omitted diode in 1/2-wave PS and blew PT fuse

[tubeswell]

Dammit.

Ever done this?

I had spent about 4 days on a build fastidiously concentrating on the layout and schematic, and forgot to put the rectifier diode in the bias supply. When I switched it on for the first time, there was a distinctive humming for about 2 seconds and then the Mains fuse blew. I realised that the ~50VAC on the bias winding must've killed the filter caps in the bias supply.

The PT has a separate 50V bias winding. So now the question I'm pondering over is, if the bias winding is fecked (and I haven't re-tested it yet so it might still be okay), would I still be safe tapping a bias supply off the HT winding? The PT is quite sturdily made. I realise I will have to put new filter caps in the bias PS.

Or should I ditch the PT?

(Embarrassing mistake I know)

[Steve Conner]

If the bias winding is "fecked" it probably has shorted turns where the insulation melted.

If it has failed open, I still wouldn't trust it as it might develop into a short in the future, and that is a fire risk.

If the transformer works and doesn't draw excessive current unloaded and/or smell of burning, I'd just go with it. And count yourself lucky, the mains fuse isn't guaranteed to blow before the bias or heater windings do.