[Amps] Is after-powerdown cooling desirable?

[John Lyles]

All the large ceramic/metal tubes that I use require a cool-down period 
after the filament is completely extinguished, before fans blowing 
through the base of the tube, and anode water stop.
I've designed with 3 different Thales tubes, all require >5 minutes of 
air after filament is ramped down to zero volts. Without doing this, the 
warranty is voided. These are much larger tubes than hams use, but 
still, the principle may be similar.
73
John
K5PRO


> Message: 2
> Date: Sun, 18 Dec 2011 09:55:51 -0700
> From: "Jim  Garland" <4cx250b at muohio.edu>
> Subject: Re: [Amps] Is after-powerdown cooling desirable?
> To: <amps at contesting.com>
> Message-ID: <B4029EE509664DDC8D7925A3CD51A52C at JimsOffice>
> Content-Type: text/plain;	charset="us-ascii"
>
> 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.