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

Subject: Re: [Amps] Should a blower run after amp shutdown?
From: "David C. Hallam" <>
Date: Fri, 03 Jun 2011 08:01:46 -0400
List-post: <">>
Does running the blower after shut down apply to a tube like the 
4CX1500A [5V @ 42A]?  I don't see any thing about that in the Eimac spec 

I've wondered about that for the amp I am building.  I have a variac on 
the filament transformer because I wanted to set the voltage and my 
filament transformer is over-sized for the requirements of the tube.  
Your design sounds good because I have wondered how to prevent 
forgetting to reset the variac after shut down.  I never thought of a 
constant voltage transformer on the filament circuit.  That's a good 
idea too.

On the other hand, if I revise the filament circuit to 240 VAC I have a 
properly sized filament transformer for the tube so I don't have to 
worry about inrush current.


On 6/3/2011 6:44 AM, Jim Thomson wrote:
> Date: Thu, 2 Jun 2011 15:11:39 -0600
> From: "Jim  Garland"<>
> Subject: [Amps] Should a blower run after amp shutdown?
> To:<>
> Message-ID:<AEA880D1565E4930BDDA3599FE9CA146@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
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