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Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Mads Barnkob wrote ...
If the filament is not warm enough, you will risc cathode depletion as the thorium on the tungsten filament will vaporize as its not hot enough to give off ions instead.
Just like running a vacuum tube with too low voltage on the filament .
I would rather run my tubes with a filament voltage of 1 volt more than 1 volt less... More voltage will make the filament break faster due to higher heat i suppose, though still slower than cathode depletion, while less will cause cathode depletion and the thorium coated is ruined.
Some tubes are rated for 12v and still are able to tolerate +-1v. Just always get within maybe 100mv or so for a filament in the lower tens of volts or less.
I have only killed a few tubes, most of them where small and died because of a grid circuit failure, making the grid melt. I also once killed an 811A, a metal box touched the tube and broke the glass from thermal shock. It takes awhile to kill a tube. Just be good to the filament and grid!
*sigh* Even when i use a limiting resistor to heat the filament slowly before bypassing, i see sparks inside of my 'brand new' GU-81M when i first start the filament up. More resistance i say! Almost no real resistance when the tubes are cold, ALWAYS use a current limiting resistor... Maybe even two different ones to heat it up in 3 different steps. Let the current rise more gradually. If you used DC on the filament with a capacitor for smoothing somewhere along the line, if you switched it on with no limiting, i am sure it would break very fast.
Anyway, enough of my pointless mumbling, good luck!
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Ok, thanks for the replies.
I was wondering how is the maximum plate voltage of a tube determined? Is it only to avoid internal sparking or something else? How can a spark form in vacuum anyway?
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Dr. Kilovolt wrote ...
I was wondering how is the maximum plate voltage of a tube determined? Is it only to avoid internal sparking or something else? How can a spark form in vacuum anyway?
Arcing/sparking can and does happen in vacuum tubes, and there are a number of ways in which breakdown can occur:
The vacuum in vacuum tubes isn't quite perfect. High voltages can accelerate the residual heavy atoms into the cathode causing damage.
The insulating supports between the anode and neighbouring structures can break down. Both Mica and ceramic supports can be prone to this mode of failure.
Gettering on the inside of the envelope can encourage internal arcing, or flashing over on the inside of the envelope, as can metal films, that have deposited on the inside of the envelope, from either production processes, or general use.
Then of course there is the physical separation of pins, top caps, and lead in wires to take into account.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Maximum plate voltage "are limiting values above which the serviceability of the valve may be impared from the viwewpoint of life and satisfactory performance" Lanford -Smith , Radiotrons designers handbook.4 the ed. P77. Ilfe & Sons London 1953
How they made tubes is damned hard to find out. IRE papers can be ferreted out, and the standards for making "test tube parts" which were assembled into tubes to test and certify materials, cathode sleeves, grid wires, batches cathode coating etc can be found in ASTM standards.
The silvering metal on the inside of a tube was to scourge the tube of contaminants boiled out of metals and other parts of tubes during service life. Tube culture has been a long time.
image from Rathheiser, LRundfunk - Rohren-Eigenschaften und Anwendung Union Deutsche Verlagsgesellschaft Berlin Roth & Co l939
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Vacuum tube life in tesla coil use
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