[Boatanchors] Question about testing tubes

[Randy and Sherry Guttery]

On 1/30/2012 10:49 AM, Richard Knoppow wrote:

The service life of some types of cathodes can be extended 
by cooking the filament at higher than normal voltage for a 
time. I think this applies exclusively to thoriated tungsten 
type filamentary cathodes.

Heating for extended periods yes - but this is not the same 
as "blasting" a cathode - which indeed works with indirect 
heaters as well as filament as plate construction (whether 
thoriated tungsten or not).

"Blasting" (or beaming as it's called when used on CRTs) is 
the process of literally blasting crud off the surface(s) of 
the tube.  The process when done on a TV-10 is to run the 
filament up a notch (or two - for subsequent attempts) from 
"normal" - allowing the tube to come to the higher temp - 
pressing and holding the normal emission button (P3 in most 
cases) and then "just tapping" the OZ4 button (P6).  This 
hits the tube with a very high voltage that can (and 
hopefully does) literally blast new emission area on the 
cathode- clear some grid issues, etc.  Sometimes you can see 
little flashes zip off - some times you can see the grid 
evaporate - or similar destruction.   This is why I don't 
"recommend" this procedure - it can flat destroy a tube (and 
often does - esp. if done repeatedly - or at higher filament 
voltages). I tell people "try this as an absolutely *last 
resort* - and be prepared to toss the tube" option for a 
tube that is past usefulness.

As noted - same as "beaming" an old CRT...

best regards...

-- 
randy guttery

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