Hi Steve,
I use another variac myself also with an analog AC voltmeter hooked up full
time.
I use the variac to vary the voltage for different tubes on my current 10V
10A filament transformer within reason of course when I try different
various tubes. I also initially check with a good digital voltmeter and
"calibrate" the analog.
Yes, running low is not good, in addition to inrush. Running low can hurt
output of coil also besides not being good.
I like to keep the tube about .2 to .4 max over rated voltage of filament
for load fluctuations, so it will not drop too low. The load from the
running coil can pull AC 120v line voltage down some, which will drop this
also. So I then adjust the filament voltage. And monitor under different
load conditions.
You will have to decide the method you want to use.
Me, I test too many different tubes to make a dedicated circuit.
Chris
Sent from my LG V20
On Feb 27, 2018 8:31 AM, "David" <zipo@xxxxxxxxxxx> wrote:
> A small variac to your filament transformer and a ac volt meter to monitor
> the voltage on the filament. Small 2 amp 115vac variacs aren't that
> expensive.
>
> On 2/26/2018 11:49 PM, Steve White wrote:
>
>> I have become recently concerned about maximizing the lifetime of my VTTC
>> 833A tube. I want to to apply a soft-start to the filament to reduce the
>> in-rush current. I measured the in-rush current on the 833A and it is over
>> 40 amps! Yikes! It then quickly settles to the required 10 amps after the
>> filament heats up. I have been mulling 2 different methods. One method
>> involves a thermistor and the other method relies on a fixed resistor for
>> pre-heat which is then manually switched out after the pre-heat period.
>>
>> I had a lot of hope for the thermistor method but after trying it, it
>> didn't work well enough. I placed a 1 ohm initial resistance thermistor in
>> series with the 10 volt filament supply which would limit the in-rush
>> current to 10 amps. This thermistor has a final resistance of 0.06 ohms.
>> This doesn't sound like much, but when 10 amps is passing through it, it
>> drops the filament voltage by 0.6 volts which resulted in 9.5 volts
>> supplying my filament which is too low for maximum filament emission. I
>> could add another secondary turn to my homemade MOT-based filament
>> transformer to compensate. I don't like that idea though because it would
>> make my filament supply highly dependent on the final resistance of the
>> thermistor which has a 20% tolerance. A related idea was to place the
>> thermistor on the primary (120 volts) side of the filament transformer.
>> Unfortunately, because MOTs are so inefficient, my homemade MOT filament
>> transformer draws 6.5 amps on the primary side. I would ne
>>
> ed
>
>> about a 100 ohm initial resistance thermistor capable of handling 6.5
>> amps steady state. All of the thermistors that I have found with this level
>> of resistance only handle 2 or 3 amps steady state. So I am giving up on
>> the thermistor method.
>>
>> This brings me to the other method that I will use. I will use a fixed
>> 150 ohm, 100 watt resistor on the primary (120 volt) side of the filament
>> transformer. This will reduce the in-rush current to the filament below 10
>> amps. After the filament has pre-heated, I will then use a switch to bypass
>> the resistor to obtain full heating.
>>
>> Does anyone have a better method?
>>
>> Steve
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>
>
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