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@comcast.net>
To: amps@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
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