[Amps] 8874/3cx400a7

[Paul Decker]

A question I'm sure the experienced amp builders can answer.  I was reading through the Eimac 8874 datasheet last night and found this paragraph: 



quote: 

HEATER/CATHODE OPERATION - The rated heater voltage for the 8873, 8874, and 8875 is 6.3 volts, as measured at the base of the tube, and variations should be restriced to plus or minus 0.3 volt for long life and consistent performance.  At frequencies above approximately 300 MHz it may be necessary to reduce heater voltage to compensate for rf transit-time heating of the cathode after dynamic operation of the tube has started.  The back heating is a function of frequency, grid current, grid-bias, anode current, duty cycle, and circuit design and adjustment.  The following heater operating voltages are recommended: 



Frequency (MHz)     Heater Voltage 

300 or lower             6.3 

301 to 400                6.0 

401 to 500                5.7 

:end quote 



In my new 70cm amp, my heater voltage is set to 6.3 volts.  Reading this, there are many variables to take into consideration of which there is no "formula" to figure out the ideal heater voltage.  Should I simply reduce my heater voltage to 5.7 volts?  What effect would this have on the amplifier performance?  Does anyone have experience with lowering the heater voltage in this type of situation? 



Thanks, 

Paul 

kg7hf