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Hope someone can help me on this. One of my M60 stopped making music. I checked the 3 fuses on the back of the amp next to the power cord entry. The fuse closest to the power cord entry (the third fuse as you look at the back of the amp reading from left to right) is blown. I read the owners manual but it is confusing. I think it is the 0.5 amp slow blow fuse for the driver circuit, according to the manual. The fuse itself has 5A 230V stamped on it; 5A seems too high. When I go to buy a 0.5 amp slow blow fuse, is the voltage rating important? Radioshack has a 0.5 amp slow blow 250V fuse. Thanks.
We have Jack down below who has identified the fuse as one that protects one of the two output stage power supplies. It makes perfect sense that this would be a 5A fuse. On the other hand, I cannot think of anything routine that would take out this fuse. If indeed it is an 0.5A fuse, then it would be protecting the input circuit, which has its own higher voltage/lower current supply (compared to the output supplies). We also have you saying that you are getting no music. Did the fuse blow while you were listening? What happened at the moment the fuse blew? Is the amp dead silent now, with that blown fuse?
Ralph got in touch with me. It was the 3 A fuse. I got a magnifying glass and looked at the rating on the blown fuse. It said 3 A, not 5. Ralph confirmed it. I usually warm up my amp for about an hour before listening. It blew while I was out of my audio room working on my computer.
is the question. Did Ralph have any ideas?
So far in 35 years the only source of arcing in tubes has been due to the tubes themselves.
The way the setup works is that any of our amps (since the beginning of the MkII era in 1996) have three fuses by the power cord. The first one is for the B+ for the power tubes and can blow if a tube arcs. The value is chosen so that if the output at the time of the arc exceeds the amp's normal power output, the fuse will blow.
The other two fuses are for the power tube filaments (2nd fuse) and the driver circuit power supply (3rd fuse). These rarely get blown, in fact we know of no cases where the 2nd fuse has ever blown.
This approach is very simple and proven effective to protect both the speaker and the amp in the event of a tube shorting.
The value of the 1st fuse is listed in each manual, but is
5A slow for S-30 (3AG style)
3A slow for M-60 "
5A slow for MA-1 "
8A slow for MA-2 (FNM style, the 'bottom' fuse)
Once a fuse has blown, you have to see if the guilty tube is still up to its tricks. I usually just install the fuse, warm up the amps and then turn on the B+ (red light). Sometimes the tube will misbehave right away, sometimes it won't. I knock on the tops of the tubes with my knuckles, something that any technician working with tubes learns to do fairly early on. Usually the tube will react and this way lets me know which one it is.
If I don't have a replacement on hand I simply operate the amp with a reduced complement by pulling a tube from the opposite tube bank as well as the defective one. This will get you by if the tube arcs at the most likely time, 5 minutes to 5:PM on a Friday :) However back in the old days it was a fairly common practice to have a few spares on hand for just such a situation, and its still a good idea today.
So, you don't need separate fuses for each of the supplies to the circlotron? I have never had an issue, so I never had reason even to think about this. Perhaps not if the two supplies run off a single power transformer with dual secondaries, which I think is the way mine are set up. Also, as I (apparently incorrectly) wrote, I would have thought that the internal fuse in the culpable 6AS7 would blow first, saving the main fuse from ever blowing. That's what always happened with my amps using 6C33Cs, except the 6C33C uses an external fuse at each tube socket, as you know.
The Mk. 1 amplifiers had separate fuses for each half of the Circlotron output. These days when we see them, we bypass those fuses, and rely entirely on the AC power fuse instead. In a nutshell it just makes the amp more reliable and there are less fuses to look out for.
Use only the rated fuses.
As indicated in another post, most likely one of the power tubes blew. The 6AS7 tubes are a different breed, and will still generally light up even if blown. If you don't have a tube tester to check them out, go to the Atma-Sphere website and read about the visual examination procedure.
If in fact a tube blew, just pull a pair out of each amp and run them that way until you can get a replacement tube.
Do make sure to check the DC offset when you do that.
Also, read the manual carefully for the unit, and make sure that you give the tubes plenty of time to warm up before applying full power.
Admittedly there may be a mechanism that I am not dreaming of, but ordinarily the demise of a single output tube ought not to take out a 5A slo-blo fuse, especially in the case of the 6AS7, which has an internal fuse wire inside each tube. Current goes up too high, fuse wire burns up, tube dies. No harm, no foul to the rest of the amplifier. This is one reason Ralph likes the 6AS7.
Sorry to hear about your problem--especially on a holiday weekend. According to your manual, the fuse closest to the IEC plug is the output stage B+. This fuse is a 3 to 5 amp slow blow fuse not a .500. I would replace with what is in there now--which seems to be a 5 amp slow blow.
The cause most likely if I were to guess--is that you have had one or more
output tubes to arc over. This is not such a big deal. I would check the DC offset and if it's difficult to zero out on your meter--then you may have one or more output tubes go south. Just replace the tubes if you have some and if not then do a statistical swap with the tubes that are in there now til you get the DC offset to settle down.
Thanks. It was the 3 A fuse(I checked the rating with a magnifying glass). Ralph told me it was probably an arced tube that blew the fuse. This is the third bad tube since I got the amp 6 months ago.
Jack, Is that a fuse for the output stage B+, aka the low voltage supply(ies)? In that case there should be two fuses, one for each of the two output supplies. (I don't have either a manual OR an amplifier to look at.) I have had output tube arc-ing, but it never took out a major fuse like that, I think because the arc-ing is a transient event so the overload should not persist long enough to take our a slo-blo fuse. The tube should take itself out, in the case of the 6AS7 via its internal fuse wire. Even with 6C33Cs, which can also arc and which are each externally fused, the worst that ever happened was to take out that external single fuse for the single tube that arc'd. As you know, this does not affect the ability to use the amplifier or to get sound out of it. It's a puzzlement what happened here. But I have to say I would err on the side of caution.
It's Christmas eve, so best wishes to all.
I want to be able to listen to music over the weekend, so I wanted a quick answer. I've called and emailed Atmasphere today but no response yet.
Forgot to mention my M60 is the latest version, I bought from a dealer 6 months ago.
If the amplifier is that new, then the factory is standing behind you, and your dealer should be also. So why ask here? 250V rating is typical for a fuse to be used in the AC line, but the question is why did the fuse blow in the first place? Answer: there is no way any of us can know. You need some advice from Atma-sphere and/or your dealer to seek the source of the problem.
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