On Wednesday, 7 November 2018 19:13:17 UTC, wrote:
On Wednesday, November 7, 2018 at 2:02:27 PM UTC-5, tabby wrote:

Not so. The trick is to meter the B+ and wind the variac up uberslowly around the point when the rectifier begins to conduct. That way the rectifier passes only tiny current and you can indeed wind the B+ up from close to nothing. BTDT. Of course there may be easier ways now.

It's true that their uses are very few nowadays. But marginally more than zero.

Rectifiers are go/no-go devices.

valve rectifiers are not. The current they pass depends on filament temperature/voltage.

Variacs are not current-limiting devices.

they are voltage controllers. The rectifier is a current controller when run undervolted.

The voltage a (tube) rectifier passes will be a function of the minimum trigger voltage on the filament as a percentage of the "correct" filament voltage x the "correct" B+ voltage. So, if the rectifier filament trigger voltage is 4 V vs. the nominal 5 V (80%), then the passed voltage will be nominal B+ x 0.80. There is no 'winding' up.

I'm not at all clear about your argument, but they certainly do wind up on a variac. I've done it.

This is the same argument as "just a little bit pregnant". The logic is just as faulty.

Peter Wieck
Melrose Park, PA

Try it some time. You'll find out what you got wrong.


NT