On Monday, 15 April 2019 09:35:07 UTC+1, dennis@home wrote:
On 14/04/2019 23:21, tabbypurr wrote:
On Sunday, 14 April 2019 19:46:03 UTC+1, TheChief wrote:

Thanks Jeff

I suspect you are right about the heat issue.
Although the fitting is as-supplied, the glass shades are
horizontal 38mm I/dia
around 50mm long.
Also, being in the hall they have foot traffic regularly above.
So maybe vibration also plays a part.

Phil

The hotter the filament is, the weaker it is, so reducing voltage slightly can help resist vibration to a [to me] unknown extent.


NT


But also reduces the temp of the quartz envelope so the halides have
less effect and cause blackening of the bulb.

You have to knock more than 10v off to get that happen.