5F2A and 5F2 fuse ratings

[tubeswell]

This might sound dumb (but that would be because I am dumb )

But why does a 5F2 have a 2 amp fuse and a 5F2A have a 3/4 amp fuse?

Is it to do with the use of a filter choke in the former and a 10k/10W resistor in the latter?

Can somebody please explain this to me - I am contemplating building a 5F1 with a tone control (- borrowing the 5F2A part of the circuit for this), and want to know - should I keep the 2 amp fuse?

Cheers

[Chuck H]

The fuse has to be rated above the max draw of the amp and below the min rating for the componants to be protected. Either value was probably suitable in that amp (at that time). The 3/4 amp fuse was likely a slo blo type. In the modern world with an amp like that running wide open alot of the time your probably better off with a 1 or 2 amp fuse.

Chuck

[Enzo]

And the fuse is there more to prevent the amp catching fire than it is to actually protect the amp.

[tubeswell]

The fuse has to be rated above the max draw of the amp and below the min rating for the componants to be protected.

Chuck

Thanks Chuck

I will be running a NOS Toshiba 5Y3GT and a New Sensor 6V6s "Mullard Reissue". The power tranny I have on order is being made for about a 3A rating on the recto winding (although I believe only likely to see 1.9A max draw?) and 1.5A on the heater winding (for a 1A max draw). I can't recall what the B+ winding current draw rating is.

I will be using a 20uF 500V reservoir cap and two 22uF 500V decoupling caps. (I can't get the original fender values parts here, but I gather that that will be likely to reduce the hum a bit anyhow). There is no choke in the design I am contemplating, only a 10W/10k resistor (or rather two 4k7 10W resistors in series) after the reservoir cap. I guess a 2A fuse is okay.

[tubeswell]

And the fuse is there more to prevent the amp catching fire than it is to actually protect the amp.

Thanks Enzo

Very reassuring

[Chuck H]

A 2A is generous for that circuit. It would only stress a 3/4A fuse on peaks with the amp maxed out. If you use a slo blo type @ 2A you may never need to replace it.

"I will be using a 20uF 500V reservoir cap and two 22uF 500V decoupling caps."

I'm guessing you mean 22uf filter caps to the preamp stages. That should be fine. In a typical amp the main filter (resevior) cap is the highest value. But it shouldn't matter much from 22uf to 20uf.

"There is no choke in the design I am contemplating, only a 10W/10k resistor (or rather two 4k7 10W resistors in series) after the reservoir cap."

FWIW the original resistor in those amps was a 1W/10k. But if you plan to run full tilt often a 3 or 5W resistor is better. Go ahead and use the 10W if you already have it though. Unless you'll be using higher than normal voltages you probably don't need to run two 4.7k Rs in series. One will do. Also, If you wanted the 10W rating (which you don't need) you would only need to use two 5W Rs in series. Two 10W Rs of equal value in series is good for 20W. Which is about 2000% over rated for what you need in that part of the circuit.

Chuck

[tubeswell]

I have a new Sprague Atom 20uF 500V electrolytic (monster of a thing) for the first filter cap.

This will see about 335V according to what the transformer winding guy has told me. The tech I saw last week recommended using a 10W resistor arrangement in this location. I must admit it was hard finding one, but I did manage to find two 4k7 10W resistors. But since you mention it, I have seen examples in other Champs where they didn't have 10W resistors in this location.

I am curious to understand the rationale behind the power ratings.

[tubeswell]

The 3/4 amp fuse was likely a slo blo type. In the modern world with an amp like that running wide open alot of the time your probably better off with a 1 or 2 amp fuse.

Chuck

Hi again Chuck

Does that mean that I can get away with a fast blo 2A fuse?

[Chuck H]

You don't need to clutter up the wiring with two big series resistors. One of those 4.7k Rs will be fine.

The wattage rating of the resistor should be based on the current draw X voltage through the resistor, then double that # and you have an appropriate wattage rating. That resistor might get 1 watt through it at worst. So a two watt resistor would be fine. A ten watt resistor is just costing more and taking up extra space. Two of them in series is good for 20 watts, and thats just getting silly. If you put two resistors of equal value in series, the first resistor "sees" half the current because the flow is blocked by the second resistor in line. The second resistor "sees" half the current because it first has to pass through the first resistor. So each resistor shares the load. If you stagger series resistor values you then have to calculate the ratio of resistance to know which resistor will be handling most of the load. The same thing applies to changing a resistors value. If for example you have spec'd a two watt resistor for a purpose because you expect it to have 1 watt across it. But now you want to increase it's resistance. You may need to increase it's wattage rating because the higher resistance will drop more voltage across the resistor and that will increase the amount of current passing through the resistor.

Thats my half baked low tech version of it.

Chuck

[Chuck H]

You probably can. But I wouldn't. I've tried to run fast blow fuses at twice the spec before and they do blow. Not right away, but sooner than you would like. Probably something to do with voltage spikes from the power source or current spikes due to speaker impadance loading or some such. Suffice it to say that the current in a guitar amp isn't a nice smooth operation. It bounces and shakes alot. So look at it like this. A slo blo fuse would be wood. And a fast blow fuse is glass. Both are rated for the same load. If the load suddenly and briefly increases the wood will flex and return but the glass will break and therefore cannot return. Of course if the load increases and stays increased, the wood will break and fail. Thats what a slo blo fuse is made to do and thats why things with abrupt current changes, like audio gear, use them.

Chuck