On Wed, 30 Apr 2014, gareth wrote:

Having now borrowed my friend's Variac, what is seen as
good practice for ramping up old valve radios to reform the
capacitors?

Start off at, say, 50VAC and then increase by 50VAC every
1/2 hour until 240V is reached?



There's two things going on here. One is that if you apply full power
(and current), if something is shorted, like that filter capacitor after
the rectifier tube, then you may take out something important (and hard to
replace) like the transformer.

It's complicated since you will be varying the B+ supply as well as the
filament, and until the filament gets heated enough, the rectifier won't
work, so it won't be linear output voltage versus input voltage. At some
point the rectifier will kick in, and you don't need the voltage any
higher to make sure there's no shorting.

The other thing is about forming the electrolytic after the rectifier
tube. There's debate on that, but the notion is that a dried out
electrolytic may come back to life if it's eased back into operation.

One thing you can do is take out the rectifier tube, and watch the
secondary of the transformer, make sure that is going up okay. For that
matter, before applying power, see if there's a short from where the
output of the rectifier would be and ground.

Michael