra-linear arrangements running about 40 watts peak in Hi-fi service were popular.

It's voltage ratings are somewhat limited and the data sheet maximum values, in particular grid 2 and anode dissipation limits, must be strictly observed for this type.

Pentode
Plate Voltage ................................. 800 Volts
Grid No. 2 Voltage ............................ 425 Volts
Plate Dissipation ............................. 25 Watts
Grid No. 2 Dissipation ........................ 8 Watts

Push-pull (cathode bias)

Push Pull Class AB1 Amplifier
Plate Voltage ................................. 450 Volts
Grid No. 2 Voltage ............................ 450 Volts
Grid No. 1 Voltage Derived From
  Cathode Bias Resistor ....................... 232 Ohms
Plate Current (Zero Signal) ................... 120 Ma.
Grid No. 2 Current (Zero Signal) .............. 20 Ma.
Load Resistance ............................... 6500 Ohms
Power Output (approx) ......................... 40 Watts

Typical application circuit (Moody BA40)


Click for full size 40k jpg

Push-pull (fixed bias)

The 6CA7 is particularly prone to two forms of failure. The first is over-dissipation in grid 2 leading to its spectacular collapse.

The second is due to excessive anode dissipation leading to the glass softening and puncturing inwards, as illustrated here.

Collapsed glass envelope
Source: Rockinghouse
What can happen if your grid bias is incorrectly set
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