Larryi..That sounds hard to believe. I would think the line fuse would blow before the diodes short out. Most manufacturers use rectifier diodes that are at least two times higher in current than are needed. That being said, as AC line voltage goes up current draw goes down and vice versa. So running your tube amp for a while at half voltage is not a good idea. This is partly the reason why I pull the output tubes before running an amp up on a variac and never exceed 110Vac on my variac and always on the advise of the manufacturer. I always set the voltage on my variac using an accurate DVM, note where that is on the variac dial as it varies with line voltage fluctuations, and then shut the variac off, and set the variac back to zero before I plug the amp into it. I carefully monitor the plate voltage at the output tube socket and compare it to the circuit diagram to make sure I'm not exceeding that voltage. I don't recommend doing this if you don't have expertise in measuring high voltage equipment. These voltages can be lethal. One other thing...If the amp has a start up relay and/or ramp up circuit at start up, that's a whole different kettle of fish and I would suggest contacting the manufacturer before attempting to power it up using a variac. Always make sure the amp is fused with the proper value. Apologies to anyone I may have bored.
