[Amps] Should a blower run after amp shutdown?

[Jim Thomson]

Date: Thu, 2 Jun 2011 15:11:39 -0600
From: "Jim  Garland" <4cx250b at muohio.edu>
Subject: [Amps] Should a blower run after amp shutdown?
To: <amps at contesting.com>
Message-ID: <AEA880D1565E4930BDDA3599FE9CA146 at JimsOffice>
Content-Type: text/plain; charset="us-ascii"

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 big external anode tubes like the 3CX-3000A7 / 3CX-6000A7, 3x10 etc...
all with thoriated tungsten fils, Eimac sez to run the blower for  15 mins  after the
amp is shut off...and that includes the fil  being shut off as well. 

##  here's the procedure I use on both a 3CX-3000A7 [ 7.5v @ 50A]...
 and also a 3CX-6000A7  [7.0V @ 80A] .  Separate 240 vac feed into the
input of a 1kva Sola constant voltage xfmr.   Output of Sola xfmr is fed into the
input of a 0-240 vac @ 5A  variac.    Output of variac feeds the  input of the
dahl fil  xfmr.   Variac has a cam + microswitch on it's  shaft.  Microswitch contacts
activate the coil of a dpdt relay. 

##  1 x toggle switch for blower, and a 2nd toggle switch for the fil.  Input of 2nd toggle gets it's power from the
output of the 1st toggle.   It's then a simple interlock.  I can have the blower on and no fil, but not the other way around. 
 The variac is used to slowly ramp up  the fil V  to desired level.   When  done for the night... the amp is placed into standby, B+
is shut down, and then  the fil is slowly ramped down to zero V.   Blower is left on for 15 mins..then shut off. 

## The  15 min blower over run is all automatic, done with an adjustable 'delay on' type timer.  If I forget to ramp the variac down
at 3 AM, I don't have to worry about  fil inrush the next night.  The cam + microswitch on the shaft of the variac takes care of that problem. 
The fil  CAN'T  come on until the variac shaft is rotated  fully CCW,..and engages the cam / microswitch!   Once the variac shaft is
rotated fully CCW, the microswitch engages, and then feeds the juice to the coil of a dpdt relay.... whose contacts pass the regulated 240 vac
into the  input of the variac.  It sounds more complicated than it actually is.   It's all quite simple really. 

##  Cold resistance of a light bulb is  only 1/10th it's  hot resistance.    Thoriated tungsten tubes are worse, only 1/12th. 

## One day I had just the  fil + blower only [ no B+]  running on my  3CX-3000A7 amp.   I shut off  both the fil + blower
at the same time..and removed the tube from socket 1  min later.   I had the tube between my legs, while on a chair... adjusting
a hose clamp around the top of the tube [ which held several layers of silicone rubber, used to shim the oversized chimney]. 

## The coaxial fil   on the bottom of the 3x3 tube  was warm... THEN started getting hotter and hotter and hotter.  The  fil
was abt to burn my stomach !    My theory is/was... the heat inside the fil  had migrated back out onto the coaxial fil. 

##  and the amp was in stand-by and NO B+  on at all...just fil + blower.    From then on in...... blower over run was on  for 15 mins
as Eimac suggested..and with the fil OFF  too.  No point in having  blower over run, with amp on standby..and fil still  on.....not
with 375/560 watt  type  fils. 

later.... Jim   VE7RF