[R-390] Black Marker on Tubes

[Tim Shoppa]

> Is it safe to use a Black Marker to mark the tube number on the
> glass of a tube? Safe in the sense that it would affect
> emissions inside the tube. I have found radios where someone
> used a piece of paper taped to a tube with the shield over it! I
> don't think that is safe.

For high-power (transmitter, >20W) glass tubes there might be
some issues having to do with differential heating that could
crack the tube.

Hard to see how it could do anything to change emissions.

Most "black marker" inks are actually transparent to infrared
and probably won't do much to change cooling, and like I said
I cannot see how it could affect emissions at all.

The marker ink might not stand up to long-term heat or shield
insertion very well.  When I've seen people mark their own tubes
(often good/bad or ranking for emission/transconductance)
it was with paint and they put little dots on the top.

Tim.