Keep Blowing Fuses - Hoffman Princeton Reverb

[Magnatron]

I'm stumped.

I've performed a number of point-to-point checks on my build. The voltages all check out per a Princeton Reverb schematic and Rob Robinette's startup procedure using a light bulb current limiter (the bulb is always dim during testing). The transformers all check out right. I'm using an 8 ohm dummy load on the output. But when I pull the current limiter and turn the unit on, the fuse blows. It's happened now 5 times.

Argh! HELP!

[Stevem]

You have to narrow it down to a wiring issue or a tube issue.

Pull out the output tubes replace the fuse and plug it in direct , does the fuse then blow?

That amp takes a 1 amp slow blow fuse, is that what you have in it?

Another thing you can do to narrow things down is to yank out all the tubes. Install a 1/2 ( .5 ) amp fuse and plug the amp in direct.
If the fuse then blows you then for sure have a wiring issue or maybe a bad power supply filter ( shorted fully or somewhat) or a filter wired in backwards.

Is the bias filter wired in with its positive going to ground as it should be?

I have seen this mistake a lot on new builds when the builder is a inexperienced.

[Magnatron]

You have to narrow it down to a wiring issue or a tube issue.

Pull out the output tubes replace the fuse and plug it in direct , does the fuse then blow?

That amp takes a 1 amp slow blow fuse, is that what you have in it?

Another thing you can do to narrow things down is to yank out all the tubes. Install a 1/2 ( .5 ) amp fuse and plug the amp in direct.
If the fuse then blows you then for sure have a wiring issue or maybe a bad power supply filter ( shorted fully or somewhat) or a filter wired in backwards.

Is the bias filter wired in with its positive going to ground as it should be?

I have seen this mistake a lot on new builds when the builder is a inexperienced.

Thanks Steve. I'll check these. I am a bit inexperienced. While I have a background in electronics, it's a little rusty and this is only my second amp build.

[sluckey]

I'm using an 8 ohm dummy load on the output. But when I pull the current limiter and turn the unit on, the fuse blows. It's happened now 5 times.

That dummy load could be hiding some important clues. Use a real speaker so you can hear if the amp is howling or making any other unusual sounds. Your ears are a good troubleshooting tool.

[xtian]

turn the unit on, the fuse blows. It's happened now 5 times.

When does the fuse blow, immediately when you throw the power switch, or 10-20 seconds later, after tubes heat up? Do you have a standby switch, and does it relate to the problem? What fuses are you using, amerpage, quick or slow?

[R.G.]

Have you been through the procedure titled "Blows fuses debug" listed at the top of the forum?

[Stevem]

Please note that if you keep on blowing fuses of the correct 1 amp slow blow type there’s a real good chance of shorting out the power transformer.

These can only take do much before going south!

[Magnatron]

Have you been through the procedure titled "Blows fuses debug" listed at the top of the forum?

I'm running through that right now. I didn't see it until I posted.

[R.G.]

No problem.
Fuse blowing debugging is hard because when the fuse blows, all work stops until it's replaced. Using a light bulb limiter can help because it can keep the fuse from blowing until you can find a problem, but then figuring out exactly what is eating all that current is hard because tools to follow currents along specific paths are not common. Opening conductors to isolate an overcurrent is at least something we can do with soldering irons.

[GAStan]

Actually test tools are getting less expensive. At work we use some Fluke 77x milliamp clamp meters but these are prohibitively expensive for the hobbyist.

However searching Amazon for "milliamp clamp meter" returns several options, some relatively inexpensive. Here's a $65 one that connects to an o'scope that I think would be useful. You would need to keep in mind you are looking at current vs voltage. Biggest challenge I see with it might be the size of the clamp in the tight quarters of an amp.
https://www.amazon.com/dp/B06W2KFZLW?st ... 922XD05G7M