[Boatanchors] Question about testing tubes

[Randy and Sherry Guttery]

On 1/29/2012 3:52 PM, Richard Knoppow wrote:


>  A good test for tubes with weak filaments is to lower the 
filament
>  voltage a step and watch the gm, if the tube is OK it will 
go up a
>  bit at first and then fall off. I don't know the physics 
but its a
>  legit test.

Actually - "life test" is a standard test used to determine 
a tube's health (therefore potential "life") - but is a 
"measure" of the cathode's emission ability (not filament 
"weakness"). "Life test" is usually associated with mutual 
Dynamic testers - though it's valid for emission testers as 
well.

Note that in this discussion - emission and mutual 
conductance are two totally different things.

The idea is that as a cathode ages - it's ability to "emit" 
electrons drops - and as it gets worse - emission becomes 
more and more effected by cathode temperature. Testers such 
as B&K Dynajets have a switch that reduces filament voltage 
- while other testers (TV-7 TV-10, etc.) suggest that the 
filament voltage switch be adjusted to the next lower 
voltage to accomplish the same thing. As noted - in a new 
tube - reducing filament voltage has very little effect on 
cathode emission; while in a tube whose cathode is nearing 
"end life" becomes quite susceptible to slight reduction in 
cathode temperature.

Rick Poole asked:

>  Sometimes, the meter will rise to a value and then slowly fall off
>  (whereas, for another example of the same type of tube, the reading
>  remains steady).

This may be perfectly normal - and a combination of how the tube tester is made - and how much power the tube draws under test. Take the case of a TV-7, TV-10, etc. - they use a variable resistor in series with the power transformer to "calibrate" the reading.  Many of us insert the tube to be tested - and THEN set the "line set" to the red line.  This insures that the tester is seeing the correct voltage with the filament load of the tube.  Once the tube has heated - the test button pressed - and the reading taken.  But let's say that the button continues to be held down. The tube is being driven pretty hard (hence as Richard pointed out - warnings are made about possibly damaging the tube - because they are being hammered pretty hard); which means it's adding it's conduction load to the circuit.  If the tube is capable of drawing a fairly substantial load (something like a 6L6 or such) - that additional load may be enough to cause the overall voltage out of the power transformer to droop - which includes the filament. If the tube is sensitive to filament temperature - yeah - it's emission will drop - and the reading "fall off".

Rick continued:

>  Assuming the reading does not fall below the
>  minimum specified in the tube test settings, is the tube OK to use,
>  or does the falling off indicate the tube is on its last legs?

Yes - and maybe.  If it falls off but stays above minimum for "life test" - then the tube is getting old - but plenty of service life left.  If it drops below "life test" minimum - then the tube maybe on suspect. However - it would be best to do a "proper" life test - as those tend to be done given "known" values, etc.  Again - as Richard noted - holding the test button down a long time is not a good idea.

Then again - if the tube is below minimums - I've been known to "blast" the cathode - and often salvage a lot of good use out of a tube doing so. I've also been known to destroy tubes in the process as well - so like all things - YMMV.


best regards...

-- 
randy guttery

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