-
I'm still a tube guy as long as I don't have to carry it around. My good audio system is powered by a Prima Luna Prologue 5 amp that I bought new in 2012. PL makes seriously great sounding stuff that rivals many of the top boutique audio amps selling for 10 times its cost (although PL ain't cheap these days). Fortuntely, it's very well designed and built. Praise be that my newly deceased KT88 (may it rest in peace) didn't take out anything else on its way down the tubes
I clicked "play" on Pasquale Grasso's new album "Fervency" and sat down with an espesso and the New York Times to relax and read. There was no sound. I checked my music management program (I use Roon for general listening) and everything looked fine. All the associated hardware was fine - server, DAC, router, etc. The Raspberry Pi I use as an audio endpoint in the living room was on, connected to the network, and working properly. The next step in the diagnostic process is a careful, thorough inspection of the amp. There was no sound (fizzing, popping, etc), smoke, or smell to suggest catastrophic failure. One of the KT88 output tubes was dark, which was a clue to the source of the problem. But only that channel should have been affected, and the right channel should have been fine. So the output tubes were not getting B+ voltage. I popped in an old tube as a quick and easy way to make sure the heater was getting juice, and it lit up fine. So the cause of the silence was almost certainly failure of an element in the B+ voltage supply to the plates of all the output tubes.
Proceeding from here is a process of looking or testing for the likely causes, starting with the most probable and working backwards through the list in decreasing order of likelihood of occurence. About 80% of power tube failures are not dramatic and cause no damage. But the other 20% can take out anything from cathode and/or screen resistors (the most common collateral damage) to a transformer (much less likely). In good audio amps, the voltage to the output tube plates is fused to prevent this damage, so that fuse is the place to look first. Bingo! The plate fuse was blown. Further inspection revealed no other signs of damage inside - no browning or signs of overheating resistors or caps, no burnt smell, no melted or softened insulation on transformer wires, etc. So I'm assuming that the fuse did its job and died to save the rest of the amplifier.
The culprit is a 5x20mm 1/4 amp glass slow blow fuse, which is physically smaller than the power fuses in most tube amps Anybody tried to buy an unusual fuse recently? There are no electronics supply shops anywhere anymore. 25 years ago, I'd have run down to Kass Electronics or Radio Shack or Soundex and gotten one for 50 cents. I called every audio dealer in the Philadelphia area and none could (or maybe would) sell me a fuse. I finally ordered 10 (@ 41 cents each) from Mouser online, and they'll arrive on Thursday or Friday (hopefully). Meanwhile, I'll check the voltage to the fuse to make sure the tranny is OK (which is 99.9% likely). I have a fresh set of matched KT88s that I bought years ago, so I fully expect to pop them and the new plate fuse in and have glorious sound again.
There's an important message here for all. Guitar amps don't usually have plate fuses. So if you blow an output tube on the stand and you're in the 20% that have collateral damage, you're probably SOL for the rest of the gig and may need significant repairs. Even if your amp has a plate fuse, you're out of business unless you know where it is and have both a spare tube and a spare fuse with you. Even SS and class D amps blow fuses. Make sure you find all the fuses are in your equipment, and know exactly what they are - size, voltage, amperage, etc. There are fast blowing fuses, slow blowing fuses, and some odd types in between. Buy spares and carry them with you. Do not use a mismatched fuse just because it fits the holder and you don't have the right one. A 40 cent fuse can save the gig along with hundreds of dolllars in repairs for transformers, tube sockets, resistors, capacitors, wiring etc.
PS: Why didn't I have a spare, you might well ask? I do.....somewhere. I had a huge collection of fuses for cars, audio equipment, etc in my shop in the house that we sold a decade ago. I brought as many of my parts cabinets as I could to our apartment and put them in our storage locker in the basement. I carry fuses for my guitar amps in a parts and tool kit in my trunk. But I can't find the box of oddball stuff for audio and offbeat appliances. I was just too lazy and unconcerned to make sure I had a fresh set of spares.
-
06-15-2025 01:21 PM
-
Where to put fuses in tube amps is certainly an interesting subject. In Europe, tube amps are required to have fuses on their heater supplies, not so in the USA. Screen grids are another common source of failure in a power tube. I've never seen an amp with an actual fuse on the screen, but many have screen grid resistors, which essentially function the same way (they just require a soldering iron to change, not something you can do while out on a gig). But there are plenty of very famous amps out there without screen grid resistors like the Princeton Reverb and Tweed Deluxe.
-
Tube guy here, too; my good audio amplification is four chassis totaling 14 tubes. My other audio amp is a very nice 52 year old Sansui AU-6500.
I have six Fender tube amps, keep my Deluxe Reverb in my wagon as a spare. I don't want to be troubleshooting / repairing anything away from home.
-
In my 45+ years of playing live I've had ONE blown fuse on the gig : a freak spike or something blew the fuse in my 80's Polytone Brute and I fixed it with a piece of tin foil, got me through the night.
I guess the wiring and the basic stability of the powerlines here (western) Europe play a big part here....
-
And I'm glad you lived to tell the tale. My advice to anyone else is not to try the aluminium foil bypass. Fuses blow for a good reason. The number reason is to prevent you from taking your own life.
Originally Posted by gitman
-
I've been playing tube amps for 50 years , never had a failure . Fender/ Mesa/ PRS .
-
My tube/valve amps have two octets of 6550s (ARC Classic 150 monobloks). They were $10000 a pair...in 1990. I could not afford to retube/revalve them so I kept them in storage. Originally Posted by nevershouldhavesoldit
-
That’s serious sound! I built a Citation 2 and did the ARC mods on a PAS-3 and a Stereo 70 back in the ‘70s, and I’ve had some classics over the years (7 & 8b, MC75s, 275, Yamaha B3, Hafler 500 etc). But I never sprung for ARC because stuff that good requires speakers that good to fully appreciate. Originally Posted by Jabberwocky
My primary speakers from the day I bought them in ‘75 until a few years ago were LS3/5as. They did justice to the range they could reproduce, but they have obvious limitations. I had some world class speakers for brief periods until my wife begged me to get them out of the living room, like Infinity Reference Standards that I got on the cheap because they’d been bought and returned twice before for the same reason
The PrimaLuna drives my Focal 726 towers very nicely. I’ve loved the sound since I got both 13 years ago. And I still love the TD125 / SME I bought in 1969.
-
…and that’s the norm. I’ve had one failure - I lost a 6L6 in my B15N in 1964. There was no damage to the amp, and a fresh one let me finish the gig. I used a Boogie HunRee I bought new in ‘78 for over 30 years without a failure. I did put a Rotron fan in it to be safe, but that was probably a waste of money. I changed the tubes once maybe 15 years in, just because I thought I should. That was also a foolish waste of $. Originally Posted by Greywolf
It’s the odd blown fuses, 20% of output tube failures that are dramatic and damaging, etc that concern me. Nothing is likely to happen to a well maintained tube amp for years. But the occasional failure will ruin the gig unless you have the parts to fix it or a backup amp with you.
-
I drove a pair of ProAc Response 2s sat on a pair of 4-post Target stands with the ARC CL150s. Love squeezing a quart out of a pint pot. Hifi should be fun; within the frequency limitations the ProAc Res2s performed like a bastard. Couldn't bond with most American behemoth loudspeakers save the Minnesota Magneplanars. ARC and Maggies were fellow Minnesotans made for each other. Sadly, with the passing of Bill Johnson ARC got sold to some Italian investment company which also bought Mcintosh of Binghamton, NY. Mcintosh has since been sold on; I don't know about ARC. Originally Posted by nevershouldhavesoldit
That’s serious sound! I built a Citation 2 and did the ARC mods on a PAS-3 and a Stereo 70 back in the ‘70s, and I’ve had some classics over the years (7 & 8b, MC75s, 275, Yamaha B3, Hafler 500 etc). But I never sprung for ARC because stuff that good requires speakers that good to fully appreciate.
My primary speakers from the day I bought them in ‘75 until a few years ago were LS3/5as. They did justice to the range they could reproduce, but they have obvious limitations. I had some world class speakers for brief periods until my wife begged me to get them out of the living room, like Infinity Reference Standards that I got on the cheap because they’d been bought and returned twice before for the same reason
The PrimaLuna drives my Focal 726 towers very nicely. I’ve loved the sound since I got both 13 years ago. And I still love the TD125 / SME I bought in 1969.
I had a pair of the Harbeth LS3/5as in Macassar Ebony. Sold it to a nutjob who collects LS3/5as. Begged me for them so I let him have them for $600. Joke's on me because they are fetching big bucks today!
Gave up on pursuing "The Absolute Sound". I run a pair of active Genelec 8040Bs streaming music via Spotify. I laugh at guys who spend $10 000 on a Shunyata power cord today. They in turn laugh at me and my <$3000 Genelecs; "can't even buy a decent interconnect for $3000, you poor thing."
Reply With Quote
Recommandations for Hollowbodies for $600 and under?
Today, 05:20 AM in Guitar, Amps & Gizmos