Fuse types for 6v6 build.

[wozt]

For a 2x6v6 build I have a 3amp fuse fitted in the plug and on the chassis fuse holder I have used a 250ma fuse (which was what I had in my bitsbox). The later has now blown twice so I'm wondering if 500ma or 1 amp ones will be better. Should this fuse be a slow blow type or anti surge type? Thanks.

[Tom Phillips]

If it was my amp I'd use a 1A Slo-Blo

[Enzo]

Since you say you have a fuse "in the plug", does that mean you are on 240v mains?

[Chuck H]

If you have a 3A fuse "in the plug", what is the 250mA fuse "on the chassis" fusing? That would be pertinent info to know what fuse would be appropriate.

[wozt]

Yes 240 mains, UK. Don't all amps have a fused plug and an internal fuse?

[Dave H]

Yes 240 mains, UK. Don't all amps have a fused plug and an internal fuse?

Is the internal (chassis?) fuse on the primary or HV secondary of the PT? My 2 x EL84 amp has 3A in the plug, 1A T HRC on the PT primary and 500mA T on the HV secondary.

[Chuck H]

Yes 240 mains, UK. Don't all amps have a fused plug and an internal fuse?

You still haven't said what the second fuse is doing.?. Do you effectively have two fuses in series on the mains? Are you fusing the HT as Dave mentioned? Or something else?

EDIT: Enzo's question above (and your corresponding answer) just clued me in, I think. If you have a fused plug and "Don't all amps have a fused plug and an internal fuse?" then I'll guess you have two AC mains fuses in series. One in the cord and another on the chassis at the end of the cord before the power transformer. If this is the case you'll want to as Tom said. A 1A slo will probably be fine. Here on the other side of "the pond" most such amps on 120V mains use a 2A slo on the chassis for this.

If, OTOH, this is an HT fuse and you're popping 250mA's there's probably a problem with the amp.

[Leo_Gnardo]

Yes 240 mains, UK. Don't all amps have a fused plug and an internal fuse?

I've only seen those big fused plugs in UK and South Africa. Typically there's a 13 amp fuse but yours is fitted with the smallest common replacement value 3 amps. Note they look much like typical 1/4 x 1 1/4" cartridge fuses but not a good fit so you have to use fuse values you can get at your electric or DIY shop. 3A is the smallest I've ever seen. So far so good. BTW most Americans have never seen such a thing, that may result in some confusion.

It looks like the 1A fuse is the "line fuse" for the whole amp, and an expected value for a 2x 6V6 amp run on 240V.

The smaller fuse, best guess is it's on the hi voltage supply, intended to open if an output tube shorts. Selecting a value that will protect your transformer from damage is tricky. Dave H mentions a 500mA T - the T suffix indicates a time-delay AKA slo blo fuse.

If you're sure the 6V6's aren't dodgy, no intermittent shorts, no loose socket connexions - and the bias circuit is working properly, you could sneak up the value of that HT fuse. There are available values between 250 and 500 mA for instance 315 and 400 mA. Whether you can find them on the shelf at local parts shops, I don't know. For occasions like this I keep a variety collection of fuses and get them from Mouser. AFAIK Mouser does have a European branch. The tricky thing is selecting a value that will protect your power transformer but not open up in typical operation. You would think 250 mA is plenty enough for a pair of 6V6's but apparently it isn't. Also, there's plenty of 2x6V6 amps in this world that have no HT fuse and they work just fine year after year, think Fender Deluxe & Princeton. Note the 120V fuses for these are typically 1 and 1.5A, that would be 500mA and 750mA for 240V powered versions. The lesson to take from that is, if there's no HT fuse, then try to put better protection on the AC line side with a smaller fuse there.

And that 3A in the plug, that's the "gone to hell" fuse in case of a short in the line cord or some other catastrophic failure. 720 watts into a 6V6 amp would mean it's just about on fire.

[Justin Thomas]

Line Plug Fuse = Dunderhead Fuse. For the idiots who would leave a 30A fuse in the 67 Bassman when they sell it.

Justin

[Chuck H]

Line Plug Fuse = Dunderhead Fuse. For the idiots who would leave a 30A fuse in the 67 Bassman when they sell it.

because you never bought one like that

[Leo_Gnardo]

Line Plug Fuse = Dunderhead Fuse. For the idiots who would leave a 30A fuse in the 67 Bassman when they sell it.

Justin

I got a big jar of 30A fuses, 25, 20, 15, too. I'll never have to buy one. Gear comes in for repair like that. And in UK lucky 13 amps is the value you commonly find in the big 240V plugs, you'd have to draw 3120 watts to pop one. By that time any guitar/bass amp would be reduced to a smoking space heater rubble. It's been said by the experts here, the fuse isn't to protect your amp or you, it's trying to prevent the house from catching fire. Maybe. Big from the fire inspector.

[Justin Thomas]

Just think. If everyone would use proper value fuses, we'd make a lot less money doing repairs... transformer cloners would be set way back... firefighters would have to rescue more cats to justify their existence... dang. Sounds like I'm putting even more hard-working Americans out of work. I rescind my comments - 30A fuses for everything!

Justin

[g1]

You can't just go with 30A for everything .
Here's a handy chart Dr.Gonz posted a while back:

[Leo_Gnardo]

Just think. If everyone would use proper value fuses, we'd make a lot less money doing repairs... transformer cloners would be set way back... firefighters would have to rescue more cats to justify their existence... dang. Sounds like I'm putting even more hard-working Americans out of work. I rescind my comments - 30A fuses for everything! Justin

There was plenty of gear made with no fuses at all 30's - 70's including hot-chassis amps & hi fi. No, let's not return to the bad old days.

Firefighters around here say "the cat climbed up there, when it's hungry enough it'll climb right back down." You've never seen cats falling from trees, or any cat skeletons stuck up there, right? He's right.

- - - - - - -

Somehow I missed the Laughing Cow Cheese on Dr. Gonz's fuse chart the first time around. Should the foil be applied with the cheese in it or is it OK to eat the cheese then use the foil? Cheese-in, you could get an odor alert when the current's running high, "smells like a grilled cheese in here, check the fuse!"

[Dave H]

Those 350A fuses are not available in the UK, we have to use 1.25” spacers. The 2000A can be quite spectacular when they blow. I once had one slide down my car and end up across the battery.