Wow, these Marshall 900 amps
have a real tendency to run unbelievably hot!!
One of the guys has
the amp running so hot it was melting the grill on top of the amp, he
keeps replacing it and is asking to install a fan inside to get rid of
This isn't the way to
fix it, the amp should not be THAT hot.
Marshall decided to
use a fairly good trick in providing a bias supply in the 900 series
BUT, did not specifiy parts that were rugged enough for the
The bias is tapped off
of the high voltage secondary with a .047 capacitor and a voltage
divider network after it.
This is a good
and valid system and provides the 5 or 10 millivolts of
negative voltage from the bias diode, but it is 100% dependent on the
"bleed off" capacitor.
capacitor is rated for far, far too low voltage and fails - too bad,
because when it fails the entire bias system is disconnected and the
power tubes have NO BIAS AT ALL.
They run cherry red
hot, into total meltdown - we have seen them being so hot the glass has
MELTED and been pulled inward by the vacuum inside the tube (until the
glass parts and loses the vacuum)
Tubes that fail in
this manner often go to terrible shorts that take out power and/or
output transformers, leading to the reputation that 900's have bad
transformers - NOT TRUE, nothing can survive this kind of operation and
a melted tube will take out even a really good Hammond or Mercury trans
as easily as it can kill the good quality Marshall trans.
modification/Repair is fairly easy if no
other damage has happened.
900 IF IT IS RUNNING TOO HOT. it will not cure
and new tubes won't fix the problem.
have a good tech replace the suspect
capacitor with a new .047 1000V cap -
final notes on web page
Legendary Book "Inside Tube Amps" is now
available on Amazon/Kindle
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