Troubleshooting after power tubes die.

[Chill]

yeah, scope the raw B+ to make sure the rectifiers are not leaky. Any AC on those caps will not make them happy.

Hmm, it blew a 2A sloblo fuse at the show, which means the rectifier diodes may have been damaged so I replaced all 4 of them. They're cheaper than getting a scope.

What voltage appears at the CT without the first filter but with the choke and second filter connected to screens and all the other stuff? What voltage appears at the screen node?

414V at the CT/choke node when disconnected from the first cap. 414V at the screen node (with the lead to the sockets/ screen R's still disconnected).

Does that first filter ALONE still blow the fuse now?

Yep.

Could it be an issue of an inrush of current which blows the fuse, but it might otherwise hold?

ANd one side when used as the first filter pops the fuse?

Before, it was using both sides of a 47uF/47uF cap can, this last time I only connected one side in the hope of not having to replace another cap can.

You say the cap was used. OK, why was it pulled from something in the first place? I don't usually remove parts that are good...I would also say that if you replace something with a used part, you ought to mention that.

The last cap worked fine in an amp running at about 380V DC which I gutted because of bad layout on the chassis. Some of the parts made their way into another amp, the caps did not.

And just not to assume anything, we are sure we used the proper terminal for ground on it?

Yep.

[Chill]

Could it be a fault in the PT when the high voltage secondary is connected?

[Alex R]

I bet it's still the used can cap. I also had trouble with those new-manufacture can caps in the past and now I don't use them. The standard 450v caps are fine. For more than 450v, 2 x 350v in series, each one paralleled with a 220K, at double the required uF. Easy enough to wire up the radial types on a tag strip.

Now whenever I replace old can caps I leave them in situ for appearances and to block up the hole in the chassis, shear off their tags, and fit tag strips with modern standard caps underneath. No problems with them. They are manufactured in much bigger numbers than the cans, and for much wider markets. My instinct is they will be that much more reliable. Cheaper and smaller too.