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@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@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@muohio.edu>
> To:<TexasRF@aol.com>;<amps@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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