Blowing fuses

General discussion area for tube amps.

Moderators: pompeiisneaks, Colossal

Post Reply
User avatar
MakerDP
Posts: 281
Joined: Thu Jul 30, 2015 6:36 pm
Location: Central California
Contact:

Blowing fuses

Post by MakerDP »

I have an amp that's blowing the main fuse. I thought I saw a place where my bias circuit might have shorted but that didn't fix it.

EDIT:
OK I'm an idiot... if you read the original post please ignore. Still trying to track this down though.
R.G.
Posts: 1223
Joined: Tue Dec 02, 2014 9:01 pm

Re: Blowing fuses

Post by R.G. »

While you're doing your background work, if it turns out that you come back for more help, it would sure help use help you if you would note in your post:
> What amp??
> Better, a schematic of the amp you're working on.
> Which fuse? AC mains? B+? Does it have other fuses?
> In what manner does the fuse blow? Is there just a little segment melted out of the middle, or did an electrical arc explosion blast metal vapor all over the inside of the glass tube, mirrorizing it?
> How did this start? Did it work fine till one day, it blows fuses and does it every time? Did it "act funny" in some way leading up to the fuse blowing?
> What condition are the tubes in?
> Do you have any way to measure the bias voltages and currents on each output tube? That would sure help.

Just as background, the DC power supply and output amplifier are really two parts of the same thing. The output amp uses probably 90% of the DC power produced by the power supply; the preamp and driver stuff is a smallish wart on the power amp/power supply, from a DC power use standpoint.

Common flaws that cause fuse blowing are:
> Wrong AC fuse rating, especially normal blow versus slow-blow/time-delay
> Rectifier failures
> DC filter cap leakage, especially in older amps
> B+ bypass electrolytic caps
> Bias power supply problems in generating the bias voltages
> Bias wiring/socket issues getting bias to the power tube(s)
> Bad tubes - there's a reason they're in sockets.

UN-common flaws are many things, but include:
> It's probably not your power transformer (but, rarely, it could be)
> It's probably not your output transformer (but, rarely, it could be)
> It's probably not your choke (but, rarely, it could be)
> It's probably not your B+ wiring and bypass (but, rarely, it could be)
> It's probably not a shorted preamp tube (but, rarely, it could be)
> then there is a whole mess of everything else in the amp that it's probably not, but, rarely, it could be.
User avatar
MakerDP
Posts: 281
Joined: Thu Jul 30, 2015 6:36 pm
Location: Central California
Contact:

Re: Blowing fuses

Post by MakerDP »

Yeah I found the issue and fixed it. The original unedited post was a specific question that was just a stupid thought I had. That's why there was no detail.

Thanks.
User avatar
xtian
Posts: 6990
Joined: Mon Apr 19, 2010 8:15 pm
Location: NorCal
Contact:

Re: Blowing fuses

Post by xtian »

Hey, R.G., what an exhaustive post! If you've got that much free time, will you please sort my resistor pile? :mrgreen:
I build and repair tube amps. http://amps.monkeymatic.com
R.G.
Posts: 1223
Joined: Tue Dec 02, 2014 9:01 pm

Re: Blowing fuses

Post by R.G. »

Comes with practice. That was typed straight out. Easy when you've written about the same thing several dozen times in various places. :lol:
Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

Re: Blowing fuses

Post by Stevem »

I just went thru my resistor and washer pile on Monday, and neither was as much fun as typing out something to help someone. Lol!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
R.G.
Posts: 1223
Joined: Tue Dec 02, 2014 9:01 pm

Re: Blowing fuses

Post by R.G. »

And while I'm thinking about it, here's a helper for sorting that resistor pile. :D

Take a block of wood and drive in four nails in a small square, leaving the heads protruding about 1" above the surface. Solder some big-ish copper wire (from bits of Romex is very nice) across pairs of nail heads, making parallel raised rails. Clip your ohmmeter to two of the nails so you can read resistance between the two raised rails.

Now take a resistor by the body, and lay the leads across the rails. Instant resistor reading. Toss it into the appropriate pile. Next resistor.

It's better if your ohmmeter is auto-ranging, of course. Someday I'll get one of those. :lol:
tubeswell
Posts: 2337
Joined: Thu Mar 27, 2008 6:42 am
Location: Wellington. NZ

Re: Blowing fuses

Post by tubeswell »

1) Pull all the tubes out and see if it still blows the mains fuse.

2) Look for possible current pathways between the phase and neutral main wires on primary side of the PT which could cause heavy current draw on the mains fuse, such as a shorted mains lamp (if its wired with a mains lamp).

3) Look for possible current pathways between the circuit on secondary side of the PT and the ground return which could cause heavy current draw. e.g. it could be a shorted rectifier/filter cap. Or it could be some sort of other short from one of the secondaries to the ground return at some point in the wiring (like a short between a heater pin and a cathode pin on one of your output tube sockets).

Or it could be a shorted PT turn...
He who dies with the most tubes... wins
Post Reply