"n cook" wrote in message
...
Arfa Daily wrote in message
...



Yes, a gentle blue glow is entirely normal ( although a brighter ' hard '
glow can indicate that the vacuum is ' soft ' )

For some interesting stuff on this, see

http://www.jacmusic.com/html/article...w/blueglow.htm

and

http://members.aol.com/larrysb/blue_glow.html

Arfa



Replaced that dual cap and runs cool and no hum after 10 minutes.
But after an hour a hum develops, intrusive but not as bad as before and
at
the same time a serious glow forms on the zinc coloured metalwork of one
pair of the EL34s.
All 4 EL34s tested good , otherwise just 2 off 1.5K resistors to the grids
of each EL34, correct values cold.
The push and the pull from the preamp are AC coupled, leaking cap here?
Swap pairs of EL34s to see if the core glowing ones are in the new
position
or the original position ?


Yes, most likely problem, if it's not the EL34s themselves ( and if both on
one half are doing it, it's probably not ) is the coupling cap between the
phase splitter valve and output valves' grids. It is very common, and will
often result in hotspots on the anodes and even grid glow. I would just
replace both caps anyway. Check also that the bias is not drifting, and if
there is a balance pot for the two halves of the output, check that this is
set correctly. Check also, any resistors involved in splitting the bias
between the two sets of valves. As far as I recall, Marshall usually use a
single -50v bias supply, adjustable with a single preset, then distributed
to both grid circuits via a couple of fixed resistors. Check that someone
hasn't turned the bias down so that the valves are thrashing a bit anyway,
and the problem is then just being caused by ' normal ' thermal
characteristic drift of the valves, exacerbated by the increased dissipation
from low bias.

Arfa