Fuse with a difference! - UK Vintage Radio Repair and Restoration Discussion Forum

ITAM805 · 8th Apr 2015, 4:47 pm

I have a Mesa guitar amp in for repair and found this in the mains fuse position - a 2R 1/4W !

It's a new dodge for me, what were they thinking?

G6Tanuki · 8th Apr 2015, 4:52 pm

In some ways it's a sensible idea.

See the spring at the end? That's under tension - if there's a sustained overload not-quite-enough to blow a traditional fuse, it will nevertheless heat the resistor, melt the solder, and the spring will then retract, breaking the circuit.

A similar idea was used in a lot of 1950s/1960s TVs: a chunky wirewound resistor with a soldered-closed leaf-spring on the side. Prolonged overload melted the solder and the spring did the rest.

mark pirate · 8th Apr 2015, 5:26 pm

I have some similar fuses in the workshop, I think they were removed from a video recorder I scrapped.
Mine too are spring loaded at one end.

Mark

ITAM805 · 8th Apr 2015, 10:18 pm

thanks guys

interesting as it obviously has been working, but if someone said to me can I put a 1/4W resistor in the 240V mains supply of a 20W valve amp (probably drawing 30+ watts with the heaters etc) my gut instinct would be - NO!

But as ever, we live and learn

Alistair D · 8th Apr 2015, 10:36 pm

1/4 watt resistors are normally rated to 250V DC. Under short circuit conditions I suspect that the resistor would burn out before the the temperature could rise enough to melt the solder. In that case the carbon blackened resistor would be subjected to 350V peak. Not 100% happy with this idea.

Al

broadgage · 8th Apr 2015, 10:53 pm

Quote:

Originally Posted by ITAM805

interesting as it obviously has been working, but if someone said to me can I put a 1/4W resistor in the 240V mains supply of a 20W valve amp (probably drawing 30+ watts with the heaters etc) my gut instinct would be - NO!

It would depend upon the value of the resistor.
In this case 2 ohms. If the mains input current was 0.1 amp, then 0.2 of a watt would be dissipated in the resistor, within its rating.
At 0.15 amp mains input (more probable IMHO) the dissipation would be 0.3 of a watt, somewhat in excess of the nominal rating but arguably acceptable if the purpose is overload protection rather than a stable resistance as part of a circuit.

McMurdo · 9th Apr 2015, 12:15 am

That's quite a common type of commercial fuse, it's a time delay fuse and has a normal fuse wire and surge spring attached to the resistor. I suspect it's to modify the fuse's I2t characteristic.

http://uk.farnell.com/littelfuse/031...slo/dp/1149695

hans · 9th Apr 2015, 12:26 am

As far as I can see the resistor is 1.2 ohm.

ITAM805 · 9th Apr 2015, 10:28 am

Thanks Kevin

I've never seen one before, certainly not in an amp. Little Fuse know their stuff so it must be a sound idea, and from what mark pirate said it's not a new idea either?

Hans, yes you're correct but the fuse measures 2R

MrElectronicman · 9th Apr 2015, 10:49 pm

I have come across this before in VCR's, they used a low value low wattage resistor as a type of Slow-Blow fuse. Took me some time to figure it out as in my case the resistor was soldered into to the PCB and looked like a normal component

Biggles · 9th Apr 2015, 11:32 pm

Could it be a fuseable resistor? I have used these in place of a "proper" fuse on projects over the years. They are available from RS and Farnell etc and come in various values to suit different overload currents.

8th Apr 2015, 4:47 pm	#1
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Fuse with a difference! I have a Mesa guitar amp in for repair and found this in the mains fuse position - a 2R 1/4W ! It's a new dodge for me, what were they thinking? Attached Thumbnails

8th Apr 2015, 4:52 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	Re: Fuse with a difference! In some ways it's a sensible idea. See the spring at the end? That's under tension - if there's a sustained overload not-quite-enough to blow a traditional fuse, it will nevertheless heat the resistor, melt the solder, and the spring will then retract, breaking the circuit. A similar idea was used in a lot of 1950s/1960s TVs: a chunky wirewound resistor with a soldered-closed leaf-spring on the side. Prolonged overload melted the solder and the spring did the rest.

8th Apr 2015, 5:26 pm	#3
mark pirate Dekatron Join Date: Mar 2008 Location: Worthing, West Sussex, UK Posts: 5,185	Re: Fuse with a difference! I have some similar fuses in the workshop, I think they were removed from a video recorder I scrapped. Mine too are spring loaded at one end. Mark

8th Apr 2015, 10:18 pm	#4
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Fuse with a difference! thanks guys interesting as it obviously has been working, but if someone said to me can I put a 1/4W resistor in the 240V mains supply of a 20W valve amp (probably drawing 30+ watts with the heaters etc) my gut instinct would be - NO! But as ever, we live and learn

8th Apr 2015, 10:36 pm	#5
Alistair D Nonode Join Date: Jan 2010 Location: Edinburgh, UK. Posts: 2,008	Re: Fuse with a difference! 1/4 watt resistors are normally rated to 250V DC. Under short circuit conditions I suspect that the resistor would burn out before the the temperature could rise enough to melt the solder. In that case the carbon blackened resistor would be subjected to 350V peak. Not 100% happy with this idea. Al __________________ I won't tell you how I discovered that.

9th Apr 2015, 12:15 am	#7
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,269	Re: Fuse with a difference! That's quite a common type of commercial fuse, it's a time delay fuse and has a normal fuse wire and surge spring attached to the resistor. I suspect it's to modify the fuse's I2t characteristic. http://uk.farnell.com/littelfuse/031...slo/dp/1149695 __________________ Kevin Last edited by McMurdo; 9th Apr 2015 at 12:22 am.

9th Apr 2015, 12:26 am	#8
hans Heptode Join Date: Jan 2008 Location: Oslo, Norway. Posts: 632	Re: Fuse with a difference! As far as I can see the resistor is 1.2 ohm.

9th Apr 2015, 10:28 am	#9
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Fuse with a difference! Thanks Kevin I've never seen one before, certainly not in an amp. Little Fuse know their stuff so it must be a sound idea, and from what mark pirate said it's not a new idea either? Hans, yes you're correct but the fuse measures 2R

9th Apr 2015, 10:49 pm	#10
MrElectronicman Heptode Join Date: Feb 2011 Location: Washington DC, USA Posts: 619	Re: Fuse with a difference! I have come across this before in VCR's, they used a low value low wattage resistor as a type of Slow-Blow fuse. Took me some time to figure it out as in my case the resistor was soldered into to the PCB and looked like a normal component __________________ David

9th Apr 2015, 11:32 pm	#11
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Fuse with a difference! Could it be a fuseable resistor? I have used these in place of a "proper" fuse on projects over the years. They are available from RS and Farnell etc and come in various values to suit different overload currents.