Fuse Questions

[Siegfried Meier]

Hey everyone,

Got a Marshall JCM2000 TSL that has blown the HT fuse (not ever sure what this stands for, if anyone knows I'd love to know myself...) that has a value of T1A (slo blow).  The filament is rather thick, so I went to our local Radio Shack/Circuit City and they have Slo Blo 1A fuses, but the filament is really thin.  I tried it out, and it seems to be ok but I'm worried it will blow at an inopportune time.

Can anyone tell me the difference of the thin vs thick filament and if it will actually make a difference?

Thanks!
Sig

 

[CJ]

maybe somebody before you put the big fuse in there,

inrush current into the caps on a solid state rect can be 10 times the normal current, so you need a fuse that will not pop on the inrush, a regular fuse would get stressed and fail possibly after a few on and off cycles,

HT stands for high tension, like the B+ tube supply,

 

[Siegfried Meier]

Hmm ok thanks CJ.

Is there a value that I'm looking for regarding the thickness of the filament?  I don't see anything labelled on the fuse itself or on packages at Radio Shack...

Thanks,
Sig

 

[CJ]

push pull circuits can draw at least twice the idle current under the right conditions, so a 6l6/el34 amp might draw 150 ma plate current for a pair, so 300 ma max, i would use a 750 ma fuse,

you can also skip the fuse and use resistors in the cathode circuit that fry and break the circuit when the tube shorts plate to cathode, but nowadays the bars are all no smoking, so you might want the fuse.

 

[Walrus]

HT means "high tension" i.e. lots and lots of volts!  ;D
"T" on a fuse means, semi-delay or slo-blow, and it's construction can vary as to how it is made. Some have the fuse element wound on a former which makes them look thick but are still only 1A rated. Others will look like a thin wire, but will be made of a material that gives it's slowness.

As long as the fuse is marked with a T or says slo-blow, then it will be fine.

 

[PRR]

> the filament is really thin.

There must be 50 ways to slow the blow. Eyeball doesn't tell you.

A push-pull audio circuit can, in principle, idle at near-zero current, so "at least twice the idle" is not a reliable guide. You go to the tube-book and look-up the bottles in question, allow some leeway for g-amps being hot-rodded somewhere past what the tube makers ever intended.

Most of the big-boy tubes draw (as CJ says) 200mA-400mA per pair at FULL roar.

The fuse is not intended to protect against slight overload. Typically it limits damage in a Dead Short. Such as you melting the screen-grid inside the tube causing a short across the HT supply. For reasonable normal performance, the HT will supply 10X normal current into a short, 2A-4A. The PT can stand this for some seconds, and does stand over an Amp when cold-starting the power capacitors.

1A Slo is a reasonable value.

> blow at an inopportune time.

Buy and carry a 5-pack of fuses, also any tool you need to get in there.