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.

[singapol]

Toriodal transformers are notorious for turn on surge/transients. It helps to have a slow start which you
mentioned. Apart from this the normal protection fusing will be at the transformer dual vopltage rails or at the DC side feeding the amp. plus and minus rails. Depending on if it's pure class A or B. Class A/B is just marketing.

Example a 50W class B amp will have a 3A fuse so I'm guessing this amp is 200 -250W ? So scale up accordingly. You can use slow blow to take account of the turn on  surge.

PS You can ignore the small 60VA toriod but if you are a perfectionist then calculate similarly.

 

[b_force]

Depending on if it's pure class A or B. Class A/B is just marketing.

I don't know where you get that from, but that's simply not true.
Class A/B simply means that the amplifier is in Class A mode until a certain amount of power. Above that it goes into Class B mode.
For the same reason you can "morph" a class B a tiny bit more into Class A to get rid of the so called crossover distortion.
Pretty common knowledge and standard building blocks.

[drussell]

Depending on if it's pure class A or B. Class A/B is just marketing.

Uhhh...  No, it most certainly is not!

Class A means it is biased to always draw the same current.  At idle, the tube plates or transistors themselves will be dissipating all the power.

Class B means that it is biased just to cutoff so at idle there is no power being dissipated in the output devices.

For tube gear, Class AB₁ means biased such that the current flow at idle is less than full Class A but more than B (which helps prevent distortion when you take a tube through the non-linear region from 0 conduction upwards.) but never allowing the grid to swing into the positive region and causing grid current to flow.  Class AB₂ means the grid may be driven positive with respect to the cathode, thus grid current will flow during this time so a power amplifier must be used to drive it rather than just a voltage amplifier and it must be capable of handling the varying load resistance that results if severe distortion is to be avoided.

Transistors don't have the grid current flow issues like tubes, so it is simply called Class AB, as there is no need to differentiate between Class AB₁ and AB₂ driving characteristics like there is with tubes.

[james_s]

A 500VA transformer will almost certainly trip a 20A breaker if there is a short circuit, though the branch circuit breaker may trip first.

Is there any fusing on the output of the transformer? That's where I'd add it if there isn't.

[Armadillo]

For a portable equipment rated at 500VA, a equipment type slow blow fuse T5A would have sufficed and is properly discriminated against the 13A BS1362 Mains plug.

A good designed high end amplifier has many forms of protection, unbalance trip, high temperature shut down etc.. aside from short circuit protection.

You may read some insights here https://elektrotanya.com/?q=en/content/repair-gryphon-reference-one-power-amplifier

[b_force]

A 500VA transformer will almost certainly trip a 20A breaker if there is a short circuit, though the branch circuit breaker may trip first.

Is there any fusing on the output of the transformer? That's where I'd add it if there isn't.

Well, that really depends.
People with 110/115Vac have much more problems with their fuse breakers than with 220-230V