Electronic – Step down transformer for vintage tube amplifier

amplifierstep-downtransformertubevacuum-tube

Hi so I've just bought a 60's tube guitar amp from the U.S, I live in Australia. Through searches across the internet I've found a few potential issues with the safety of running the amp through a 240-110v step down transformer.

Firstly, the amp has no power transformer, it rectifies the line voltage directly. A common safety modification is to add an isolation transformer, I figured I could avoid doing this by using an isolated step down transformer

Secondly, the amp isn't grounded so if a certain capacitor inside the amp fails, the chassis of the amp becomes live. The standard fix is to add a 3 prong plug and ground the chassis, but I've read that the ground plug in isolation transformers dont actually ground.

Thirdly, would the 50hz of Australian mains damage the output transformer or any other component of the amp, designed for 60hz?

Thanks in advance

Here is the schematic for the amp

http://www.magnatoneamps.com/schematics/magnatone_401_412.pdf

Best Answer

Since the amplifier is mains connected you can use an ordinary step down transformer. An isolated stepdown is nice but not needed. The extra safety introduced helps to prevent electrical hazard in case the input or output transformer of the amplifier fails.

If all is good then the common return from the amplifier is not connected to the chassis. The chassis itself can therefore be connected to earth in both situations. Without isolation transformer or standard stepdown transformer.

Finally there is no problem in connecting the amplifier to 50 in stead of 60 Hz mains.

Warning. If you are working on mains operated equipment be careful and respect the fact that a mains voltage can kill.