Nomination: Worst fuse holders ever

[Rhodesplyr]

A couple of days ago, I spent about an hour locked in battle with the Mains and H.T. fuse holders on a Marshall JCM800 head. The amp was working--I was simply checking the fuses to make sure the right values were installed, but when I tried to power the amp back up, there was no power. The fuses were good, but the fuse holder circuits were open.

These holders are snap-in types with a metal sleeve that fits over the fuse and a spring in the end-cap. I think the idea was that the spring presses the metal sleeve and spring against another contact inside the holder. The problem seems to be that if there's ANY binding between the plastic end-cap and the metal sleeve, no contact is made.

I finally got them to work by 1) seating the fuse completely in the holder (not the cap) 2) pressing the sleeve into position around the fuse and 3) fastening the end cap with spring inside. I can't imagine that this was how they were designed to work. And, of course, the springs have an annoying tendency to fall out of the end caps.

My conclusion was that these were an attempt at a clever new fuse holder design, but I've never had this much trouble with one in all the years I've been working on electronics.

[Enzo]

Or the particular holder in that amp was defective. Some little something was bent inside, or a little piece fo metal somewhere was broken off, or whatever.

[teemuk]

I hate the practice of not using fuse holders at all but instead soldering the fuse in place.

[Phostenix]

Originally Posted by teemuk

I hate the practice of not using fuse holders at all but instead soldering the fuse in place.

Preferably with an opaque sleeve over it so that you can't tell whether it's good without a meter. I guess the upside is that it's truly an idiot fuse. Most people won't even notice it....

[Rhodesplyr]

Originally Posted by Enzo

Or the particular holder in that amp was defective. Some little something was bent inside, or a little piece fo metal somewhere was broken off, or whatever.

It was BOTH holders. I think the design would work if the metal sleeve vital for making contact moved freely inside the plastic end-cap, but it's possible that the plastic warped slightly over time or that the sleeve got bent at some point. But my point is that even an imperceptible distortion of the metal sleeve or the end cap makes the holder not work like it's supposed to.

[Rhodesplyr]

Originally Posted by teemuk

I hate the practice of not using fuse holders at all but instead soldering the fuse in place.

How about using no fuses whatsoever--like the Dynaco PAS preamps?

Do I smell something burning....

[Sweetfinger]

Just as all 1/4" plugs aren't quite the same diameter and length, maybe some fuses are just enough off of the original fuseholder spec to cause problems.

[Don Moose]

Why not breakers?

Weber sells 'em for the same price as a fuseholder.
You can't put the wrong value in, or defeat them without opening the chassis.
You can't run out of spares.

They're marginally larger, but not longer or wider than a traditional fuse holder.

[Steve Conner]

Annoyances.org - How many forum members does it takes to change a light bulb?
(substitute fuse for light bulb in the above )

[Enzo]

That's great.

[drewl]

Originally Posted by teemuk

I hate the practice of not using fuse holders at all but instead soldering the fuse in place.

My friend who played bass with me in high school never fails to remind me of when I put tinfoil around the fuse in his Bassman when we were 15.

Weber sells breakers? great. I need one for a Randall I'm working on.

[olddawg]

Originally Posted by drewl

My friend who played bass with me in high school never fails to remind me of when I put tinfoil around the fuse in his Bassman when we were 15.

Every once and a while an amp came across my bench with 1/4" aluminum dowel cut to the length of an AGC fuses in the fuse holders. The PT was always bad of course. BTW, I always like pigtail soldered in fuses. There are no bad connections and it forces you to use a meter and not rely on looks. I guess it's a pain if you have to pull the chassis. If the chassis is out I prefer it. Besides, you can always install a twist type fuse holder like they use on car stereos.

[Enzo]

Had a burnt up amp come in once with a section of 1/4" bolt sawn off and clipped in the fuse holder. but it was good audio quality stuff, it was one of those gold plated 1/4" bolts like they do power connections with in car stereos.

I don't mind pigtails, but for situations I have a box of twin fuse clips - back to back clip sets. You clip it onto the body of the open pigtail fuse, which is the size of a common fuse. Then in the empty clips on the other side of it, you snap in a common fuse. Takes two seconds, no soldering. yes of course you can abuse it, but it gets you going fast.

Mouser 504-3434. A drawing of it is on the Bussmann fuse holder page, item N on mine.

[olddawg]

That's basically what I meant with the twist fuse holder and an alligator clip at each end.

[tedmich]

circuit breakers can be great, but often its just the house fire they avoid. The cheap thermal (bi metallic) circuit breakers usually used are designed to trip within one hour at 140% of its rating (?!!)

Much better are magnetic circuit breakers that trip in 10 ms or less, or hybrid thermal/magnetic...guess which cost more?

A central fuse block (3-4 fuses, internal) is good for most amps but the panel mount units can be convenient (and/or a pain).

[Mark Black]

My nomination for the worst fuseholders ever would have to be the practice of using only a fuse embedded in the power supply transformer, as in some DBX and Behringer signal processing gear, with no other external or internal fuse in circuit.

So, fuse blown = new transformer (unless you are skilled and/or lucky enough to dig out the bad fuse & replace it).

Maybe someone can explain to me how this is a good idea...

[olddawg]

Originally Posted by Mark Black

My nomination for the worst fuseholders ever would have to be the practice of using only a fuse embedded in the power supply transformer, as in some DBX and Behringer signal processing gear, with no other external or internal fuse in circuit.

So, fuse blown = new transformer (unless you are skilled and/or lucky enough to dig out the bad fuse & replace it).

Maybe someone can explain to me how this is a good idea...

Usually that's a termal fuse. Remmber....FUSES ARE THERE TO PREVENT FIRES NOT TO PROTECT EQUIPMENT. Usually you can remove the paper from the transformer and jump the fuse. Just make sure you install an external fuse and tell the customer you did it. I only did it when a replacement transformer was not available (or if it was my own stuff) and made the customer sign a disclaimer. Truthfully though, I've fixed a lot of abandoned equipment like this.

[drewl]

i've replaced those thermal breakers that are in power trannys like GK amps.
And back to the original post, I've come across the same problem a couple times with the Marshall Fuse holders.
Two minutes to pop a new one in.
I've got a ton I've scrounged holders from junk Telco equipment at work.

[R.G.]

Fuse holders are always problematical, especially with high current paths - like heaters.

You are inserting two metal-to-metal contacts of NON noble metals into the current path, and hoping that without sufficient pressure to make gas-tight connections and a variable amount of spring tension that they'll stay permanently highly conductive, and that any plating or solder in them will not have flex failures, corrosion or plastic creep. And you have to do it with the important action parts (the fuse) being cheap enough to throw away.

Fuse holders are also a safety issue. A lot of fuse holders have a somewhat contorted design to prevent you from ever being able to contact a live wire if you simply stick a fuse into the holder by hand. Since the fuse needs to be in the hot side to prevent fires, that means that inserting a fuse with the power cord in the wall automatically makes the outside end of a new fuse hot to the AC line. External fuse holders are designed (these days at least) to keep you from ever touching the end of a new fuse if the first-inserted end has touched the inside contact.

I prefer soldered-in fuses for heater currents. Too many times the fuse in a heater circuit will heat up and soften the springs on its contacts, go high resistance, and cause intermittent or soft operation of the heaters. And I prefer to always fuse heater circuits because of the danger of burning out the power transformer if there's a heater short. The AC power line fuse is there, as was said, to prevent fires. Fuses on the secondaries of the power transformer can be designed to protect the PT from overloads if you're good at calculating fusing.

Fuses *may* if properly designed protect the output transformer from overcurrents in the primary, like from shorted output tubes or lost bias. May.

It's a little sad that the people most interested in keeping tube amps going are the least interested in using modern technology to do that. It's not hard to design current limits and monitors for things like B+ currents and heater currents, as well as ramp up/down circuits. I personally like the image of hundreds of solid state slaves making the world safe for tubes.

[Steve Conner]

Originally Posted by R.G.

Since the fuse needs to be in the hot side to prevent fires, that means that inserting a fuse with the power cord in the wall automatically makes the outside end of a new fuse hot to the AC line. External fuse holders are designed (these days at least) to keep you from ever touching the end of a new fuse if the first-inserted end has touched the inside contact.

The fuse holder on that old Crown amp I restored is from before the safety craze. If you put a new fuse in, the live end sticks right out. And if you don't have a fuse in, you can touch the other terminal. So no matter which way you wire it up, you can get electrocuted somehow. I didn't notice until I'd completed the restoration, and couldn't be bothered replacing it afterwards. But it makes me uneasy, especially with our 240 volt mains.

I personally like the image of hundreds of solid state slaves making the world safe for tubes.

Well, it's an honourable idea, but (to me at least) part of the appeal of tubes is that they're crude and agricultural enough that they don't need anyone to look out for them. In the long term (was it you who thought up the Immortal Amplifier concept?) I think a pure tube circuit would be more maintainable than some hybrid thing. What if it blows a MOSFET 50 years down the line, and nobody makes MOSFETs any more because the power supply industry has moved to silicon carbide quantum modulators?

The 6L6 has been in production continuously since the 1930s, can you name a 600 volt MOSFET that's been in production that long?

Having said that, I'm surprised that more tube amps don't use a switching power supply. It's easy to build all the features you mentioned into those, at hardly any extra cost. But again, not so easy to fix if it blows some specialty diode 20 years down the line, and you can't get a replacement.

[Enzo]

Well, the newer Crates are doing that. I had to stock up on 2A ultrafast diodes for the secondary. If a power tube shorts, it can stress the B+ rectifiers. Frankly I am all for it.