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Anonymous asked in Science & MathematicsEngineering · 1 month ago

Why is the 50C5 pentode Beam Power tube biased at half its max rated output power in most schematics?

The 50C5 spec sheet says it supports a maximum plate current of 7 watts/48mA but states the typical signal output power is only 2.3 watts/24mA.  

In a Class-A, single-ended configuration using 120v plate supply, a 2.5K ohm primary/8 ohm secondary output transformer rated at 24mA will produce a maximum of 3watts audio power.  A 2.5K ohm primary/8 ohm secondary output transformer rated at 48mA will produce a maximum of 6 watts audio power. Note that the plate current is increased by reducing the value of the cathode resistor not by increasing the plate voltage. 

Will driving the tube near the maximum plate current reduce its lifespan or audio quality.  If not, is there some other reason why most schematics show the tube biased at half its maximum plate current?


2 Answers

  • 1 month ago
    Favourite answer

    the data sheet I have has no max plate current, it has a typical operating plate current of 50 mA. The important spec is plate dissipation of 7 watts max.  with 120 VDC plate voltage, that is 58 mA.

    yes, of course operating at higher power will reduce the life.

  • ?
    Lv 6
    1 month ago

    Today, pentodes are used in high dollar audio amps because the music sounds sweet. Biased half to prevent plate saturation. Want louder sound buy a bigger pentode. SONAR had huge pentodes and you could watch them pulse when active. Kill radius for a swimmer, 100 feet. Liquid water doesn't compress but the inverse square law still applies.

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