Series fuses and other oddities

[maxkracht]

I have a few marginally related fuse questions/observations to ask the hivemind.

1: I worked on a couple amps in the past week that have series mains fuses, presumably to hedge against user error. Curious if anyone has thoughts or opinions about this. One was in an swr workingmans amp, the other was a musicman. The SWR was just a regular fuse, the musicman has a glass tube with a bimetallic strip inside. Musicman also had a 20A mains fuse installed and a choke that failed open which someone jumpered across...

2: Curious if anyone has experience using micro radial plastic fuses above their rated voltage? I think I have seen this in production amps before, but can't remember which. I know fender uses pico fuses in a few places, but within their rated specs. 5mm glass fuses seem fine for ht supplies, would the micro fuses work past 250v? https://www.mouser.com/ProductDetail/Sc ... W3VA%3D%3D

3: Anyone use fusable resistors for power tube cathodes or artificial heater centertap? I usually stick with 100r .5w film for heaters incase something bad happens but picked up some proper fusable ones. 1/3w 10r also seems like a decent idea for power tube bias check resistors. Maybe another resistor and LED in parallel for an indicator.

[R.G.]

1: I worked on a couple amps in the past week that have series mains fuses, presumably to hedge against user error. Curious if anyone has thoughts or opinions about this. One was in an swr workingmans amp, the other was a musicman. The SWR was just a regular fuse, the musicman has a glass tube with a bimetallic strip inside. Musicman also had a 20A mains fuse installed and a choke that failed open which someone jumpered across...

Weird. Was this factory wiring?
Bimetal strips are weird in an amp. I wonder if it's a thermal cutout for the whole amp when the inside air is too hot, just in a glass tube package.

2: Curious if anyone has experience using micro radial plastic fuses above their rated voltage? I think I have seen this in production amps before, but can't remember which. I know fender uses pico fuses in a few places, but within their rated specs. 5mm glass fuses seem fine for ht supplies, would the micro fuses work past 250v? https://www.mouser.com/ProductDetail/Sc ... eVMW3VA%3D%

Using any fuse above its voltage rating is a bad idea. In fact, if it's conducting DC and not pure AC, there is a different and usually lower voltage rating. AC reverses polarity at mains frequency, so the current goes through zero twice per AC cycle. This gives any arc the opportunity to quench and have the arc not re-strike, letting it block. DC never lets the arc current go to zero, so it takes longer to melt the element back to a distance that quenches the arc. There is some possibility of legal liability to the manufacturer if they used a fuse above its (AC or DC) voltage rating and someone has a fire. The plaintiff has a valid claim of design defects. If the fuse has a voltage rating appropriate to its use, it's a pretty solid defense on that particular grounds. Sad, but today's litigious society forces legal considerations in electronic designs.
As to the micro fuses in the link, I had a look at the datasheet, and the term "DC" does not appear there. I personally would not use that fuse with HT at all. It might, maybe, could, even probably would, work just fine, but I would not bet my customer's life and property on it.
On the 5mm glass fuses; there are 5mmx20mm fuses stocked at Mouser (that's my informal personal test for "does this part exist in the real world?"; Digikey is similar) that do offer 450 and 500V DC ratings. However, these are only available in ceramic tubes, and the lowest value stocked is 3.15A. It could take a long delivery time and cost more to get a fuse that is (a) rated for DC as well as AC and (b) in the right time-to-blow. All of the ones at Mouser were slow blow and a minimum of 3.15A, up to 8A - maybe not the best choice. I was looking for 0.5A to 1.0A rated fast blows.

3: Anyone use fusable resistors for power tube cathodes or artificial heater centertap? I usually stick with 100r .5w film for heaters incase something bad happens but picked up some proper fusable ones. 1/3w 10r also seems like a decent idea for power tube bias check resistors. Maybe another resistor and LED in parallel for an indicator.

Same issues as for fuses: are they rated for the current, time delay, voltage, and DC current/voltage that they'll be expected to clear? If they're a safety item, are they safety rated.

Yeah, I know this is officious sounding doom and gloom, but it's also by-the-book engineering. If you're putting in something for a protection circuit, it only makes sense to use parts that are rated for plausibly working in the circuit as per their data sheets.

[xtian]

The Fender Bassman RI (tweed 4x10) has a 3A fuse (accessible outside chassis) followed by a 4A fuse in series, inside the chassis. Strangely, the 4A fuse had gone open when the client brought it in.

[maxkracht]

Weird. Was this factory wiring?
Bimetal strips are weird in an amp. I wonder if it's a thermal cutout for the whole amp when the inside air is too hot, just in a glass tube package.

Glass tube is stock to the Musicman. I have worked on several, all have it. I'm speculating about bimetallic strip, but don't know what else it would be. I don't have the musicman readily available, but here's a pic of a similar device I pulled from a low voltage power supply. Looks like a big neon with a switch inside.

IMG_2246.jpg

Using any fuse above its voltage rating is a bad idea

Agreed, but isn't this what is done with nearly every HT fuse? I am comfortable enough with 5mm glass fuses, prefer ceramic and/or 3AG but both are less common.

[maxkracht]

The Fender Bassman RI (tweed 4x10) has a 3A fuse (accessible outside chassis) followed by a 4A fuse in series, inside the chassis. Strangely, the 4A fuse had gone open when the client brought it in.

Weird, do you know if the client replaced the 3A?

[R.G.]

Glass tube is stock to the Musicman. I have worked on several, all have it. I'm speculating about bimetallic strip, but don't know what else it would be. I don't have the musicman readily available, but here's a pic of a similar device I pulled from a low voltage power supply. Looks like a big neon with a switch inside.

Thanks for the picture. I think it's one of these:
https://www.piris.com/wp-content/upload ... sb-esb.pdf
They're stated as both temperature and current protectors. I've not run into those before.

Agreed, but isn't this what is done with nearly every HT fuse? I am comfortable enough with 5mm glass fuses, prefer ceramic and/or 3AG but both are less common.

Yeah, probably. Certainly with replacements. The manufacturer could get a pass in that there do exist some 5x20 fuses that are rated correctly, but owners and techs will probably use the lower voltage and cheaper ones. They certainly work mostly. It might be that a detailed analysis of the PT impedance, limited charge stored in the filter caps, etc could prove the case that a lower voltage rated fuse would work. But that is going to involve a lot of assumptions and math.

Well, to be fair, this is not a safety related position now that I think about it. Safety fusing is the AC mains fuse. This is a protection fuse, with the aim of protecting (probably!) the OT and maybe the PT secondaries, rectifiers, and filter caps. So mostly getting away with it may meet the needs.

[maxkracht]

Thanks for the picture. I think it's one of these:
https://www.piris.com/wp-content/upload ... sb-esb.pdf
They're stated as both temperature and current protectors. I've not run into those before.

Thanks for the link! I know how bimetallic strips work but didn't think of thermal protection. I guess I had my mind set on resettable fuses.


I mainly want to use those plastic fuses for the convenient package. Seems easy to add a pair before backup rectifier diodes hanging off of the tube socket (derated for temperature of course) or on terminal strips in point to point builds. I guess I can still do that on lower powered amps without exceeding 250v by much.