(RESOLVED) New build kept blowing fuses

[Snicksound]

FINAL EDIT: The cap is fine, but after installing a PT with more current capacity it seems the inrush current was stronger than before. So I did what Marshall did for years and moved the B+ fuse to after the first cap. All is good now.

I've never formed caps in my life, I only use new stock. Currently rebuilding a 50W Plexi and I used brand new F&T cap cans (the blue ones). Dual 50s for power amp and PI, dual 32s for the preamp.

Powered it up with no tubes to check everything, mains fuse instantly blew. No big deal, it was a 2A and I usually use a 3A in those (figured 2A would be enough with no tubes). Pop in a 3A and now the HV fuse blows. It was a 315mA, again I usually use a 500mA for these so I pop in a new 500mA... blows right away again.

I check everything, test everything, I can't find a fault. So I disconnect the choke and OT, put a 1M to ground resistor in its place, so now it's just the fuse, the dual 50uF caps and a resistor to ground. Bam, fuse blows again.

So I put a big fat resistor in lieu of the fuse (470k) so I can measure current across it. Everything seems normal. I gradually lower the resistance, reconnect each node one by one, until I only have a 1K and what little current is pulled across it is perfectly normal. I put a fuse back in... it works.

So all I can think of it, by putting a smaller and smaller resistor between the diodes and the caps I basically slowly increased voltage (while also limiting current) on those caps (from 180V ish to 460V). Aka... I formed them.

These were brand new F&Ts, vendor hasn't been selling these for that long so they couldn't have sat on the shelves for more than a few months.

Should I be worried?

[sluckey]

Electrolytics will dry out just as fast sitting on a shelf as they will sitting inside an amp. It's not uncommon to buy a new one that's already a prune. When it comes to electrolytics I like to buy from dealers that move a lot of stock. Better chance of getting a fresh cap.

[Stevem]

Electrolytic type caps of these types ( foil) produce there own insulation to keep there foil wrapped up plates from shorting out to each other by means of a chemical process that can only take place by means of a voltage placed across them.

This is why when you bring up newly installed power supply caps slowly with a veriac over the course of many hours to even a day in small voltage steps, the amps filter better and sound better.

I have proven this to myself over and over after I have installed new filter caps' so it's no Wives tail!

This all being said I would keep a Eye on those new filters for one that seems like it's running hot, or better yet if you have cap test meter that can read out the ESR value then unhook and fully ( slowly) discharge each filter and make a ESR test looking for a filter that test above 3 ohms of resistance on that test out.

Everyone toying with amps should have one of these meters!

There come in a mostly assembled kit form for all of like 30 bucks and do a whole bunch of test, including ESR / leakage!

You just need to be sure that when testing caps that can hold a charge that the cap is darn near fully discharged!

[pdf64]

Maybe you received a cap that had slipped through the factory forming process?
An unformed cap is a different thing to an ecap that has dried out; the latter is a show stopping failure, cap requires replacement. Whereas an unformed cap should be able to achieve full spec if it’s suitably formed / reformed.
No need for a variac, just use a suitably rated, high value series resistor (eg 100k) to limit the charging current to a safe value. When the voltage across the resistor drops to a low level (eg 10V) then the cap/s are formed.

EDIT - Though the forming / deforming thing is applicable to vintage ecaps, I remember RG Keen suggesting that modern ecaps are made using a different process (foil surface etched?), and it's irrelevant to them.

[Snicksound]

Ok... so it might not be the cap after all.

See, after "forming" the cap as I thought, when it did work without blowing the fuse, it still had a 1K resistor in parallel with the fuse (no voltage was dropped across it since the fuse is an open circuit at that point). This worked a few times. But when I removed the 1K resistor... bam fuse starts blowing up again... but only after a certain voltage has built up (not sure exactly how much, but there was still over 200VDC on the caps a few seconds after).

Now, true that the fuses are only rated for 250V, but that has never been a problem before. But then I realized the one thing that IS new: Littelfuse 345 series holder
A quick googlefu and I realize there are two versions of these, one of which is called HV for high voltage and rated for 600VDC. That's not the ones I got, I got the 250V ones (to be fair, both series are rated 250V according to Mouser's search engine, have to study the datasheets to figure this one out).

So, looks like it could simply be the fuse holder, which would be a new one to me. But odd that having a resistor across the fuse would fix that. Inspecting the fuse holder I see no signs of arcing.
I usually get the no-name fuse holders from Tube Depot or AES, which are also rated only for 250V as far as I know.

(Note that even though the fuse holder I'm using is not technically rated for the 460VDC in this amp, it does specifify "Insulation Resistance: 10,000 megohm minimum at
500 VDC")

[Snicksound]

Ok... so it might not be the cap after all.

See, after "forming" the cap as I thought, when it did work without blowing the fuse, it still had a 1K resistor in parallel with the fuse (no voltage was dropped across it since the fuse is an open circuit at that point). This worked a few times. But when I removed the 1K resistor... bam fuse starts blowing up again... but only after a certain voltage has built up (not sure exactly how much, but there was still over 200VDC on the caps a few seconds after).

Now, true that the fuses are only rated for 250V, but that has never been a problem before. But then I realized the one thing that IS new: Littelfuse 345 series holder
A quick googlefu and I realize there are two versions of these, one of which is called HV for high voltage and rated for 600VDC. That's not the ones I got, I got the 250V ones (to be fair, both series are rated 250V according to Mouser's search engine, have to study the datasheets to figure this one out).

So, looks like it could simply be the fuse holder, which would be a new one to me. But odd that having a resistor across the fuse would fix that. Inspecting the fuse holder I see no signs of arcing.
I usually get the no-name fuse holders from Tube Depot or AES, which are also rated only for 250V as far as I know.

(Note that even though the fuse holder I'm using is not technically rated for the 460VDC in this amp, it does specifify "Insulation Resistance: 10,000 megohm minimum at
500 VDC")

Well... nevermind this. I put the fuse holder that was there before the rebuild in its place and it's still blowing the fuse.

So if anyone has a theory to explain why having a 1K resistor in parallel with the fuse could prevent it from blowing, I'm all ears.

One other thing that has changed is I replaced the PT with one with slightly lower voltage (Hammond 290DX in place of 290GX), and the new one does have a slightly higher current rating (230 vs 213mA) and lower DCR (70 ohm vs 91.5 ohms) which means it has the ability to provide a stronger initial surge of current.

[Snicksound]

So anyway, got tired of blowing fuses, so I moved the fuse to after the first cap. It's not best practice, but Marshall shipped thousands of amps this way. Everything is fine now.

I'm gonna blame this on the stronger current capacity of the new PT (and associated lower DCR)