[Amps] Is after-powerdown cooling desirable?

[Jim Thomson]

Date: Sun, 18 Dec 2011 09:55:51 -0700
From: "Jim  Garland" <4cx250b at 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