[AMPS] 3-500z tubes

[Rich Measures]

>Good morning,
>I would appreciate some advice on a pair of new Eimac 
>3-500z tubes which I tested recently and both have failed.
>One has an internal grid-filament short; the other has
>lost vacuum and emitted metal oxide vapor when plugged in.
>The tubes were purchased some years ago as spares; both tubes
>are unused and still in original cartons with warranty sheets.
>
>Since Eimac sold the glass tube division, is there any 
>warranty or replacement policy available?
>
>Regards,
>Martin ZL1ANJ

IMO, the gassy tube may have a defective glass to metal seal.  Another 
possibility is a glass fracture around a glass to metal seal -- typically 
due to mishandling (lateral movement) during tube removal from the 
socket.    

IMO, bent filament helices can be caused by:
1.  Aeronautical mobile operation in an aeroplane capable of 10G turns.
2.  Intermittent parasitic oscillations at 90 to 155 MHz.  To determine 
if this was the case, measure the parasitic suppressor resistor's 
resistance.  If the resistance is higher than it should be, a vhf 
parasite was likely the reason.  Also, I would measure the resistance of 
the grid current meter shunt resistor and the front to back ratio of the 
zener bias diode. {a photograph of a grid/filament shorted 3-500Z is on 
page 15 in the September, 1990 issue of *QST*}  

In the days when Eitel and Macullough owned Eimac, the warranty policy 
was indeed liberal.  However, after Varian bought the company, the 
warranty policy became RIGID.  For example:  during the last 7 or so 
years, Eimac built some 3-500Zs with bad spotwelds in the anode cooler. 
-  How can one spot a bad spotweld?  Good spotwelds look a bit rough, due 
to the metal having melted.  Bad spotwelds look smooth and clean due to 
the absence of metal having been melted during the welding operation.   
With normal use, defective spotwelds tend to break, allowing the anode 
cooler to drop down and short against the grounded grid.  This situation 
is quite obviously a manufacturing defect.  However, if such a short 
occurred even slightly beyond the 12 month warranty, Eimac refused to do 
the right thing and replace the shoddily-made tube.   
Rich---

R. L. Measures, 805-386-3734, AG6K   


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