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Re: [TCML] Methods for VTTC soft filament startup



The other problem with using a small variac on my VTTC is the inefficiency of my homemade MOT filament transformer. It draws 6.5 amps under load! This means I can't use one of the small variacs and would have to use a larger variac which I certainly want to avoid for size and weight reasons.

MOTs are really inefficient. I thought it was just mine but I did some investigation and found out they are all like that more or less. They will get very hot even with no load. The manufacturers are so cost sensitive that they minimize any copper or iron. They are so cheap that if they would have added about 20 more turns to the primary, the efficiency would be significantly higher. I have even considered adding about 20 more turns myself to the primary just to get the current draw down.

----- Original Message -----
From: "David" <zipo@xxxxxxxxxxx>
To: tesla@xxxxxxxxxx
Sent: Tuesday, February 27, 2018 8:31:15 AM
Subject: Re: [TCML] Methods for VTTC soft filament startup

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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