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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi,
I put this in Tesla coils because most members here wouldn't use a tube for anything else
Well, the question is simple, does a soft-start circuit increase the lifetime of a directly heated thoriated tungsten filament? In most tube sheets, there is not a single word about soft starting the filament, only in sheets of large power tubes (over a few kW of plate dissipation), maximum inrush current is sometimes specified.
I read somewhere that undervolted filament tends to sag, so you would want to avoid undervoltaging it, and this goes against the use of soft start circuits... What do you think?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Dr. Dark Current wrote ...
I put this in Tesla coils because most members here wouldn't use a tube for anything else
I'm not sure our resident tube expert builds TC's
I understood that all directly heated tubes were soft start, and that is one of the reasons why TV's and radios took such a long time to warm up. I may be mistaken though, so don't take my word for it.
Registered Member #2899
Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 255
Ash Small wrote ...
Dr. Dark Current wrote ...
I put this in Tesla coils because most members here wouldn't use a tube for anything else
I'm not sure our resident tube expert builds TC's
I understood that all directly heated tubes were soft start, and that is one of the reasons why TV's and radios took such a long time to warm up. I may be mistaken though, so don't take my word for it.
I think so too, and to be sure, I start with a diode in series (a811)...
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
The situation to avoid is one where HT is applied to a power valve before the filament or cathode has reached its full working temperature. Consumer products like radios and televisions tended not to go in for the luxury of HT Delay ciurcuitry, but it was used in professional equipment such as medium and high power transmitters.
One solution was to use all-in-one vacuum thermal delay switches such as DLS 17.
A common configuration popular in medium and high power transmitters was to use a vacuum relay whose switching current would only be enabled after a specified time determined by some combination of CR. Another approach was to make HT relay switching dependent on the current through a diode valve, which itself would not start conducting until its own cathode had reached its working temperature.
An additional 'soft-start' feature sometimes used in tetrode and pentode designs was time-delayed step-up of screen supplies, so the valve could only operate at full power after a given time. Switching here has the advantage that the screen supply is usually of lower current and voltage than the anode supply, so a less expensive relay can be used.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The delayed turn-on of plate supply is a sure thing, I was just wondering about the filament, if there's any advantage in limiting the inrush current to a cold filament compared to switching the filament on instantly to a full voltage.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
It is unusual for maximum filament inrush current to be specified in valve data, but it does happen. For example, Eimac say of their power triode 3-500Z, which has a thoriated filament, that "For best tube life the inrush current to the filament should be limited to two times the normal current during turn on. This will minimize thermal stress on the thoriated-tungsten filament wire, which can cause internal geometry changes during repeated cycling."
If you are concerned about filament inrush, I would limit inrush to double the normal operating current as Eimac suggest - if limitations of your power supply doesn't do it for you.
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Tube filament soft start
