I read with some laughter, dismay and sadness the follow up on Stereophile May 22, 2012. page 109 concerning the latest hype of Tuning Fuses. Besides my opinion that these are a horrible waste of good money I want to let readers know that these fuses can damage your equipment.

I repaired a RM-9 MK II where the owner had installed 8 of the $49 tuning fuses in the individual fuse holders of the amplifier. Besides spending close to $400 the fuses they cost him another $400 in repairs and $320 for a new set of output tubes. When fuses cost more than tubes something is rotten in Denmark (actually they are made in Germany.) Not only had the fuses not protected the tubes but they had also blown a cathode resistor that I have yet to see fail in any other RM-9. Upon opening the fuse I found that it was not of a high-breaking construction. In fact the construction was such that a 10 cent RadioShack glass fuse offered more protection. Whoever designed this fuse evidently does not know about breaking ratings or how to achieve them. To my knowledge they publish none of the essential engineering graphs of time VS current. Furthermore, I can see nor can I find anywhere UL, CE or any of the standard safety approvals. Given their construction, I doubt they could get any.

I called the distributor to see if there was any technical data and he referred me to a white paper on the website which is down for maintenance for a week. Let's have a look at that when it's back up.

I know it's difficult to do, but if audiophiles would take the money they are tempted to spend on useless tweaks and start a savings account for that money, in some time they would have enough to buy something like a better pair of speakers, new amplifier or something that would really make a difference in their listening.

Now that I have had my say, here's something you can do if you want to experiment. Since we know these are made of silver wire you can get a enough silver wire for $40 to make hundreds of fuses that sound even better and will protect your equipment. Since these fuses are only good for the AC line lets look at what a line fuse does and what you have to do if it blows. An AC line fuse will protect your solid state and modern tube equipment (Not your 1960's tube amp or any tube amp where there are no tube or B+ fuses) from further damage if a rectifier, main filter cap or any short in the high current portions of the product. This is a good thing. We don't want a shorted rectifier to cause capacitors to explode or transformers to burn up. However if either of these occurs, it's going to have to go the the shop to replace the shorted rectifier which is a very simple matter.

I have more audio equipment and test equipment in daily use than most of you and I assure you, other than nuisance blowing, AC line fuse replacement is a very rare event in my experience. For those who are unfamiliar with the term "nuisance blowing" it is a commonly accepted term for a fuse just wearing out. It is caused by the simple fact that every time a piece of equipment is turned on, the inrush current, which is many times the run current, expands the fuse wire eventually causing it to break. Usually it takes years and when a fuse fails in this manner one simply replaces it and goes on for another similar period. It is more common with fast blow fuses which are not good choices for line fuses for reasons given below.

Fuses are of particular interest to me and every time I replace a fuse I give consideration to what fuse I would have used had I designed the product. Though cautioned "replace fuse with same type and value as original" I often wonder how carefully the "original" fuse was chosen. I have seen a lot of bad fuse choices and have replaced many a fast blow fuse with a much lower current slow blow which often ends the nuisance failures and improves the safety of the unit. I have found many products where the specified fuse would not protect against a shorted filter capacitor or shorted rectifier because the fuse would not blow under those conditions. Instead the power transformer would overheat and fail, then it would blow the fuse. Now there is a lot of stuff to replace instead of the simple diode.

In the original RM-4 I did many tests to determine that the 250 mA slow blow fuse would protect against every possible failure. I shorted a rectifier to see what happened. I shorted a filter cap. I shorted the main supply which never blows a fuse because, unlike many regulated power supplies it is short-circuit protected.

So, how do you make your own silver fuses. Simply wrap the right size silver wire from post to post of the fuse holder and solder.

"Not only had the fuses not protected the tubes but they had also blown a cathode resistor that I have yet to see fail in any other RM-9"