[Boatanchors] tracking tube inventory?

[Brian A Clarke]

Hi Eugene,

What you want is an estimate of the risk of failure.

However, there are two failure modes for tubes - gradual ageing and sudden
death, this latter usually as a result of your doing something stupid, like
hitting the heater with too many ergs. Most well-designed circuits will
continue to operate well down on the rated emission - I have some sets whose
tubes, when tested showed about 30% emission and the radio still worked
quite well. The main exception to this is audio output tubes in high quality
applications, where the actual amount of power is critical, eg, in a cinema
or dubbing studio.

The gradual ageing failure mode is dependent on running hours at normalised
cathode current [=actual current/rated current], which may be related to the
normalised cathode-anode Voltage, and the frequency of restarts - much like
incandescent lamp bulbs. The problem is how to find out how many restarts
and how many running hours there are on the clock already. And you won't
find too many tube manufacturers telling you the failure characteristics -
most of them have died.

I would suggest that if the radio is working and the tubes are running well
within rated characteristics, then just get one or two per twenty tubes you
have in use. Then store them so the pins won't require cleaning when you
need them.

73 de Brian, VK2GCE.