Using tubes - new vs. old

W8JITom at W8JITom at
Tue Apr 16 12:34:31 EDT 1996

In a message dated 96-04-16 04:42:08 EDT, you write:

>After an 8874 tube blew in my Alpha, I ordered two - one to replace the 
>bad one, and one spare. Then I got to reading the Eimac literature 
>included, such as the warranty. Apparently they guarantee the tube to 
>some degree based on length of time from manufacture to failure.
>Sooo, the question for the group is:
>Does it make sense to replace an old tube with my new "spare" just in 
>case there's a problem with it in the not too distant future (and keep 
>the well-seasoned tube now in the amp as a spare)? Or is the failure due 
>to manufacturing defect so remote, or proving that it was a manufacturing 
>problem so tough, that I should just replace the bad tube?
>73 Barry

It's a good idea to cycle the spare tubes through the amp every few months.
One reason is if the tube has gas ingress the tube has a chance to "getter"
itself before going beyond hope.

Small external anode tubes such as yours have the gettering material (the
material that absorbs gas) on the heater-cathode assembly. Turning on the
filaments heats and activates the getter, unwanted gas is absorbed.

Internal anode tubes (like 3-500Z's) have the getter coated on the anode, and
must be run with the anodes showing color to fully activate the getter. If
you run a 3-500 without anode color it may eventually gas up enough to arc.

High vacuum arcs caused when minute amounts of gas are ionized inside the
tubes are propbaly the main cause of sudden loud pops and bangs that blow out
meters and chokes. It's a good idea to "cook" your spare tubes once in a

73 Tom

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