[Amps] 8874/3cx400a7

[Paul Decker]

Interestingly enough, the 3cx800A7 and 3cpx800A7 sheets don't have that paragraph.  I suspect because they are only spec'ed to 350 Mhz. On the other hand, the 3cx800U7 has a similar paragraph showing reduced heater voltages.  It goes even further stating that the heater voltage should only be reduced when rf drive is applied and it should be returned back to normal when rf drive is removed. 



quote: 

Heater voltage should be returned to 13.5 volts simultaneously with the removal of rf drive.  In low duty factor applications such as CW telegraphy and voice SSB, heater voltage shouldbe maintained at 13.5 volts. 

:end quote 



So maybe I shouldn't worry about it at all.  Most of my operating is CW and SSB with the occasional FM and WSJT.  I'm considering putting in a switch mechanism to switch between class AB and class C for FM and WSJT. 



Paul. 




----- Original Message ----- 
From: "Paul Decker" <kg7hf at comcast.net> 
To: amps at contesting.com 
Sent: Friday, June 17, 2011 8:20:41 AM 
Subject: 8874/3cx400a7 






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