A great many guitar amps and PA amps were, and still are, built around 6CA7/EL34's.
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)
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.
Source: Rockinghouse
What can happen if your grid bias is incorrectly set
