Marshall 2061 Lead and Bass 20W

[GregS]

I'm presently building a an amp from the Marshall 2061 circuit. The plans I'm working from have the center tap of the heater winding grounded. There is no fuse in the heater circuit. First question is: is there much value in putting a fuse in the heater circuit or is it kind of unnecessary? Next question: if putting a fuse in where should it be placed? in the center tap wire or one of the other heater wires?

Also, from the wiring diagram of the Mercury Magnetics P20JM power transformer I'm using there is a green/yellow wire that connects to is called "shield" in the diagram. The "shield" seems to be something in the core (from the diagram) and is not connected to any of the windings. This wire is not used in the plans I have. It seems logical to me that the purpose of the "shield" is to shield the transformer from noise and that this wire should be grounded. However, I want to be absolutely sure as this is an expensive transformer.

Thanks,

Greg

[olddawg]

Originally Posted by GregS

I'm presently building a an amp from the Marshall 2061 circuit. The plans I'm working from have the center tap of the heater winding grounded. There is no fuse in the heater circuit. First question is: is there much value in putting a fuse in the heater circuit or is it kind of unnecessary? Next question: if putting a fuse in where should it be placed? in the center tap wire or one of the other heater wires?
Also, from the wiring diagram of the Mercury Magnetics P20JM power transformer I'm using there is a green/yellow wire that connects to is called "shield" in the diagram. The "shield" seems to be something in the core (from the diagram) and is not connected to any of the windings. This wire is not used in the plans I have. It seems logical to me that the purpose of the "shield" is to shield the transformer from noise and that this wire should be grounded. However, I want to be absolutely sure as this is an expensive transformer.
Thanks,
Greg

From what I understand the transformer shield is typically used for instrumentation power supplies, mil spec stuff, and other low noise applications. It is earthed if used. It is probably inconsequential in a guitar amplifier. As far as fusing the filament supplies goes, it wouldn't hurt, especially if you are going to be modding and messing around inside the amp a lot. In a production amp it might not be there to keep the cost down, because there is nothing else on the filament line except maybe a lamp, and filaments tend open rather than short when they fail.

[loudthud]

Some safety agencies require a fuse in the heater winding if their tests show that shorting the heater winding won't blow the mains fuse. A guy I know once caused a short when he removed the pilot light in a blackface Bassman while the power was on. It melted the wires back to the transformer. On the schematic for some silverface Fender amps there are two 4.7 ohm resistors to limit current or act as fuses.

[GregS]

Thanks for the replies. It sounds to me like it wouldn't hurt to fuse the heater circuit. Since I posted this topic I found a number of examples of fusing the heaters in Kevin O'Connor's TUT3. Many of them have a fuse in both heater wires. The center tap is not fused. It is connected to an elevated DC supply if I understand correctly. In my case, the center tap is connected to ground.

Greg