Gents/Ladies,
Just a suggestion if running a 1-1000A and the bright red plate bothers you
as it does me. I know, many think that this tube should always run with a
plate red because its built for it. All I know is that my HB amp's tube is
still in perfect condition after 15 years of mostly "black-plate use" here at
station W8OHT.
My suggestion is based on my happy experience in the recent Volta RTTY
contest. For good results in this DX contest, as is in most, almost full RF
output is required from the amplifier when using a 4-1000A linear into a
poor-man's inverted ell on 40 meters at night, for example. My tube's biasing
is fixed at about 10 vdc as I recall.
When my homebrewed Pi loading control (ex-HRO-receiver variable with all
sections-in-parallel) was set the same for RTTY as for SSB or CW, my tube's
plate went from black to full red (not yellow) by the time my longest contest
exchange macro had completed its canned message: (his call 599 Q# Q# - 05 05
his-call). Since I couldn't allow my favorite pet tube to undergo such
treatment, I reduced the drive from the K-3 to about 11 watts. A cheap old RF
watt meter on the antenna line indicated about 300 watts to the antenna (with
no appreciable reflected power). Due to noisey nitetime conditions on 40m
this output signal was enough to get DX, but only with frequent repeats.
So I made an effort to see if I could lighten the loading control to result
in more RF voltage on the plate of the subject tube and thereby get the same
output with less DC plate current. The HRO loading control had been indicating
a reading of 200, but now I could put amplifier in tune and use more drive
power (45 watts), yet experienced no RF voltage break downs (frying sounds) in
the amplifier's output circuit (the HRO loading dial now indicated 25 (a much
higher output pi loading capacity, i.e., lighter loading). Now turning the
tuning capacitor always yielded a much easier-to-adjust dip and there was only
slight-redness in the plate at the very end of contest transmissions. Now the
output RF meter indicated 850 watts output to the antenna and the longest
sending macro for the contest resulted in plate redness only during the final
second of these hundred-percent-duty-cycle transmissions. So "Mission
accomplished". No more contest messages had to be repeated.
FYI,
John, W8OHT
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