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. GET YOUR MARSHALL 900 FIXED IF IT IS RUNNING TOO HOT. it will not cure itself and new tubes won't fix the problem. We replace the suspect capacitor with a new .047 1000V cap - that is why we first got the 1000V caps we have on our web page. Rugged stuff for this particular application, we use them in a lot of other power supply applications now. At the same time we are fixing the bias supply we adjust the bias to the correct range for the tubes in use and/or convert the amp toEL34, KT66 or KT77 power tubes for some really killer tone!! |
FREE SHIPPING ON ANY ORDER OVER $60.00 - FREE! (in the U.S. onlyNOTICE - THE ORANGE DROP CAPACITORS USED IN OUR KITS HAVE BEEN 100% DISCONTINUED BY THE MANFACTURER FOR EVER. THEY WILL NEVER BE MADE IN THE U.S. AGAINHIGH QUALITY Mallory Tubular Capacitors will be included in your kitwe apolgize for this, but no one anywhere was warned or informed in any way prior to discontinuance.We can install any and all of our kits in our custom workshop - email for info.Check out Torres Guitars.com our new info site
| ||