The VS100 is a 100-watt combo amplifier. It is part of Marshall's popular Valvestate range of amplifiers. Marshall is a British company, and the British often refer to tubes as valves. Amps in the Valvestate range use a combination of tube and transistor technology. The power section is completely transistor-powered, while the preamp section contains an ECC83 preamp valve. The valve is engaged when the boost channel is in use. To carry out repairs and modifications to your Marshall VS100, it is necessary to completely or partially dismantle the amplifier.
Turn off the power and disconnect the plug. This is extremely important. Even when turned off, guitar amps can carry a lethal current. Reduce the risk of electric shock by totally disconnecting the power supply. The Marshall Valvestate VS100 carries a mains current of 230-volts, so proceed with caution. If you have been using the amplifier, leave it to cool down for 10 minutes. If you move the amp while the tube is hot, it is likely to shatter.
Unscrew the rear panel. This will enable you to access the preamp and power section. If you need to replace the tube, you do not need to dismantle the amplifier beyond this stage. The ECC38 tube is located on top of the preamp chassis, alongside the transformers.
Remove the preamp chassis. You can partially remove this part without disconnecting the wiring. The Marshall Valvestate handbook claims that there are no user serviceable parts inside the chassis. But if you intend to access the printed circuit board you need to completely disconnect the chassis and open the lid. Unscrew the chassis and slide it towards you. Be careful not to tug it, as this can fray the wires. Melt the solder joints that fix the wires to the chassis with a soldering iron. Place the preamp chassis to one side.
Unscrew the output transformer from the rear of the chassis. To aid identification, the power transformer is black and the output transformer is metalic. Then disconnect the wires with your soldering iron and place the power amp to one side.
Remove the front grille. Use a flathead screwdriver to carefully pry the grille away from the amp cabinet. This enables you to inspect the speaker cone. If you wish to remove the speaker, you don't need to disconnect the wiring, as the power amp is already disconnected at the other end. Save time when putting the amp back together by leaving the wiring attached to the speaker. Unscrew the speaker from the internal struts. The VS100 ships with a stock 12-inch Celestion speaker. There are always four screws holding this speaker in place.
Open up the preamp chassis. This will give you access to the printed circuit board and transistors. There are 15 potentiometers for each dial on the front of the Marshall VS100 and eight toggle switches located on the inside front of the chassis. If you are performing any mods to the tone of the amplifier, this is where you'll do them. Your Marshall VS100 is now completely dismantled.
Keep all of the screws in order. This will make it easier to get them back in to their original holes.
Dismantling your amplifier may void the warranty, so always proceed with caution.