Troubleshooting with a variac

[maxhifi]

Tube amp blew main fuse. Decided to troubleshoot using
Variac (never used this method before). Turned it up to 30V already had 3A current.. Checked voltages on secondary of power transformer, compared to rated voltages. Figured out which section of transformer was loaded heavily (calculated ratio for each section with normal and test voltage) and rapidly found shorted section of bridge rectifier.

Was my first time to use a variac for troubleshooting, and I am really impressed by the result - I had the defective component identified in 15 minutes and amp back in service in 30, I know for sure it wouldn't have gone so well if I had tried to fix it without the Variac.

Conclusion: I know experienced techs probably know this already but in case someone doesn't (like I didn't before today) a variac is a great aid for troubleshooting a device which blows fuses, because you can give it just enough voltage to diagnose the issue without damaging any more components.