# Popped a fuse?



## dwagar (Mar 6, 2006)

I had a fuse pop on my 84 JCM800 last night.

What happened was, I'd set the amp up, turned the power on, set up my pedals and stuff, asked the bass player to turn on the standby switch, and the fuse blew.

I just replaced the fuse, and everything seems fine now.

Buuuuut, I'm obviously concerned with what may have caused it to blow.

Do fuses periodically need replacing? Like, every 25 years or so?

Is it possible I got smacked with a power surge? I didn't have (slap me upside the head) a surge protector on the line.

I think I'll leave it running for a few hours, just to make sure it doesn't pop again, but, does anyone have any ideas what could have caused this? Other than the fact that I let the bass player touch my Marshall, of course.


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## Greg Ellis (Oct 1, 2007)

Imagine what could have happened if the *drummer* turned it on!


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## dwagar (Mar 6, 2006)

yeah, but remember, there are two switches, one marked power, one marked standby. Could you trust the drummer to get it right?

(on the aside, our drummer has his PhD. We love making dummy drummer jokes to him)


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## Wild Bill (May 3, 2006)

dwagar said:


> I had a fuse pop on my 84 JCM800 last night.
> 
> What happened was, I'd set the amp up, turned the power on, set up my pedals and stuff, asked the bass player to turn on the standby switch, and the fuse blew.
> 
> ...


Ordinarily, fuses blow because something MAKES them blow! That being said, if you put a new fuse in usually it will then blow also.

If you were running the original fuse it wouldn't be surprising that the fuse just got old. Every time the amp is turned on there's a surge through the fuse. The damage to the fuse is tiny but it's real. After 25 years the poor thing may simply have had enough! I've had this happen with other old amps.

The other possibility is that the bias is running the idling current too high. This is hard on the tube life and runs more current through the fuse than expected when the amp is cranked. Not enough to immediately pop the fuse but after wailing the amp for a set or two it will blow.

The cure there is to have the tubes and bias checked.

:food-smiley-004:


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## dwagar (Mar 6, 2006)

Thanks Bill, the tubes seemed to check out okay, but I'll check the Bias setting tonight.

or as you said, perhaps after 25 years that fuse had just had enough, lol.


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## dwagar (Mar 6, 2006)

the tubes check okay, I checked the bias:

460 plate v
I set the bias to about 35mA (one is 34.5, the other 35.5)
so if I calc correctly, I'm running at about 64%

I'm using JJ E34Ls. 

and, she is still working okay.

So I think it was, after all, just the fuse.


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## nonreverb (Sep 19, 2006)

dwagar said:


> the tubes check okay, I checked the bias:
> 
> 460 plate v
> I set the bias to about 35mA (one is 34.5, the other 35.5)
> ...


Hey,

A couple of questions for you...was it the mains fuse or the H.T fuse and was the fuse a slo blo?

:smile:

Richard


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## dwagar (Mar 6, 2006)

mains, and yes, a slow blow.


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## Big White Tele (Feb 10, 2007)

Thats funny, I just had the exact same thing happen to me today, except no bass player. It was a 66 Deluxe Reverb, and the fuse just let go. Put a new one in, and all is well, hopefully for another 42 years.


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