Set it a 5.7V as specified; the back bombardment of electrons provides the
rest of the cathode heating.
Carl
KM1H
----- Original Message -----
From: "Paul Decker" <kg7hf@comcast.net>
To: <amps@contesting.com>
Sent: Friday, June 17, 2011 8:20 AM
Subject: [Amps] 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
>
>
>
>
>
>
>
>
>
>
>
>
>
>
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