Fuse Help

[xampworshipx]

So the guitarist in my band has a 200 watt Earth Sound Research head from the 70s. One of the fuses blew at practice the other day, and according to the back of the amp it's a "KTK 1.5 amp". We can't seem to find any 1.5 amp fuses that aren't insanely huge, anyone know where I can get a suitable replacement? As far as I can tell size wise it's the same as the pretty standard fender/marshall smaller one. Thanks!

[Bruce / Mission Amps]

So the guitarist in my band has a 200 watt Earth Sound Research head from the 70s. One of the fuses blew at practice the other day, and according to the back of the amp it's a "KTK 1.5 amp". We can't seem to find any 1.5 amp fuses that aren't insanely huge, anyone know where I can get a suitable replacement? As far as I can tell size wise it's the same as the pretty standard fender/marshall smaller one. Thanks!

I have a dead stock Earth G-1000 (four 6L6GCs and about +80 watts output) in the shop for a tune up right now and it has a standard 3a slo-blo fuse in it ... same kind that would fit a Fender tube amp.
By the way it doesn't say KTK 1.5a on this fuse holder...


Take a picture of the fuse holder in yours and send along.

[xampworshipx]

Thanks bruce, i'll try to get a picture up soon. It has one of those fuses (3a slo-blo) in addition to the KTK one, this thing is monstrous (HUGE power transformers, 4 6550s and 3 12x?7s). I think, but am not 100%, that it is the same as the plush 4000 series if you've ever run into one of those.

[Bruce / Mission Amps]

Boy, for the life of me I can't think why that fuse would be needed and it doesn't show up in any schems of other Earth amps I have.

Even the big ass Sunn amps didn't have this even though some Ampegs and the like had circuit breakers.
Regardless of what you find, I'd do a work around and not tolerate that ugly thing.
Can you scribble up a schematic of where it is in the circuit?

[xampworshipx]

Obviously we were fools. I checked tons of vintage amp store sites and all over the net for the fuse, and had a ton of trouble finding it. They have it at rat shack and on ebay, sometimes I forget just because something isn't a common value for amplifiers doesn't mean it's not a common value.

I need to restore this badboy in general, I will for sure draw a schematic when I pop it open. I haven't looked under the hood in awhile, but I remember it being pretty simple and clean. It's also ptp so i'm not worried about drawing it out.

Based on the back panel, if I had to guess, it would appear that this fuse is probably right after the output transformer before the speakers. I'd guess this is to minimize damage to speakers based on voltage spikes? I think the fact that the amp still powers on completely fine but generates zero sound makes this a fairly educated guess.

[Bruce / Mission Amps]

Well, it could be a speaker fuse... but using the oxymoron "RMS of 200 watts" across a light load of only 16 ohms, the amp would push 3.5 amps, not 1.5a.
Way worse with heavier speaker loads like 4 ohms.... about 7 amps.
Sq rt of P/R = I

So, I'm guessing it could be a high voltage, OT center tap fuse.
In that case, it could be there was no fuse holder rated to the B+ of the amp but it would seem that a slo-blo 2a fuse would be OK for a 200 watter.
I'm at a loss at why it would need that giant fuse but my little DMMs have big ass 10a fuses too!
I hope somebody else can chime in here.

[Steve Conner]

The Buss KTK fuses have a 600 volt rating, so maybe the designer thought they would be theoretically safer as a HT fuse than the garden variety ones, which aren't rated to break a DC arc at all, but every other amp in the world uses them as HT fuses regardless.

Cooper Bussmann KTK Series Fuses

They're used in DMMs for the same safety-related reasons.

[Bruce / Mission Amps]

The Buss KTK fuses have a 600 volt rating, so maybe the designer thought they would be theoretically safer as a HT fuse than the garden variety ones, which aren't rated to break a DC arc at all, but every other amp in the world uses them as HT fuses regardless.

Cooper Bussmann KTK Series Fuses

They're used in DMMs for the same safety-related reasons.

Yes sir just like the ones in my two or three DMMs.

[xampworshipx]

I put the new ratshack fuse in, and lo and behold, perfectly functional. I need to at some point down the line do an overhaul on this thing, i'm a bit curious as to why the fuse blew to begin with. I'll make a thread on the overhaul when I start it, this thing is a beast, and man does it sing.