Graphite plate tubes retain heat a long time and the glass temperature does
rise.
Carl
KM1H
----- Original Message -----
From: "Jim Garland" <4cx250b@muohio.edu>
To: <amps@contesting.com>
Sent: Thursday, June 02, 2011 5:11 PM
Subject: [Amps] Should a blower run after amp shutdown?
> 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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