Amps
[Top] [All Lists]

Re: [Amps] Should a blower run after amp shutdown?

To: 4cx250b@muohio.edu, amps@contesting.com
Subject: Re: [Amps] Should a blower run after amp shutdown?
From: TexasRF@aol.com
Date: Thu, 2 Jun 2011 17:36:48 EDT
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Hi Jim, I have always used a manual "timer" (me) to turn the blower off a  
couple of minutes after killing hv and filament. My homebrew amps have three 
 power switches; air/water on, filament/heater on and hv on. Hv on also 
closes  the tx/rx switchover circuit. The switches are daisy chained so that 
cooling has  to be on before fil/htr can be energized and fil/htr must be on 
before hv can be  energized, hv must be on before tx/rx can be energized.
 
Some tubes, such as TH327/347, caution in the data sheet to not run the  
filament without air flow. In their case, the filament power is about 200 
watts.  If 200 watts is bad for a tube then it seems latent anode heat might 
also be  bad?
 
On the issue of thermal shock with blower left on, remember every time you  
switch to standby the same scenario exists. No problem there, so how can 
there  be a problem at shut down?
 
In my view, the largest gain in tube life comes with careful management of  
filament/heater current at turn on. I keep thinking about how light bulbs  
typically fail at turn on and how that applies to transmitting tubes. One 
tube  manufacturers' web site suggests a loss of 60 hours of life during every 
on/off  cycle. Yikes!
 
These are only personal opinions, pending new information.
 
73,
Gerald K5GW
 
 
 
 
 
 
 
In a message dated 6/2/2011 4:12:15 P.M. Central Daylight Time,  
4cx250b@muohio.edu writes:

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

_______________________________________________
Amps mailing  list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

<Prev in Thread] Current Thread [Next in Thread>