Fuse Rating

[Littlewyan]

I was looking at a reissue Marshall JTM45 earlier and noticed the Mains fuse rating was 1.5A. I thought 2A was the norm for 30-50W amps?

[martin manning]

Mains fuse ratings are a compromise between surviving the start-up surge and going open in the event of a fault. If the reissue includes surge suppression devices it might well get away with less than the traditional fuse rating.

[Phil_S]

Is there an old time rule of thumb for choosing a fuse? If I copy a design, I just go with the value on the schematic. This recent SE plexi, I had to choose a value. I just added up the secondary power requirements and then divided by 120VAC mains:
HT 825VAC * 62.3W (based on Vdrop at the choke)
Heaters 6.3VAC * 2.4A = 15.1A
Total 76W / 120VAC = 630mA; choose 1A slo blo fuse. This seems to be holding just fine in bench testing. I know there is surge on startup when B+ at the first filter cap rises to 499VDC (LOL, it never seems to exceed 500V) and then drops to ~340V.

edit: corrected the decimal point problem!

[xtian]

76W / 120VAC = 63mA

There you go with your decimal placement again!

Case study, FYI: I've just repaired a Randall RT100 head (faulty DC supply to preamp heaters). Now that it's working, I can see it draws ~120watts during operation--that's 1000mA. It arrived with the wrong mains fuse (5A fast) which blew immediately. The specified fuse (6.3A slow) works fine.

[Bob S]

Couple of things to consider with fuses.
Not all slow blow fuses are equal. It's rated on a multiplication factor of the current rating for a specified time. Transformers in general have high inrush current, until the magnetic field balances.
Also watch out for the power switch - that same magnetic field tries to hold up when you switch off, causing a spark across the switch contacts as they open.
Fast snap action is more betterer.

[Littlewyan]

Martin Manning - I'm fairly certain that amp has no surge suppression devices inside. Although the GZ34 does a bit of surge suppression with it's slow start up, the Marshall handbook tells you to use the standby switch when turning the amp on (and off) so there will be an inrush current. Also some blown GZ34s I imagine! Unless they've recently added protection in or at least put a resistor across that standby switch.

Phil_S - I think I did a similar calculation with the JTM1 that I built.

Xtian - 6.3A mains fuse?!!!

Bob S - I've actually blown the fuse in my TW Express a few times by being lazy when turning off the mains power switch.

I think I'll stick with the 2A fuse for now. Although I know Ken used a 2A fuse in his Express amps which ran on 120VAC, so in theory I should be using a 1.5A fuse in mine (240VAC).

[Bob S]

I tend to think of it like it's more for catastrophic failure - anything short of smoke - than actual protection. For real protection you need something bigger and a lot more expensive. I'm ok as long as it doesn't give nuisance trips.
Or cause a fire.

[xtian]

Xtian - 6.3A mains fuse?!!!

I only say it 'cause it's true.

[R.G.]

Fuse ratings are not the current that will open the fuse. They are rated for the current they will carry forever without blowing.

For currents above the rating, fuses will blow after carrying the overload for some amount of time. Exactly how much time for how much overload is set by the fuse's time delay characteristic, and the technical term for that is the fuse's I-squared-T rating. Current squared times time is proportional to the amount of energy delivered to the fuse filament, and that's what heats the filament to make it melt.

A fuse's time delay type is a rough guide to its I-squared-T number. Mains fuses for amps are generally time delay (slow-blow) types so the input surge when starting a transformer or power supply may be much larger than the actual line current in full-power operation. Slow-blow fuses may not protect electronics circuits well, so faster fuses are generally used for secondary and other fuses.

Transformer inrush is a big deal. I once instrumented one of the lab's variacs that caused breakers to pop when turned on. It was pulling 200A peaks on some switch changes. Exactly how much varied with where in the AC cycle it was last turned off (that is, the amount and direction of the remanent core magnetization) and where in the AC cycle it was turned on (whether it had to opposed the remanence and how much of a full cycle it had to do that). The physical size of the iron core sets up much of this. That much iron has to be "charged" at turn on.

AC mains fuses are in fact intended to stop fires from starting and prevent people from being electrocuted if there is an internal short to chassis. Those are about all you can depend on an AC mains fuse to do. They may or may not protect the PT from secondary loading issues. If you want to protect secondaries, fuse the secondaries individually with proper fuses for their individual load.

The right size mains fuse is the lowest rating that will reliably let the unit power up reliably (equals "will sustain worst case inrush") and sustain maximum full power operation under worst conditions (equals "will not blow if it's turned up to 11 and running at max power out").