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.

A variac is ESSENTIAL if you're going to fix solid state amps say 100W on up. Attached is a portable variac box I built. Best to use a circuit breaker in it as you could go thru a lot of fuses.

I usually use my Powerite PR-57 in a case like this...it is a NICE ISOLATED variac...

I usually use my Powerite PR-57 in a case like this...it is a NICE ISOLATED variac...

I do the same, the PR-57 is a very handy device, with that nice big meter to monitor voltage and current.

Gregb

Here's mine

I had good luck using a variac even on a switching power supply.
I was troubleshooting a Sony BVM 1910 that wouldn't turn on.
With the variac, I was able to bring it up just enough to see a picture before it would shutdown.
The problem turned out to be a partially shorted inductor in the switching power supply.

The original owner discarded it because the knob's set screws were stripped and it would spin freely. It was from a school and I suspect students forced it. I removed the knob to use it and haven't bothered to find a new one or repair the old one yet. It would be a whole lot easier to use with a big knob, the variac needs some serious torque to control! Test equipment gets the least maintenance of any of my gear, and it should get the most!