Slight drop (10%) in output with warming is normal for most solid-state
amps. Having fans run full speed might counter that, slightly. Drop
from 1500 to 1350 is 0.5 dB in signal (which is likely only noticeable
if you are running a marginal propagation path (like eme). In fact that
is not significant on eme (normally) if you have sufficient antenna gain.
FT8 and many of the wsjt-x modes are weak-signal modes so one wants to
run as much power as needed for the path. Eg: yesterday I ran FT8 on 6m
with a friend that was running a portable station for POTA. He had 50w
into a mobile vertical. That was good for a -16 dB signal at my 6m-eme
station (two 7-element LFA yagis into a K3). I ran 1000w which was more
than needed for the path (received 599 report since he was in
contest-mode). He was in the mountains only 100-miles from me.
Most amps are most efficient at their rated power: My converted Harris
TV Linear runs 1000w at 48.6v at 50A (dc power is 2430w so 1430w is
being dissipated as heat and indicates eff of 41%). It uses 16
transistors to do that. Five 4-inch fans run full speed (always)
because I normally am running "key down" digital modes 50-seconds out a
2-minute Tx/Rx cycle. Good part of this Harris designed this amp to run
continuous in ch2 AM for TV (100% duty cycle). Keeps shack warm in winter!
You should be running fans full-speed under those conditions if you want
it to live long! When I ran a 8877 on 2m running JT65, blowers ran
continuously (and tube never failed at 1500w). Talk about noise!
Good part about digital modes is one does not listen to received
signals, so noise is not a hindrance; wear noise-cancelling headset if
it bothers you (I just remove my hearing aids).
73, Ed - KL7UW
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