Hi Jim, I have always used a manual "timer" (me) to turn the blower off a
couple of minutes after killing hv and filament. My homebrew amps have three
power switches; air/water on, filament/heater on and hv on. Hv on also
closes the tx/rx switchover circuit. The switches are daisy chained so that
cooling has to be on before fil/htr can be energized and fil/htr must be on
before hv can be energized, hv must be on before tx/rx can be energized.
Some tubes, such as TH327/347, caution in the data sheet to not run the
filament without air flow. In their case, the filament power is about 200
watts. If 200 watts is bad for a tube then it seems latent anode heat might
also be bad?
On the issue of thermal shock with blower left on, remember every time you
switch to standby the same scenario exists. No problem there, so how can
there be a problem at shut down?
In my view, the largest gain in tube life comes with careful management of
filament/heater current at turn on. I keep thinking about how light bulbs
typically fail at turn on and how that applies to transmitting tubes. One
tube manufacturers' web site suggests a loss of 60 hours of life during every
on/off cycle. Yikes!
These are only personal opinions, pending new information.
73,
Gerald K5GW
In a message dated 6/2/2011 4:12:15 P.M. Central Daylight Time,
4cx250b@muohio.edu writes:
Like many homebrewers, I've always built a timer into my amps that powers
the blower for a couple of minutes after killing power to the amplifier.
Today, however, in a conversation with another builder, I began to
question
whether this is actually a useful feature. (Obviously, exhausting hot air
from the enclosure after powering off an amplifier does no harm, but the
issue is whether any presumed benefit is worth the effort building in this
feature.) Here's my reasoning.
On external anode tubes (8877, 4cx1000, etc.) the anode is made of copper,
which is such a good thermal conductor that basically the entire anode and
cooling fin structure is at a uniform temperature. In continuous key-down
operation, the anode is in steady state equilibrium at a constant
temperature: the airflow through the fins exhausts exactly the amount of
heat generated by the tube dissipation. If the power suddenly switches off,
along with the blower, then it seems to me the tube anode would remain
initially at the same temperature and then slowly cool to room temperature
through radiation and convection. The anode and tube seals would not
increase their temperature, because no elements in the tube would be a heat
source (except the filament, which presumably is at a hotter temperature
than the anode, but which has such a low thermal mass that its effect can
be
neglected). Thus, the conclusion is that there is little to be gained with
post-powerdown cooling of external anode tubes. In fact, killing the blower
immediately might actually extend tube life, because it would allow the
tube
to cool gradually and not be subject to as much thermal shock. The same
reasoning applies to solid state amps.
However, this reasoning leads to the opposite conclusion for glass tubes,
like the 3-500Z. In glass tubes, the anode structure can be much hotter
than
the glass envelope and seals (as evidenced by the orange glow of the
plates). In that case, a sudden poweroff and loss of cooling could in
principle raise the temperature of the seals to a dangerous level, because
the tube plate can act as a heat source until its temperature falls to that
of the glass. Thus for these tubes, keeping the blower on for a minute or
so
would seem to be a good idea. Maybe this is all well known, but I don't
recall seeing any discussion of it.
73,
Jim Garland W8ZR
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