Nomination: Worst fuse holders ever

[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]

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]

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.