Blowing fuses

[Rob Livesey]

Hi Chaps,

I have an original 1965 blackface Deluxe Reverb. About 10 years ago it was recapped, serviced and retubed and has been one of my favourite and most reliable amps of all time.

Last week, the bias supply cap failed, the tubes drew loads of current (loud hum) and the mains fuse blew. The cap has now been replaced.

At the weekend, I played a festival over two days, the first day, the amp was fine, the second day it would not power up and it appeared that the fuse blew as I turned the amp off the previous day. I replaced the fuse and powered up and down a few times without any problems.

Last night, on a fresh set of power tubes, I used the amp again for a recording session and as I switched the amp off at the end of the session, it blew the fuse again.

This seems to be intermittent, what could be the possible cause of this fault?

Cheers,
Rob.

[Blue Strat]

Odd problem. It's unusual to have a fuse blow at POWER OFF.

Is the bias cap polarized correctly? Positive to ground? What bias setting did you use? 20mA is about right.

Was the recently replaced bias cap changed 10 years ago with the other caps?

[Trout]

Last night, on a fresh set of power tubes, I used the amp again for a recording session and as I switched the amp off at the end of the session, it blew the fuse again.

After the initial failure of the bias cap was the replacement fuse the a correct amperage slow blow fuse?

I have seen on several occasions NON-slow blow fuses blowing on power off. There is often a bit of flyback voltage generated by opening the circuit.

Also,

About 10 years ago it was recapped,

There is no fixed in stone service life on caps. Some last decades, some barely make it to normal service life. @ 10 years they are approaching the end of life and may be contributing to the problem. Generally that causes power up problems, but it can also cause power down blows.

Trout

[Rob Livesey]

Is the bias cap polarized correctly? Positive to ground? What bias setting did you use? 20mA is about right.

Was the recently replaced bias cap changed 10 years ago with the other caps?

I'll have to check the orientation of the cap, I didn't do this myself as I was busy rehearsing and dropped it off and picked it up the same day. I trust the guy who did the work though, he's very good.

Yes, this was a Sprague Atom that failed

After the initial failure of the bias cap was the replacement fuse the a correct amperage slow blow fuse?

I have seen on several occasions NON-slow blow fuses blowing on power off. There is often a bit of flyback voltage generated by opening the circuit.

Trout

I don't know. I have bought new fuses today to make sure of that. We'll just have to see how it pans out, I'll be carrying fuses for years after this !

Cheers,
Rob.

[Trout]

I don't know. I have bought new fuses today to make sure of that. We'll just have to see how it pans out, I'll be carrying fuses for years after this !

Cheers,
Rob.

I had an old amp Kay that had a nuisance blow about every 25th power down. I spent countless hours trying to locate the reason. After replacing every part in the amp I ended up putting a .01/1600V ceramic cap on the power switch. As much as I hated to do that, it never blew another fuse.
Marginal design combined with possibly off spec or dying PT was most likely the culprit on that old amp. It also had a tendancy to blow pilot lamps fairly often.

Good Luck!

Trout

[BassHog]

I have seen this happen many times with Fenders when a fast blow fuse is used instead of a slow blow.
I'm betting thats all it is

[stratman89]

I'm having a problem with blowing fuses too! I've got a fairly new Marshall 1974 X and when the bass player turned off his amp mine shut off. We were sharing the same power bar and the light stayed on the bar. I changed the mains fuse with the correct slow blow but it blew again. I was told it was probably the power tubes so I changed them and the fuse blew again. Any thoughts on what's going on?

[John Phillips]

Sadly, with a modern Marshall my first thought is that it's a blown transformer. I hope I'm wrong.

Try pulling all the tubes (safe on a tube-rectified amp - but don't pull the preamp tubes on a solid-state-rectified amp) and power up. If the fuse now blows, it's a near certainty that it's the PT.

If not, put in the preamp tubes and try again. (It's unlikely but just possible it could be a filament short, I came across one just a few weeks ago.)

If it's still OK, put in the rectifer. If it blows, it's probably the rectifier tube or a filter cap.

If the fuse still holds, chances are it's a power tube - far from unlikely even if they're new, unfortunately - but this would more likely blow the HT fuse not the power fuse (I'm fairly sure the amp does have one).

[Rob Livesey]

Thanks to everyone who replied. I've managed to get through a couple of gigs now without popping fuses. I still have the new power tubes installed, I might go back to the RCA blackplates I had in there before to try my luck shortly.
It appears that my new fuses are more robust than my old ones, a new box has managed to do the trick. Whether or not I had quick blow instead of slow blow is a mystery, they were labelled T1A, but the new ones are definately Slow Blow and all seems well.

Thanks again,

Rob.

[stratman89]

Sadly, with a modern Marshall my first thought is that it's a blown transformer. I hope I'm wrong.

Try pulling all the tubes (safe on a tube-rectified amp - but don't pull the preamp tubes on a solid-state-rectified amp) and power up. If the fuse now blows, it's a near certainty that it's the PT.

If not, put in the preamp tubes and try again. (It's unlikely but just possible it could be a filament short, I came across one just a few weeks ago.)

If it's still OK, put in the rectifer. If it blows, it's probably the rectifier tube or a filter cap.

If the fuse still holds, chances are it's a power tube - far from unlikely even if they're new, unfortunately - but this would more likely blow the HT fuse not the power fuse (I'm fairly sure the amp does have one).

Thanks very much for the info John, I tryed your first recommendation of pulling all the tubes. I then then powered up and the fuse blew. I guess that's a good chance the PT is gone. I took it back to the music store I bought it from and since it's still under warranty it's off for repair. I hope Marshall has an upgraded tranformer to replace the ones that so many have had trouble with.