[Amps] Should a blower run after amp shutdown?

[John G3UUT]

Another caution, this time about constant voltage transformers.  I had 
one of these and tried to use it to stabilise the mains to a signal 
generator to improve drift.  It sort of worked but the main problem with 
it was that it distorted the mains waveform quite badly, sufficiently so 
that a normal moving coil meter read very inaccurately.  I'm not sure if 
all constant voltage transformers have this problem but some certainly 
do so if you want to set the filament to a particular voltage make sure 
you use a true RMS reading meter.

73 John G3UUT

On 03/06/2011 13:07, TexasRF at aol.com wrote:
> Hi Carl,
>
> I have read/heard that about filament transformers before. I am not sure
> where that idea started but have some doubts about the validity in all  cases.
>
> The cold resistance of a filament is close to a short circuit at turn on.
> How much current can a filament transformer deliver when the secondary is
> shorted? 2X rating? 4X? 10X? Voltage is not important;  current is the
> culprit here.
>
> You are right about the 3-500Z; it is not known to have open filament
> issues. Perhaps the current requirements are low enough to limit  destructive
> magnetic fields inside the filament during turn on?
>
> Larger tubes definitely have open filament issues due to surge currents;
> learned that first hand. This leaves a bit of a dilemma: do you or don't you
> pay  attention to filament turn on surge current for a given tube?
>
> Those big tubes are expected to be turned on/off for a limited number  of
> times in their lifetime. We amateurs expect unlimited on/off cycles, a very
> different scenario.
>
> There is a lot more to be learned about this.
>
> 73,
> Gerald K5GW
>
>
>
>
> In a message dated 6/3/2011 6:15:46 A.M. Central Daylight Time,
> km1h at jeremy.mv.com writes:
>
> If the  filament transformer is sized properly it should provide sufficient
> inrush  protection at ham tube power levels. The 3-500Z certainly isnt
> known
> for  open filaments nor are any of its tetrode cousins.
>
> I do bring my  250TH's and 304TL's up with a Variac.
>
> Carl
> KM1H
>
>
> -----  Original Message -----
> From: "Jim Garland"<4cx250b at muohio.edu>
> To:<TexasRF at aol.com>;<amps at contesting.com>
> Sent: Thursday, June 02, 2011 8:24  PM
> Subject: Re: [Amps] Should a blower run after amp  shutdown?
>
>
>>
>> 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  ba
>>
>>
>>
>> You're right about tube data sheets  cautioning the need for airflow with
>> only the filaments. But that's  because the filaments disspate enough
> heat
>> to
>> raise the  temperature of the tube seals too high without cooling.
> However,
>>   that's a different situation from the latent heat of the anode, since
> that
>> can't increase once the power is killed to the amp,  assuming  the
> filament
>> power is killed simultaneously. Of course,  if the filaments remain on
> with
>> no cooling, then that's asking for  trouble!
>>
>>
>>
>> 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?
>>
>>
>>
>> Good point, Gerald. I think you're  right.
>>
>>
>>
>> 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!
>>
>>
>>
>> I've wondered about the light  bulb analogy.  Obviously, reducing the
>> inrush
>> current at  turn on is desirable, but I've hardly ever known a
> transmitting
>> tube  to have a burned out filament (excepting really old tubes, like the
>>   250TH, or tubes that were gassy). Actually, I did lose the filament on
> an
>> 8877 years ago, but that was because of a bad weld to the filament
> support
>> structure. Eimac replaced the tube, even though it was long  out of
>> warranty.
>> More typically, filaments (or cathodes) lose  emission, or the grid
> becomes
>> contaminated, or the tube becomes gassy  or shorts internally. That said,
> I
>> still think filament inrush  protection is a good idea, particularly with
>> thoriated tungsten  filaments.
>>
>>
>>
>> 73,
>>
>> Jim  W8ZR
>>
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