Single Ended Amp Problem - Fuse Blowing

[RockNRollDr]

I am building a single ended amplifier based on a 5F2A but with solid state rectification and after a few hiccups I am now stuck. After 5-10 seconds after powering the amp on the fuse blows, I took the tubes out and it did the same thing. I was lucky enough to have my guitar plugged in the first time and after the tubes warmed up the amp sounded fine but only for a second then the fuse blew. Also the second filter cap, 22uf 500V, is getting extremely hot, over 250 degrees, while the others are 90-100 degrees. Any help figuring out why this is happening would be greatly appreciated.

[kbperry810]

Have any pictures of the insides to share?

What is the make/model of the PT you are using?

Also, for the SS rectifier, are you using an internal bridge, or is there an actual octal socket with a SS rectifier plugged in (just curious)

[RockNRollDr]

I will have to snap some pictures of the insides to post, the PT is BDG-2540-60. The rectifier is bridge, attached is the schematic I did.

[leadfootdriver]

Are all' your diodes pointed the right way? Otherwise, there must be a short somewhere.

Be careful.

[RockNRollDr]

Yes all of the diodes are pointed the right way, I have had that problem before haha, I felt like an idiot.

[leadfootdriver]

I've never had a cap blow up before, but I guess it's no joke. Wear some eye protection.

[Firestorm]

Might be a bad cap. Partially shorted and drawing big current. But do post some pix in case it's something else.

[kbperry810]

I couldn't find a wiring diagram for that PT by the part number - and it looks like the schematic image is blocked at work for me so not sure how much help I'll be

I was curious as to if your PT has multiple primary leads - if so, there are a few other possible things to check; like for instance making sure you wired up the correct respective pair/pairs for your primary voltage... and/or if the two leads you are using have to follow a specific phase - in which case reversing them could do the trick (asusming the damage is not already done of course).

Otherwise, have you checked the diodes to make sure you don't have a bad one?

[ToneMerc]

Divide and conquer, pull the 2.2k in the PS rail and see if the problem persists.

TM

[RockNRollDr]

Still blows the fuse.

[Phil_S]

Good call by ToneMerc. If it blows, that narrows the problem considerably. Parts in play include the OT, rectifier, PT, two caps...is that a Zener on the PT CT? Remove the Zener. Your amp will be fine with 275-275, or raise the 120-5W if you want to drop a few volts in the B+ ladder, maybe 500 ohms +/-.

Back up to the next dropping R. Lift one side of the 120 5W and see if it still blows. That will eliminate the OT and the 2nd cap. If that blows, then lift the connection from the rectifier to the 1st cap. If that blows, you are limited to the rectifier or the PT (and how they are wired).

Have you checked your wall supply to the primary for a short?

Gut shot(s) please?

[LOUDthud]

Disconnect between the 1N4007s and that first filter cap. If it still blows a fuse, it might be the power transformer.

[VacuumVoodoo]

Are you by any chance using old electrolytic caps?
Are you sure the one getting hot isn't connected in reverse polarity?
In any case an electrolytic cap that cooked sould be replaced.
Remove that cap and try the amp, if all else is ok absence of that cap will only cause a higher level of hum.

[katopan]

A build a light bulb limiter. Search for it. Every time you blow the fuse you're doing more damage. If your PT is OK, it might not be by the time you finish all your testing.

[rdjones]

I've never had a cap blow up before, but I guess it's no joke. Wear some eye protection.

I have, right next to my head.
New guy in the shop installed a cap backwards and on power up we heard a weird static.
I leaned over to hear if it was coming directly from the amp rather than the speaker and BAM.
Confetti and shrapnel all over the shop, a piece of which hit me in the top of the head.

So, yeah, watch out for safety risks of all kinds when working with high voltages.
If the cap is getting hot there's a problem, either over voltage, reverse voltage or an internal short/leak.

Looking at this from a different angle, the 3/4 amp fuse is the smallest fuse in any SE Princeton or Champ.
Other SE Princetons and the Tweed Champs use 2A, and AA764 and later Champs use 1A.
(admittedly these all have the additional drain of a 5Y3)

We need to see some more detailed voltage and current readings.
Can you rig up a 1 or 2 amp circuit breaker to avoid going through fuses ?
Variac or bulb limiter ?

rd