Virtually all damage here from guest operators comes from undercoupling
amplifiers. They do not open the load control enough, or peak the amplifier
at low power. It is always the same operators who cause damage, because they
tune wrong.
The same amplifier that lasts me 30 years never arcing will melt tuning
capacitors in 30 minutes with certain guest ops.
Retuning the amplifier at reduced drive is not very often good. Tuning at
lower power does the following:
1.) Reduces heat in the tube by increasing tube efficiency (good with
marginal tube dissipation ratings or tube cooling)
2.) Increases voltage and heat in the tank components by increasing loaded Q
(bad always)
3.) Reduces the ability of the amp to tolerate increased drive power, which
sometimes comes accidentally (very bad)
Even with a 3 kilovolt supply, you can make over 10,000 volts in the tank by
peaking power at low drive power and then accidentally increasing drive.
That can put you off the air with capacitor or bandswitch failures.
See:
http://www.w8ji.com/Vacuum_tube_amps.htm
http://www.w8ji.com/demonstation.htm
So while retuning can reduce tube heat, it also will increase tank losses by
increasing loaded Q, which will in turn require more frequent readjustments
as you move around the band.
It is not very likely though, that the tube is hotter at reduced power even
without retuning. If that really happens (it can), it indicates a poor
design. Everything should be cooler as power is reduced.
When I adjust a bigger than needed amplifier in contest use, I just adjust
at full power and then reduce drive to the legal power. This way the
bandwidth before readjustment is required as wide as possible, and changes
in SWR as antennas are changed does not have an negative impact.
It is almost always much easier on the amplifier and other people to
overcouple (load at higher power) and then back off.
The worse thing for most amplifiers and other people on the band is to tune
right at peak efficiency, especially with a radio backed off to 30 or 50
watts. That is a bad equipment to radio match, and can easily become a
splatter box. It would be better as Mario says to use an attenuator, but the
amp should still be overcoupled so peak efficiency occurs at more power than
ever expected.
Personally, I would never retune at lower than peak power the amplifier can
run. I would reduce drive or use an attenuator. I use attenuators so my
radios can run near full power. Running the radio far below design power is
not as stupid as running beyond design power, but is still bad.
73 Tom
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