Bulletproofing an amp?

[Aurora]

Fuses for 220V are usally half the amp rating than the 110V version...
( current is halved for 220V vs 110V)
Some leeway may have to be given for turn-on surge.

[Zippy]

All this holds true if fusing the mains (input from wall) but has no bearing on the fusing downstream of the power transformer secondary.

[ChrisM]

I've been looking around at various amps and their fusing schemes.

Marshalls use HT fuses a lot.

It seems that for 50w amps a 500ma HT fuse is the norm and in a 100w amp a 1 amp fuse is used.

This is for 120vac operation.

Main's voltage doesn't matter

Fuse value will depend where it is located, what current the circuit is drawing and how much of a margin the fuse is designed for before it blows.

Could you clarify that Chris?

Fuses have voltage ratings.
I realize that it is the current limit that determines when it blows but why do amps that have international power transformers in them have different specs for what fuse to use depending on what the mains voltages is?

When I look at amps there is one rating for 220vac and another for 120vac.

Basically the 220vac fuse is double the value of 120vac.

So say on a 100w amp powered by 120vac, the fuse is generally a 4 or 5 amp slo blo.

The same amp would have double that, so 8-10 amp slo blo.

For example, the Marshall 1923 amp states on the back panel that for 230vac operation it needs a 6 amp fuse.
For 120vac operation it needs a 3 amp fuse.

On the HT fuse it doesn't matter because the voltage there is the same regardless of the mains voltage.

Check it out
http://marshallamps.com/downloads/files ... %20Eng.pdf

Very much aware of that all. That is for the mains fuse though.

You were asking and I was talking about the HT fuse.
The HV out of the PT will be the same regardless if you use a 120V or 240V PT.

[Aurora]

All mains driven apparatus are required to have an input fuse, as requested by mostly all countries regulations, for safety purposes. The purpose of this fuse is initially for safety purposes only. Tha rating of this fuse shall reflect the total / max consumption of the apparatus, eventually given som leeway for e.g. transformer surges at power on. Given that most amplifiers uses mains transformers, the fuse needs to be slow blow, and will as such not protect against internal failures.

In the HiFi crowd, DC fuses in SS amps are disputed, but not for reasons of lack of protection, but for "sound quality". For transistoer amps, these fuses will be protected by a good handfulof expensive power transistors.

As for tube amps, most power tubes will sustain a fair amount of abuse, let's say for up a few tenths of a sec. Output transformers will also generally speaking survive a short circuit for a limited time. Thus it makes sense to use suitably sized fast blow fuses in the HV line. Current rating will of course have to be "suitable". The voltage rating marked on some glass fuses are of no concern - any normal glass fuse will sustain the voltages used in guitar amps, - the design of the fuse holder is another question, external or internal too. Several high line HiFi amps use this solution. If it is concidered yes or no in the guitar amp crowd is another story.... will be interesting to follow..

[Doctordog]

If you check that PDF really careful, it seems that the Mains fuse for 230V is not 6A, but T 1.6AE, and T3AE for the 110/120V.



JB

[Zippy]

Ah, this is the kind of stuff I was hoping for - a mod to protect the OT in the instance that the speaker load goes open.

From http://ampgarage.com/forum/viewtopic.ph ... highlight=

Traynor used to use a 200ohm 10w( maybe 20w i don't remember) resistor to ground right off the tip of the output jack. I could not hear any degradation of tone or power. Very safe. Others have used similar schemes. one of the bullet proofing ideas that few use today.

Thanks, Billy!

[Structo]

If you check that PDF really careful, it seems that the Mains fuse for 230V is not 6A, but T 1.6AE, and T3AE for the 110/120V.



JB

Thanks for pointing that out, I mis-read the specs.

[Zippy]

Yet another vote (and reason) for adding internal fuses - PT and HT.

From http://ampgarage.com/forum/viewtopic.php?t=16030

I played the amp for months, and all was okay, I decided to sell it to pay for another build (of course). It did have JJ KT77s in it, which have given me nothing but nightmares in the past as EL34 replacements (the screen grid fails very easily, and I'm sure that's what happened with these).

It was 100W and had a 4 amp fuse. It did blow, but only after the PT did :/

FWIW, I've been adding HT fuses in my amps anyways after reading Merlin's power supply book in the PT secondaries.

[Luthierwnc]

I usually put a .1 630 volt cap across a 220k resistor between the guitar hardware and the jack. Since I only play my own either scratch-build or highly-monkeyed-with beaters (both amps and guitars), I'm not so confident that I got it completely right that I want to be the ground for a catastrophic failure -- or even a stray DC wire. If you were the tissue fuse for B+ with that arrangement, you'd still feel some volts, just not all of them. FWIW, sh

[Gaz]

Why across a 220k resistor?

[Luthierwnc]

Not sure why that value. I got it from Adrian Legg's book and have seen it repeated (not the same as confirmed) elsewhere. The resistor references ground for hum control. The cap blocks DC. If you got rail voltage on the input of the amp jack, the juice coming through the instrument would be knocked down to about 40 volts when it hit the strings. Not enough to smack you across the room (done that) but you'll definitely know something's amiss.

sh