Blown PT custom designed amp

[lowell]

Hey all,
So I've now blown the SECOND Mercury Mag's PT in my custom designed tube amp. I built five of these amps. All 4 other ones are healthy and played regularly in live settings. Something is obviously wrong in this one. The mains fuse is blown, and I now think the PT is shot. I've disconnected the HT, filament, bias taps on the secondary and still my bulb glows very bright. With the amp off and no power - I am getting ~56ohms across the HT taps, 0ohms across filament taps, and 6ohms from one filament tap to one HT tap. The filament taps to the OTHER HT tap is 61ohms. So this DOES sound like it's blown, correct? Finally, what would you all recommend I do to troubleshoot my wiring mistake. It's possible there was a tube/heater short I guess, but would that kill the PT? My measurements were taken with NO tubes installed. The amp has 2 6v6 and 2 6l6 power tubes, 5 preamp 12xx7 style tubes and a 4A mains fuse BTW. This may be tough to answer without a schematic, but any off the cuff suggestions welcome. thanks!

[The Dude]

Not withstanding the fact that there's a problem somewhere and without seeing the schematic, my first question would be what is the current rating on the transformer? 4 amps seems high for a mains fuse. Does the amp also have an HT fuse? If the amp is fused correctly, you should not be eating transformers. The fuse should go first. Obviously, you'll have to figure out what is drawing so much current, but even if the amp shorts somewhere, it should be designed so that the fuse blows before severe damage is done. Can you provide us with a link to or specs on the transformer and a schematic?

Edit: Your heater tap should definitely not be shorted to your HT tap.

[Enzo]

You say disconnected. Are ALL those secondary wires sticking out into the air connected to absolutely nothing? Do not assume they are disconnected if they go to an empty socket. get the wires totally disconnected from everything, and check then. There could be some little bit of wire touching something in the circuit otherwise.

I'd pull the tranny out of the amp completely, clip mains to its primary through your bulb and then see what happens.

And if the transformer still seems shorted, pull the bells off it and look inside, maybe some wire got tugged and brought two leads together.

[lowell]

Great advice guys thanks. I'll report back.