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Registered Member #1316
Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
Hello. I was experimenting with my 811a VTTC and removed the grid resistor and capacitor. I grounded one end of the feedback coil while connecting the other end of the feedback coil directly to the grid. and to my surprise the spark length drastically increased!amazed. I am using a single 811a vacuum tube and running the coil off of 600 volts ac. I have checked for grid heating and detected none. Is this harmful to my coil in any way? I really need feedback because this is for my science fair project which is due in 2 weeks. thanks.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
It may or may not destroy the grid. It's hard to tell in a directly heated tube like the 811A, because the light from the filament makes it hard to see if the grid is glowing.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Weston wrote ...
Hello. I was experimenting with my 811a VTTC and removed the grid resistor and capacitor. I grounded one end of the feedback coil while connecting the other end of the feedback coil directly to the grid. and to my surprise the spark length drastically increased!amazed. I am using a single 811a vacuum tube and running the coil off of 600 volts ac. I have checked for grid heating and detected none. Is this harmful to my coil in any way? I really need feedback because this is for my science fair project which is due in 2 weeks. thanks.
You need the R/C (Grid Leak network) in there. Modify the values of the Grid Leak network - don't eliminate it)
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
600VAC is pretty low for a 811A tube, how long sparks are you getting?
The low voltage would explain why your tube is still alive, you are running with the tube conducting for the full half wave which it wont like if you try to punish it abit.
What circuit are you using? If its build as a Class C oscillator and you remove the grid leak which really makes it class c as it controls that the conduction is below 50% if might not be optimal.
Please provide more information, pictures and video :)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Dr. Spark wrote ...
Data Sheets are your friend to follow as spent about ten years running FMAX patterns and timings/temperature to ensure datasheets are 100% of what the device can do and are based on real values.
Except for IGBTs of course, where we all know better than the manufacturers what they can take. Or something like that.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I see from the data sheet that 811A operated in self-rectifying class C has a an absolute maximum DC grid current of 25mA, so measure that together with the anode current to see that the valve is operating within its design centre ratings.
If you overdrive it, you will shorten its life, but when it will fail is anybody's guess.
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VTTC without grid capactior or resistor
