Click to enlargeA dangerous part failure can 
kill your amp

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 the heat.

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 job.

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.

Unfortunately, this 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.

The modification/Repair is fairly easy if no other damage has happened.

FIX    YOUR MARSHALL 900   IF IT IS RUNNING TOO HOT. it will not cure itself 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

Dan's Legendary Book "Inside Tube Amps" is now available on Amazon/Kindle

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