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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