RANT WARNING:

Isolation transformers are critical for any workbench, and after good
hand-tools the first thing any radio person should get... perhaps even
before said tools.

Variacs, on the other hand, are either very handy diagnostic tools or
worse-than-useless anchors fit only to let the magic smoke out of an
otherwise salvageable radio. I am not really sure if there is an in-between
other than their original function as light dimmers.

Without both current and voltage metering, they are worse-than-
useless. With the proper metering, they can be very handy tools.

Myths: A variac will help reform caps (with specific reference to tube/
valve radios).

Fact: Only on those vanishingly few radios with solid-state rectifiers
that pass B+ current at any voltage, and only if done over more time than
one cares to consider. Bench DC supplies are best for this typically futile
purpose in any case.

In the case of a tube rectifier, most of them do not start to pass DC
until the filament voltage reaches between 65% and 75% of nameplate rating. If the B+ on the particular radio is say.... 350V, that means that the first voltage the caps-to-be-reformed would see would be something between 227V and 263V... not exactly a soft-start.

Myth: A variac will allow one to apply voltage gently to a radio, so
as to discover problems before they become fatal.

Fact: No, not at all. True, a variac does allow a radio to see the
minimum amount of current to trigger its functions... an AA5 with bad filter
caps may begin to hum at 80V rather than 120V, similarly with a transformer
radio. But if the caps are not so bad as to show audible hum, the variac ceases to be useful. And a variac will *not* tell you whether a repaired radio is OK or not.

However, if the proper meters are put on the Isovariac, then some
real diagnosis can happen: An AA5 which should, by calculation draw say...
35 watts or so (0.3A), and draws 42 watts is dissipating 7 watts of heat
somewhere... perhaps the output transformer. And, unless one has calibrated
eyeballs, the difference of 7 watts will not show up on a dim-bulb tester.

Comes down to a current meter with fine enough increments to give
meaningful information. Such a tool is useful right down the line from initial diagnosis to testing the completed results.

End Rant. And it could have been much longer.

As to order-of-parts:
I keep the Heathkit IP 5220 (this one: http://www.byan-roper.org/steve/stev...-variable.html )

From what you are seeing, it is pretty clear that the Iso goes after the Variable in this design.

Peter Wieck
Melrose Park, PA