Power amplifier safety modifications

[diydidi]

I have a Large power amplifier that needs repairing for a client of mine.
It's a no-name brand stereo power amplifier and uses TO-3 lateral mosfets.
It has a large 500VA toroidal power transformer as well as a smaller 60VA toroidal transformer for the relay speaker protect circuits.
The builder didn't add any fusing except for a large 20A trip-switch wich doubles as the power-on switch too. I'm afraid the switch is too big in my opinion and will not trip during a fault situation. Removing it is not an option. Too much metal work.
Amp at least has a soft start circuit built in for the large transformer.
I would like to make it safer by adding a fuse on the mains side where the power enters.
My question is, do each of the transformers need its own primary fuse, or will one suffice.
Also how does one go about sizing the fuse/s? I assume they will all be slow blow.
Mains where I am is 230V.

[nickb]

The primary object of fusing the equipment is to prevent fire. Therefore when you have two transformers it is best to fuse both primaries. You can achieve finer control by fusing the secondaries too.

As a very rough but practical guide, to size you fuse you need to know the maximum current consumption during normal operation and choose one that is (arguably) 1.5x that current. First try a quick blow and move to an anti-surge if it keeps blowing.

[J M Fahey]

Agree and add: I´m specially worried by the 60 VA transformer: if shorted it will happily burn the house down without even tickling , let alone triggering the thermo magnetic circuit breaker, which is arguably too large even for the 500VA one.

Leave it there as an oversized power switch, but add two fuses:
* 500VA means 500/230= 2.17A nominal, so a 3A fuse should do.
If it nuisance trips at random, go slow blow ; absolute worst case go for next larger: 4A , but that does not make me too happy.
In any case, it´s WAY better than CRAZY 20A circuit breaker rating.

* 60VA means 60/230=0.25A ... 0.5A fast or slow blow should do fine; doubly so because this one will hardly be overloaded .... while the large one WILL when (didn´t say IF ) power transistor blow, not forgetting shorted supply rectifiers or large filter caps.

When that happens (hopefully in theb far future), we hope the fuses to blow at once, not making amp catch fire.

It´s an amplifier after all, not a sophisticated time bomb.

[diydidi]

Agree and add: I´m specially worried by the 60 VA transformer: if shorted it will happily burn the house down without even tickling , let alone triggering the thermo magnetic circuit breaker, which is arguably too large even for the 500VA one.

Leave it there as an oversized power switch, but add two fuses:
* 500VA means 500/230= 2.17A nominal, so a 3A fuse should do.
If it nuisance trips at random, go slow blow ; absolute worst case go for next larger: 4A , but that does not make me too happy.
In any case, it´s WAY better than CRAZY 20A circuit breaker rating.

* 60VA means 60/230=0.25A ... 0.5A fast or slow blow should do fine; doubly so because this one will hardly be overloaded .... while the large one WILL when (didn´t say IF ) power transistor blow, not forgetting shorted supply rectifiers or large filter caps.

When that happens (hopefully in theb far future), we hope the fuses to blow at once, not making amp catch fire.

It´s an amplifier after all, not a sophisticated time bomb.

Thanks JM and Nickb.
This confirms my thoughts as much.
I will use slow blow fuses and see if they work.
Should I make both fuses accesable from the outside for the User?

[J M Fahey]

It depends on how much you trust owner

If he´s the "rolled up aluminum paper solves all problems" type, mount tem inside

[diydidi]

It depends on how much you trust owner

If he´s the "rolled up aluminum paper solves all problems" type, mount tem inside

JM, it just dawned on me the that there is no indication of the transformer specs on the transformers themselves. I was mistaken.
Must have confused my initial post with another amp.
Is there a way to thumb-suck a fuse value based on measurements I can make?

[nickb]

JM, it just dawned on me the that there is no indication of the transformer specs on the transformers themselves. I was mistaken.
Must have confused my initial post with another amp.
Is there a way to thumb-suck a fuse value based on measurements I can make?

See post # 2- measure it! Then x 1.5. You will need a true RMS meter though.

[diydidi]

First of all, thank you for your replies.
I found this from numerous different sites and manufacturers.
Can anyone confirm these guidelines?
it seems like up to 300% of VA on prim for upto 2A..

[nickb]

First of all, thank you for your replies.
I found this from numerous different sites and manufacturers.
Can anyone confirm these guidelines?
it seems like up to 300% of VA on prim for upto 2A..

It says 300% maximum at < 2A and 2A at 600V = 1.2KVA to take the limiting case

For maximum protection You should choose the lowest multiplier that avoids nuisance blows, x 1.5 is a bit of a a rule of thumb. Have you measured the current yet?

[diydidi]

Nickb I haven't measured the current yet.
Can't I just use the VA of the transformer and use your x1,5 rule.
So if for instance 230VA with 230V mains supply, gives 1 Amp prim current. So I will use a 1,5A fuse??

[nickb]

Nickb I haven't measured the current yet.
Can't I just use the VA of the transformer and use your x1,5 rule.
So if for instance 230VA with 230V mains supply, gives 1 Amp prim current. So I will use a 1,5A fuse??

Yes, but earlier your said

There is no indication of the transformer specs on the transformers themselves.

I took that to mean you did not know the VA. Did I misunderstand?

[diydidi]

You are indeed correct. But the client said he will find out the VA from the builder in the meantime.
If he doesn't get hold of the builder, what would the proper procedure be to measure the current?

[nickb]

I would run in into a dummy load at max output and measure the current in. The BIG caveat it that the current waveform is very distorted and therefore you must use a true RMS meter.

If you haven't got one than I can't see a practical way forward other than start with a small A/S fuse (say 1A) and gradually increase until there are no more nuisance blows. What ever you do is going to be safer than a 20A breaker

[diydidi]

I suppose ill have to load both channels with a 4 ohm load and get it up to just clipping.
I have one of those analog moving coil meter AC amp meters that uses a current transformer. Fairly accurate. I have it in series after my variac.

[nickb]

I suppose ill have to load both channels with a 4 ohm load and get it up to just clipping.
I have one of those analog moving coil meter AC amp meters that uses a current transformer. Fairly accurate. I have it in series after my variac.

A moving coil meter will measure the average and not the RMS current. A moving iron meter, OTOH, measures RMS.