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Re: [Amps] Is after-powerdown cooling desirable?

Subject: Re: [Amps] Is after-powerdown cooling desirable?
From: "Roger (K8RI)" <>
Date: Tue, 20 Dec 2011 00:21:34 -0500
List-post: <">>
On 12/18/2011 11:55 AM, Jim Garland wrote:
> 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.
Not all tubes do this and my experience is limited to just a few.
The filament and cathode are probably the most massive structures in 
there other than the anode which has plenty of cooling.
If I kill the power on my amp which uses a tetrode (either the Chinese 
FU728F or Eimac 4CX1500B) and quickly remove the tube (take proper 
precautions) the bottom seals and ceramic will start getting warmer to 
the point of being uncomfortable to hang onto.  These are not dangerous 
temps and I believe they are unlikely to stress the tube. OTOH the 
larger tubes like the 3000 to 5000 watt dissipation get really hot. 
Still probably not hot enough to stress the tube, but I'd have to defer 
to those who have been using those tubes for some time.
> 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.

I would think that the filament and cathode of the external anode tubes 
are much like these glass tubes with their only way of cooling being 
radiation to the anode and conduction out through the leads.  So when 
the power it turned off the heat from these still continues to radiate 
and conduct to the seals.
I agree that will most amateur used tubes it is highly unlikely to be a 
problem.  I'd like to see a bit more information on the 3CX3000, 
3CX6000, 4CX3000, YC156, etc...


Roger (K8RI)

> 73,
> Jim W8ZR
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