[R-390] Waking up my non-A

[Tim Shoppa]

"David C. Hallam" <dhallam at rapidsys.com> wrote:
> In spite of all that has been written in various reflectors, etc. and
> conventional wisdom aside, it is not a good idea to "bring up slowly with a
> variac" a vacuum tube radio.  Vacuum tubes will not conduct until the
> filament temperature reaches design temperature (or close to it).  Nothing
> good can come from letting the tubes try to operate at a low line voltage
> for any extended period of time.

Well, it's not the best thing for the tubes, but they are after all
just tubes. I think what you're worried about is stripping or poisoning
the cathode, and while that is a worry with big power tubes I don't
think I've ever seen it do bad things to little receiver tubes.

The variac approach is for those who don't want to blow up capacitors
unnecessarily, but I've got a long personal history of blowing up
capacitors (usually with a vengance) and if there are any weak ones
I'd rather know sooner than later. This is not exactly conventional
radio maintenance philosophy but the process of "margining" (stressing
the components to find weak ones at maintenance time rather than at
use-time) is well established in other areas
of electronics, so if you find me using a variac it's usually to
boost line voltage a bit :-).

Now there are several components that are not so easily replacable
with off-the-shelf parts (e.g. chokes, transformers) but on a radio
with fused B+ lines they're mostly safe.

Tim.