When I said, "It cant hurt anything" My Father would say, "It cant do it any
good." Why not err on the side of caution and run the fan a few minutes after
shut down?
Charles Harpole
k4vud@hotmail.com
> From: jim.thom@telus.net
> To: amps@contesting.com
> Date: Mon, 19 Dec 2011 21:25:48 -0800
> Subject: [Amps] Is after-powerdown cooling desirable?
>
> Date: Sun, 18 Dec 2011 09:55:51 -0700
> From: "Jim Garland" <4cx250b@muohio.edu>
> Subject: Re: [Amps] Is after-powerdown cooling desirable?
>
> Roger K8RI says:
> " With external anode tubes and particularly the larger tubes we know that
> they continue to get hotter when the fan and power are killed at the
> same time."
>
> I'm very surprised to hear this. Here's a counterargument: since the
> internal filament, cathode, and grid structures of an external anode tube
> dissipate very little heat, compared to the anode itself, I'd think that
> once the power is turned off, the anode would immediately begin to cool.
> Obviously it will cool more slowly if the blower turns off with the power,
> but I don't see why the anode temperature would ever increase after turnoff.
>
> Further, since the anode is made of copper, there is a negligible
> temperature gradient between the inner and outer walls of the anode. While
> one can argue that it's nice to flush the heat out of the RF compartment
> after turnoff, that strikes me as a very modest benefit that doesn't warrant
> the complexity of a post-powerdown time delay circuit on the blower.
>
> The situation with glass tubes is different, since the internal anodes
> typically show color and run at a temperature much higher than the glass
> envelope. Thus, the heat will continue to radiate through the glass envelope
> for awhile after turnoff, and an airflow will keep the glass and seals cool
> during this time.
> 73,
> Jim W8ZR
>
> ## It's the fil that keeps getting hotter, when the fil + blower are turned
> off simultaneously.
> I did some air flow tests a while back on a hb 3CX-3000A7... with just fil +
> blower running for
> 15 mins. [ NO B+ and no drive]. I shut off the fil + blower at the same
> time, then pulled the tube
> out, and had it sitting in my lap. The coaxial fil stem [ both portions]
> kept getting hotter and hotter
> and HOTTER. My conclusion was/is... the heat in the fil was now migrating
> out to the fil stem.
> The anode is a non issue..provided you don't shut off the blower and fil, a
> few secs after running
> full bore power.
>
> ## the Eimac notes say to use "blower over run" on all these big metal,
> external anode tubes, at least those
> with directly heated fils...and when done for the session, put amp into
> standby, then shut off B+, then shut
> off the fil. The blower is to stay on for 10-15 mins.
>
> ## I ramp the fil v up..with a variac..and also ramp it down when done for
> the night. A cam on the variac
> shaft + microswitch + relay ensure that the varaic must be cranked fully CCW
> in order to activate the
> microswitch + relay. That way, if I forget to ramp it down from the night
> before, I don't get a fil surge. Fil won't
> come on at all, until the variac is 1st cranked fully CCW. After that, the
> variac now has power, and can be used in
> the normal fashion. A fluke 87 is used to measure the fil V...right at the
> socket, via a pair of 200 uh chokes.
>
> ## the kenwood Tl-922 also had blower over run.
>
> later... Jim VE7RF
>
>
>
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