Marshall Amp Forum - View Single Post

thebigclunk · 10-22-2009, 03:01 PM

l have just had to replace the eight 6550 power tubes in my 10 year old Marshall VBA400 and bought a new set from Marshall via my local stockist (l strongly reccomend that Marshall matched sets are used - they are the best bet and not very expensive. lf you must source elsewhere l reccomend a matched set of Svetlana 6550's and not KT88's).
l obtained the instructions from Marshall technical on the correct way to set up the bias.
see the attached biasing instructions - you will need an accurate DVM, an audio signal generator an oscilloscope and a 4 ohm dummy load - DO NOT ATTEMPT THIS WITHOUT THE CORRECT EQUIPMENT OR ELECTRONIC OR ELECTRICAL ENGINEERING EXPERIENCE AND WATCH OUT? THE POWER SUPPLIES OF THESE AMPS ARE LETHAL! 750VDC AT 1AMP IS LETHAL TO MOST PEOPLE.
IF YOU DON'T HAVE THIS EXPERIENCE YOU MUST TAKE THE AMP TO A QUALIFIED TECHNICIAN OR, BETTER STILL TO MARSHALL THEMSELVES WHO WILL BE VERY HAPPY TO DO IT FOR YOU VERY CHEAPLY.

10-22-2009, 03:01 PM	#1 (permalink)
thebigclunk Junior Member Join Date: Oct 2009 Location: kettering northants UK Posts: 3	Biasing 6550 tubes in VBA400 l have just had to replace the eight 6550 power tubes in my 10 year old Marshall VBA400 and bought a new set from Marshall via my local stockist (l strongly reccomend that Marshall matched sets are used - they are the best bet and not very expensive. lf you must source elsewhere l reccomend a matched set of Svetlana 6550's and not KT88's). l obtained the instructions from Marshall technical on the correct way to set up the bias. see the attached biasing instructions - you will need an accurate DVM, an audio signal generator an oscilloscope and a 4 ohm dummy load - DO NOT ATTEMPT THIS WITHOUT THE CORRECT EQUIPMENT OR ELECTRONIC OR ELECTRICAL ENGINEERING EXPERIENCE AND WATCH OUT? THE POWER SUPPLIES OF THESE AMPS ARE LETHAL! 750VDC AT 1AMP IS LETHAL TO MOST PEOPLE. IF YOU DON'T HAVE THIS EXPERIENCE YOU MUST TAKE THE AMP TO A QUALIFIED TECHNICIAN OR, BETTER STILL TO MARSHALL THEMSELVES WHO WILL BE VERY HAPPY TO DO IT FOR YOU VERY CHEAPLY.