Blowing a fuse

thisisguitar

My Cornell Plexi blew a fuse this evening… I'm blaming the attenuator fully for it.

Finding the right fuse online took half an hour of searching to get them in the UK. eBay wasn't helpful, unless ordering from the other side of the world, Maplins have a fucking useless search function, and when I did finally find them on their site they are rated at 60vAc. Much gaffing finally led to Amazon, and now I have 10 on the way.

Moral of the story… make sure you have all the fuses you need for amps, its a ball ache you don't need, especially if you have a gig coming up. 

(as a side note, I ordered the wrong size first, and was most pleased with myself for finding it within a minute... If anyone needs a T1a 250 20mm x 5mm fuse, come talk to me!)

ecc83

Err? Well under a minute. No. link won't work (why dat mods?) but pt # GL59P

Dave.

richardhomer

There's such an obsession with Health and Safety now. Replacing a blown fuse with rolled up silver paper from a fag packet was where tone lived in my day....





(I AM joking btw!)

ICBM

thisisguitar said:

My Cornell Plexi blew a fuse this evening… I'm blaming the attenuator fully for it.

What attenuator is it?

An attenuator *shouldn't* cause a fuse to blow if the amp is capable of being cranked into a speaker - although some definitely are harder on the amp than they should be. Or it could be biased too hot, which will increase the risk of overloading a valve and causing a temporary arc.

Which fuse was it - T1A sounds like a mains fuse unless it's a 100W amp. Normally an attenuator-related problem would blow the HT fuse - assuming the amp has one. Blowing a mains fuse (assuming there is an HT fuse which didn't blow) could mean something more serious is up with it.

Either way a valve could have an intermittent short which will do it again when you least want it, even without the attenuator.

richardhomer said:

There's such an obsession with Health and Safety now. Replacing a blown fuse with rolled up silver paper from a fag packet was where tone lived in my day....

No, that's where the smoke lives!

thisisguitar

It's that Ultimate Attenuator, which is rubbish. The amp has been run continuously for 3 hrs at a time when running in my new speakers, no problems at all. Within 10 minutes of being hooked up with that it blew the mains fuse. 

The amp is 10 months old, and hasn't really seen much use at all. My 5150 used to throw a fuse every now and then, so I'm hoping that it'll be fine when I change it. The amp does have a HT fuse too. 

WIll report back when I've had a chance to test it, will also have a look inside before I change the fuse though, see if there are any obvious problems.

thisisguitar

ecc83 said:

Err? Well under a minute. No. link won't work (why dat mods?) but pt # GL59P

Dave.

Those are the wrong ones, I have ordered a packet of them from eBay… thanks for trying though :-)

My amp uses the 32x6mm size, hence the mistake. 

ICBM

thisisguitar said:

It's that Ultimate Attenuator, which is rubbish. The amp has been run continuously for 3 hrs at a time when running in my new speakers, no problems at all. Within 10 minutes of being hooked up with that it blew the mains fuse.

Ah.

What impedance did you have the amp set to? Anything below 16 ohms is too low for that "attenuator", regardless of what the maker says. The load resistance in it is 30 ohms, which is just about within the correct range for a 16-ohm amp, but no lower.

Over-high load impedance can cause arcing in the amp. Hopefully it will have just been in a valve rather than anything more serious and somehow blown the mains fuse first, even though it shouldn't. What value is the HT fuse?

The other main possibility is that it's arced across one of the power valve sockets from plate to filament, which would be more likely to blow the mains fuse. If you're unlucky that will entail changing the socket, so hopefully not.

thisisguitar

Amp was running at 8ohms, the HT fuse is 500mA.

I'll open her up and have a look tomorrow or late tonight if I have a chance. My mate said, 'put the attenuator in so we can crank it and hear it without earplugs'. 'I don't like attenuators, especially that one', I replied. 'Go on'…

Should have trusted my intuition, I remember @ICBM you told me they weren't very good :-(

ICBM

thisisguitar said:

Amp was running at 8ohms, the HT fuse is 500mA.

Should have trusted my intuition, I remember @ICBM you told me they weren't very good :-(

In my opinion, they're poorly designed, poorly made and dishonestly described. It's not an attenuator at all - it's a simple dummy load and a solid-state power amp 'reamp system' in a box.

The dummy load is 30 ohms, and comprised of two 15-ohm 50W resistors so it can't possibly take a 100W amp (which can put out up to double that when fully cranked) as claimed - worse, in the one I fixed the resistors, which are meant to be heatsinked to achieve the 50W rating, were intentionally mounted on small spacers off the chassis! I did not change it though - even though I really wanted to - in case there was a stability issue or something like that… I can't think of any other reason for it.

The rest of the thing was poorly made too - the one I fixed stopped working because one of the very poor-quality jacks which are fitted with no shakeproof washers had come loose. Not only that, the mains wiring was borderline acceptable and would not have passed a CE test - so it's not surprising that it is not sold 'officially' in the EU - hopefully yours is not as bad. The rest of it was a shoddy rat's nest of wiring too.

If you really want to risk using it again, set the amp to 16 ohms only.

thisisguitar

Had to replace the fan in it before I even tried it to be honest. Won't be using it again, ever.

Thanks for the info @ICBM, as usual you're a credit to the forum. Much appreciated :-)

ecc83

thisisguitar said:

ecc83 said:

Err? Well under a minute. No. link won't work (why dat mods?) but pt # GL59P

Dave.

Those are the wrong ones, I have ordered a packet of them from eBay… thanks for trying though :-)

My amp uses the 32x6mm size, hence the mistake. 

Ooops! Sorry, did not know it was OLD kit!

I suppose you found GL917 1A Time delay 31x6mm? Yes these are rated at 60V ac but I cannot understand why? I have been fitting 1/1/4" fuses to all sorts of gear for the best part of 50 years, never occurred to me that they might be the "wrong" voltage rating?

I think it must be an error, I mean, WTF used 60Vac back then? In any case, standard 20mm fuses are rated at 250V but we use them in HT circuits of 500V+ (well over 700V in one case!)

Dave.

ICBM

ecc83 said:

I suppose you found GL917 1A Time delay 31x6mm? Yes these are rated at 60V ac but I cannot understand why? I have been fitting 1/1/4" fuses to all sorts of gear for the best part of 50 years, never occurred to me that they might be the "wrong" voltage rating? 

I think it must be an error, I mean, WTF used 60Vac back then? In any case, standard 20mm fuses are rated at 250V but we use them in HT circuits of 500V+ (well over 700V in one case!)

I think it's a regulation rating rather than an electrical rating as such - because the 1-1/4" fuseholder fails the "toddler's finger test" by being a larger hole than 6mm. By restricting the fuse rating they theoretically prevent use of the fuseholder at over 60V as well.

If you think about it, a 1-1/4" fuse must have a higher absolute electrical rating than a 20mm one since the gap between the caps is bigger once the fuse has blown. I can't see any practical reason why a glass fuse isn't good for at least a couple of thousand volts in terms of what it would take to actually arc past it.

thisisguitar

@ICBM - New fuses have arrived. Amp powers on, but as soon as you hit standby it immediately blows :-(

ICBM

thisisguitar said:

@ICBM - New fuses have arrived. Amp powers on, but as soon as you hit standby it immediately blows :-(

It's got a blown power valve (probably) or output transformer (hopefully not…).

Remove the power valves and power up again. If it blows the fuse when you take it off standby, contact Denis Cornell .

thisisguitar

@ICBM - Tried your idea, you are right it's power valve(s) - Took them out and no fuses blow. 

A thousand thank you's!! 

ICBM

If you have enough fuses, you can easily find which of the valves is blown - this amp is safe to run with only one power valve, just put one in and see if it blows the fuse. If not, it's the other one . If it does, try the other valve - chances are that only one is damaged and the other will do as a spare.

Just glad to hear it's not the OT, you couldn't rule that out.

Are you going to Ebay the attenuator? If so, I would strongly suggest warning the buyer that it's not suitable for running at 8 ohms...

thisisguitar

I think I'll bin it.. couldn't live with knowing someone else may do the same!

ICBM

thisisguitar said:

I think I'll bin it.. couldn't live with knowing someone else may do the same!

If you're actually going to do that, I'll give you something for it. Not a lot though! I'd quite like to find out if the re-amp part has any value, apparently it's quite a clever circuit - I didn't bother to check on the one I fixed. I might even be able to rebuild it and make it into something usable.

PM me if you want.

strtdv

Glad to hear the OT didn't get taken out. I had 2 fuses blow in one afternoon on an amp once, never happened since. Apparently there was a known issue where a batch of cheap and slightly overrated fuses was used.

thisisguitar

Thanks @strtdv :-)

Sassafras

Ahha! The "old I think I'll bin it ploy" for a quick sale!