FWIW, We’ve had boutique fuse manufacturers sending us fuses for years. At least one of them was a customer of ours. Funny thing- none of them have mentioned anything about directionality of the fuse (although one of them is marked as if it is).
So we did test these fuses. The ones that are marked with a diode symbol on them are from Germany. Measurements of the fuse show that it has exactly the same characteristics in either direction- in both regular resistance mode of the DVM and also on the diode test function (where it shows a dead short).
What we are dealing with here is something called Expectation Bias. This is where there is an expectation that something might be so (regardless of the reason for the expectation) and so a predisposition to perceive that phenomena exists. However, the fact is that in the case of fuses there is simply no difference.
This Expectation Bias can arise out of someone with an agenda telling you that a fuse is directional (which might be done by marking the fuse with a symbol); what you are experiencing is some sort of scam or fraud.
Again, we do have good reports from our customers regarding such fuses and all reports we have are quite consistent, with one exception:
One amplifier came in with one of these ’tuning’ fuses which was blown. Usually when the particular fuse location in question is blown, it indicates a service issue, but in this case the amp was fine- once a good fuse (we use Buss and Littlefuse for the most part) was installed. As this fuse was made in Germany, I suspect its the same fuse as Roger Modjesky must have encountered. Even though marked correctly the fuse was not up to its spec! So Roger likely has good reason to not recommend these fuses.
Since the installation of this fuse caused a customer to have downtime and the cost of shipping (about $170.00), I have to agree that these fuses can be problematic! While we are not saying they can’t be used in our gear, I am saying that you may experience fuse failures that are premature. So this aspect should be on your radar should you choose to install such fuses.
FWIW, we detected audible issues with fuses about 26 years ago. This was when we were developing and releasing the MA-2 amplifier. Since boutique fuses did not exist back then, we used a different style fuse that uses a larger fuseholder. The fuse is a type FNM (Google it) . It has substantially larger contacts on the fuse and much higher contact pressures in the fuseholder. We still use this system on the MA-2 today.
