additional fuses for Fender style circuits

[nsubulysses]

I looked at all the datasheets connected to those links - imho they would all be fine for your application.

A fuse holder is rated for a range of fuses - the datasheets identify standards compliance for parts up to 6.3A or 10A parts, where the fuse usually dissipates more power as the current rating goes up, so the capability of the clip end and the max temp are most stressful for 6.3-10A fuses - as you would be using a fuse <1A then the design margin is well in your favour.

The linked datasheets show particular compliance references for 250Vac and 300Vac ratings - the Mouser webpage reference to 600V is either a typo (as the datasheets make no such reference) or related to some other application code or compliance that is not identified. The reference to 1500V in the datasheet is the creepage/clearance capability of being able to apply a test voltage of 1500V min between the fuse and the chassis/earth (as is sometimes required when doing insulation resistance testing on a product using test equipment such as a 'megger').

As indicated in the fuse article (with a Littlefuse reference), for amp applications where the fuse is on the secondary side of a transformer, a 250Vac rated fuse can be fine for a much higher working voltage (such as 360Vac), especially as the fuse current rating will be low. Similarly, a 250Vac rated fuse holder would also be fine for that application.

This bolded section gives me a question.

Seems a 4 power tube amp would typically have a HV secondary rating of 400-600mA and I thought the typical thing to do would be make the fuse double the working current rating. So would that be 200-250mA per winding, meaning a 500mA fuse is best for EACH HV winding? Or would the best thing to do be to use a 1A or 1.5A fuse in series with each HV winding?
I remember checking around in the past and seeing manufacturers using anywhere from 500mA to 2A fast blow fuses for HV secondary depending on rectifier style. Peavey 5150 uses 1A fast blow in series with both HV secondary windings.

It seems like a typical 4 power tube fuse in series with B+ is 1A fast blow, I would guess from each pair of power tubes conducting 210mA each maximum as per the 6L6GC datasheet for two tubes in class AB1 -- http://www.mif.pg.gda.pl/homepages/f...93/6/6L6GC.pdf

Maybe I should just read the article.

[trobbins]

The article takes the approach of reverse engineering the power supply circuit and max loading, and using PSUD2 to then identify the likely turn-on and continuous rms currents through a fuse, and then to choose a type and value of fuse from what is available. This approach includes various margins.

With the Fender using a secondary tapping for bias, it is appropriate to use a fuse in each secondary arm (rather than the simpler CT location) - so the fuse rms current levels are different (from a CT location). The type of rectifier/circuit will also alter the rms current levels experienced by a fuse.

The type of fuse (IEC or UL) will also modify the value seen (or chosen). Similarly for the practical fuse values available.

Verifying the situation is another issue. Some may have made in-circuit measurements of rms current flowing through the fuse link, or just used a fuse that was in stock, or looked at other circuits, or appreciated what different valves and mains voltage tolerances could be used in to the future, or .....

Ciao, Tim

[SoulFetish]

Thanks!!! Oh man I searched through so many parts, didn't come even close to that, Exactly what the Dr Ordered.

So, here is a helpful tip if you are going to use this style fuse holder: when you solder these, insert a blown fuse(or working fuse) into the clip to sink some of the heat at the junction of the clips and plastic. The plastic cannot stand up to much heat at all, and it doesn't take much exposure to deform the mold and cause the tabs to fall out. Plus, it may be even more important is to use the dummy fuse while you are wiring and handling these. The fuse really helps to secure the tabs. You'll notice that with no fuse, the metal inserts move quite a bit and can easily break or be dislodged.

[SoulFetish]

Thanks!!! Oh man I searched through so many parts, didn't come even close to that, Exactly what the Dr Ordered.

*duplicate post. wouldn't delete for some reason

[catalin gramada]

The best protection can get with fuses : use fast type fuses in secondary side for nominal max rated current of you circuit. Replace standby switch with a "shunt" switch over a 22-100 ohm power resistor (by choice) to limit inrush current in PT and cap batery at startup. That get more effective protection but have just one incovenience : if you don.t respect the startup protocol the fuses will blow for sure. So, need to have easy acces to fuses from outside of amp and a fuses reserve just in case you flip the switches in wrong order accidentaly

[SoulFetish]

Thanks for the tips. Oh yeah, back in the day, I remember finding a paper clip wrapped in aluminum foil in one fuse holder. That amp wasn't producing any sound at that time. Thanks for the tips on fuse holders and fuses. Not very easy to find the right fuse holders, for me anyway. Sifting through hundreds of items on the big electronics sites (Mouser, Digikey, etc). Most seem to be rated 120/240 vac or similar. A few I found by accident 300 range. But didn't see one 350. stil looking. Also, reading up on agc. ...

This just takes some time, so you're putting in the right work. The parametric search parameters can be frustrating on sites like Mouser, Digikey, Newark, etc., when you're specifically targeting one certain criteria. Sometimes there are several different voltage figures for a given device, and sometimes there can be errors and/or omissions. More often than not they are fine. But, if I'm looking for a component with specific application requirements (ie suitable for HV secondary voltages), I go right to the manufacturers websites. They usually have great resources available in application notes, datasheets, tutorials, and others. This is helpful with fuse selection, as they explain fuse de-rating, how to select fuses according to I^2t. Often, if you need to use a fuse in a higher voltage than specified, you can talk to a design engineer or tech support and they will make some adjustments to de-rate the fuse figures to compensate for your circuit.